Download Changes of meteorological parameters that influence tornadoes and

Changes of meteorological parameters that
influence tornadoes and thunderstorms in climate
simulations with models from the fifth phase of the
Coupled Model Intercomparison Project (CMIP5)
Sindy Klotzsche
1799799
adv: Prof. Dr. J. Quaas
coadv: Dr. M. Salzmann
Master of Science, Meteorologie
September 24, 2013
Contents
1 Abstract
1
2 Introduction
3
2.1
Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2.2
Formation of thunderstorms and tornadoes . . . . . . . . . . . . . . .
5
2.3
Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Data and Methods
13
4 Results
19
4.1
Results of the MPI-ESM-LR . . . . . . . . . . . . . . . . . . . . . . . 19
4.2
Model Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5 Summary and Discussion
41
Bibliography
45
3
1 Abstract
Severe thunderstorms and tornadoes are a great threat for human lives and cause
high damage to property. Therefore, changes in the number and in the strength of
this phenomena by changes in the concentration of greenhouse gases are examined
in this study for the United States of America. General Circulation Models (GCM)
of the CMIP5 are used to examine five parameters, which play a major role in the
formation of thunderstorms and tornadoes. An increase in Convective Available
Potential Energy (CAPE) during the second half of the 21th century was found.
The largest changes was found in the coastal regions of the Gulf of Mexico and the
Gulf Stream as well as for the ”Tornado Alley”, which extend from North Texas
across Oklahoma and Kansas to Nebraska. This is attributed to the increase in
water vapor in the lower atmosphere. CAPE and Convective Inhibition (CIN) are
interdependent, due to the existence of CIN, CAPE may increase and if CIN is not
too large, severe thunderstorms can form. The Lifting Condensation Level (LCL)
is decreased for parts of the USA where CAPE is increasing, that facilitate the
formation of tornadoes. The vertical wind shear (VWS) is slightly decreasing in the
future for most parts of the eastern U.S., which impede the formation of organized
storms and tornadoes. However, the decrease of shear is more than compensated by
the increase of CAPE, so that an increase in severe thunderstorms also in connection
with tornadoes can be assumed.
1
2 Introduction
2.1 Motivation
Thunderstorms and tornadoes are a great threat to human lives and objects. Annually occur about 365000 thunderstorms around the world and a few thousand people
die of the effects of flashes [Goruma, 2010]. About 60 people die every year in the
United States of America as a result of the tornadoes and the damage to property
amounts to million US-Dollars [Sävert]. At the last years many severe storms were
observed in connection with high mean temperatures. An example of this is the tornado series in March 2012. On the 2. and 3. March 2012 more than 100 tornadoes
crossed several federal states of the USA [Tagesschau, 2010]. The reports of severe
weather events are shown in figure 2.1. Most of these storms moved across the states
Indiana and Kentucky. The consequences were disastrous: 39 casualties, more than
2000 strongly injured people and a loss of several million US-Dollars. Some cities
were completely destroyed for example Marysville in Indiana [Tagesschau, 2012].
In this part of the USA tornadoes occur mostly in summer, such high numbers in
tornadoes are uncommon, especially for the early spring. The series of storms were
connected with the highest mean temperatures for March since weather recording
for North America. At some places, like a few parts of Indiana and Kentucky, which
belongs to the extended Tornado Alley, the temperature values were 9◦ C higher
than the mean temperatures of March, since weather records began (Fig. 2.2). This
example is not a individual case, many extreme weather events occur in connection
with uncommon high temperatures. Several studies suggested a connection between
the unusually temperatures and the large number of storms in this part of the USA
[Trapp, 2007; Solomon, 2013]. The interest in research increased, because of the re3
CHAPTER 2. INTRODUCTION
lation between the increase in number of severe storms and the climate change, this
is under investigation with climate models. Large-scale parameters have to be used
to examine thunderstorms and tornadoes in climate models, because the resolution
of the models is to rough.
Figure 2.1: Reports of severe weather events. source: National Weather Service (2012)
Figure 2.2:
Temperature anomalies for North America March 2012.
source:
http://www.wetteronline.de/wotexte/redaktion/extremwetter/2012/04/0401 rm RekordMaerzin-den-USA.htm
4
2.2. FORMATION OF THUNDERSTORMS AND TORNADOES
2.2 Formation of thunderstorms and tornadoes
To examine thunderstorms and tornadoes in GCM’s, first understand the formation
conditions of them. There are two types of thunderstorms, which are distinguished
by the trigger. Cold front thunderstorms caused by coincidence of warm, moist air
and cold, dry air . The cold front forces the warm air to rise.
The trigger of heat storms is intensive solar radiation. The ground is heated by
radiation, therefore the air near the ground is heated by terrestrial radiation of the
surface and if the heat is strong enough, than the air parcel rises independently. If it
is not strong enough, the lifting can be enforced for instance on a hill or mountain.
The next steps of genesis are equal for both types (Fig. 2.3).
Figure 2.3: Formation of a thunderstorm.
source: http://www.vde.com/de/Ausschuesse/Blitzschutz/FAQ/fo/PublishingImages
/Gewitterentstehung/21awww1.gif
The warm, moist air parcel expands during ascent, therefore it cools and that is
why it can no longer hold all water vapor. If the temperature is equal to the dew
point, condensation starts. The water vapor is deposited on the condensation nuclei
and they liquefy to little water droplets. During the process latent heat is released
and adds the parcel an additional buoyancy. This is the genesis of Cumulonimbus
5
CHAPTER 2. INTRODUCTION
clouds (Cb’s).
Under the warm air package prevails low atmospheric pressure, that is why warm air
is converges. Warm air is pulled in, rises and forms the updraft tower of the thunderstorm. Eventually the air parcel can not rise higher, because its temperature has
become equal to the temperature of its surrounding (level of neutral buoyancy) or its
bordered to the tropopause. The clouds flows apart at the upper edge, whereby the
so-called anvil is visible. If the air parcel still have momentum at the tropopause,
it can overshoot the troposphere. This applies for many supercells in the midwest
of the USA. A thunderstorm is born, if lightning and thunder are formed at the Cb’s.
Tornadoes are small-scale cyclones, which are rapidly rotating and extend from the
convective parentcloud to the earth surface. Most tornadoes occur in the Tornado
Alley in the USA, because of the ideal development conditions with the warm moist
air coming from the Gulf of Mexico and the cold, dry air coming from the North
West. In addition to the already mentioned states of the Tornado Alley (Texas,
Oklahoma, Kansas and Nebraska), tornadoes also occur more frequently in parts
of North and South Dakota, Iowa, Illinois, Missouri, Arkansas, Louisiana, Indiana,
Ohio, Kentucky and Tennessee. The highest frequency of tornadoes per unit of
all US-states is in Oklahoma. Tornadoes are also often found in Great Britain,
Argentina, Australia, Eastern Europe, Italy, Germany and Japan (Fig. 2.4).
6
2.2. FORMATION OF THUNDERSTORMS AND TORNADOES
Figure 2.4: Occurence of tornadoes worldwide. source: Beppler (2010)
The diameter and the duration of tornadoes varies greatly. Most of them have a
width of about 60 meters to 150 meters and some tornadoes can be broadly up to 2
kilometers. Most small-scale cyclones go on less than 15 minutes, but they also can
stay for a few hours. The length of the traveled distance also varies strongly, some
tornadoes are nearly stationary and other ones move a large distance. An example
is the tornado on the 21th of December 1947, which moved across Arkansas and
covered a distance of 386 kilometers [Ludlum, 1989]. The intensity of a tornado is
indicated at the Fujita Scale, developed by Professor Doctor T. Theodore Fujita in
1971. It includes 12 categories (F1-F12) depending on the rotational wind speed,
which is measured by a radar or is estimated from the destruction caused by such
a storm [Kraus and Ebel, 2003]. Category 5 is limited by 513
km
h
from the fastest
tornado, which was ever recorded. It was the Oklahoma City tornado on 03.05.1999.
Currently, the values of F6 to F12 are only theoretical.
There are two types of tornadoes with regard to their genesis. The first type is
a supercell tornado. A supercell is a special appearance form of strong convective
storms. It is a durable single thunderstorm cell and the essential element of this cell
is a vortex within the parent cloud. The most tornadoes in the USA are supercell
tornadoes, because the conditions of form such single thunderstorms are optimal,
7
CHAPTER 2. INTRODUCTION
above all in the Tornado Alley. A deep pressure system above the central plains
pulls warm, moist air from the Gulf of Mexico. Cold, dry air flows from Canada
to the central plains with high wind speeds over the Rocky Mountains, that is why
the cold air receives an additional drying by the sliding on the leeward side of the
mountains. Thus, the differences between the warm, moist air and the cold, dry air
are even greater. Over the Tornado Alley the cold air lies above the warm air and
stops the warm air from rising first, but if there is a uplift trigger, the warm, moist
air fires explosively in the height and Cb’s are formed. Warm air parcels often break
through the tropopause and can tower it a few kilometers, because the temperature
gradient between the air parcel and its surrounding is really large.
Figure 2.5: Fully developed thunderstorm cell with a roller gust.
source: http://www.seewetter-kiel.de/bilder/wetter/gwzelle.gif]
The thunderstorm cell can begin to rotate if vertical shear is available. If the horizontal wind increases with height, it results in a roll motion (Fig. 2.5). So-called
roller gusts are formed, this is a rotation around a horizontal axis, but for supercells
and tornadoes a rotation around a vertical axis is needed.
8
2.2. FORMATION OF THUNDERSTORMS AND TORNADOES
(a) Tilting of the rotation
(b) Genesis of a supercell
Figure 2.6: Tilting of the horizontal vorticity into the vertical to form a supercell or a
tornado. source: Nikolai Dotzek
This happens when the roller gusts passes into the region of the thunderstorm updraft, the gusts are stretched and the horizontal rotation is tilted into the vertical
(Fig. 2.6 a+b)). The upwind zone of the storm is now forced to rotate, a supercell is
born (Fig. 2.6 b). This effect is enhanced by a wind direction shear, for example in
the Tornado Alley, where the warm air comes from southeast and the cold air from
northwest. Under the raised roller guests is a strong pressure drop. To compensate
the pressure drop, the easiest way is, that the air tube is moved toward surface, this
stadium of tornado genesis is called funnelcloud. Only if the air tube has contact
with the ground and the rotation of the supercell is strong enough to put the whole
air tube into rotation, then a tornado is born.
The genesis of non-supercell tornadoes is similarly, but without a rotating parent
cloud. This type occurs especially at a convergence line. By the convergence the
air masses can forced to rise and so Cb’s could form. If there is a wind speed shear
near the ground through turbulences, then roller guests can be formed and if they
reach in the uplift area of the Cb’s, tornadoes can be formed as well as the supercell
tornadoes.
9
CHAPTER 2. INTRODUCTION
2.3 Parameters
As already mentioned the lenght scales of thunderstorms and their by-products like
tornadoes are to small to investigate them in global climate models, because their
resolution is to rough. Based on the genesis mechanism of convective storms is
clear, that meteorological variables of the large-scale surrounding are important.
To investigate the surrounding of convection, moisture near the surface is needed
and is therefore examined. Additionally, the Convective Available Potential Energy
(CAPE) is used as well as the vertical wind shear near the ground, it is needed
to form organized storms and tornadoes [Rasmussen and Blanchard, 1998; Craven,
2002]. The Lifting Condensation Level (LCL) is also used, which favors the tornado
formation the smaller it is and is thus suitable to distinguish between tornadic and
non-tornadic conditions [Brooks, 2003]. The Convective Inhibition is also considered.
CAPE is one of the most important parameters in the thunderstorm forecast, because it gives the direct measure of the available energy maximum. CAPE provides
information about how stable or unstable the atmosphere is to estimate the intensity of convection and the storm potential. To form thunderstorms and tornadoes,
the stratification of the atmosphere has to be unstable. The warmer the air parcel is compared to its surrounding air, the higher it can rise and the stronger the
convection and thus also the storm can be. Another criterion for a thunderstorm is
high moisture in the lower atmosphere. It is the basic prerequisite for the genesis
of a thunderstorm and high values of CAPE. The higher the temperatures in low
levels are, the more the air can take up moisture and thus provide enough fuel for a
strong storm. For a tornado, a further condition is necessary, a vertical wind shear
(VWS). The wind velocity has to increase with altitude so that a vortex can form.
The collision of the two air masses from different directions supports especially the
10
2.3. PARAMETERS
supercell formation. The vertical wind shear is computed by the following equation:
δ~v ∼ g
k̂ × δta
=
δz
f ta
(2.1)
where g is the gravitational acceleration, f is the Coriolis parameter, ta the temperature and δta is the horizontal gradient of temperature from equator to pole [Trapp et
al., 2007].
δṽ
δz
decrease increasingly in the future, because the temperature gradually
increases. Another reason is the decline of the meridional gradient of temperature
which results from greater warming of the pols than the equator regions. Through
the increase of temperature the ice at the poles melts which reduces the albedo.
That means less radiation is scattered back and more will be absorbed whereby the
greater increase in temperature is adopted.
In addition to those already mentioned necessary conditions a low LCL is advantageous. It is the altitude where the temperature and the dew point are equal and
the cloud formation begins. The LCL depends on the vertical temperature profile
and the humidity of the air parcel. The lower the LCL, the easier it is to form a
tornado, because the distance to the ground is smaller.
The studys of Trapp et al. (2007) and Trapp et al. (2009) examined also the
changes in the frequency of thunderstorms and their by-products caused by climate
change. Both found an increase in mean values of CAPE for the future period almost everywhere in the U.S., which was attributed to the growth of the specific
humidity. The vertical wind shear, between 0 and 6 km, showed a decrease for
almost the entire USA. They associated the increase in CAPE with an increase in
the frequency of severe thunderstorms, but the decrease in shear acts hindrance on
the formation of severe storms. Therefore, they have used an additional parameter,
the product of CAPE and vertical wind shear. Based on Brooks et al., they said if
this product is greater than a threshold of 10000, the storm risk is increasing. This
indice showed an increase for the entire U.S., the strongest growth was found near
the Gulf of Mexico and the Atlantic coastel regions.
These results are simular to this study, but for this work additionally parameters
11
CHAPTER 2. INTRODUCTION
are involved to examine if the numbers as well as the strength of thunderstorms and
tornadoes are increasing in the future by an increase in greenhouse gas concentrations. CIN is considered in this study, to examine if convection can be triggered and
thus severe thunderstorms can form as well as the LCL. This is important to estimate, if a tornado can form. The use of models from the fifth phase of the Coupled
Model Intercomparison Project (CMIP5), to investigate the changes in number and
strength of thunderstorms and tornadoes, is new.
12
3 Data and Methods
To investigate potential changes in the number of thunderstorms and tornadoes,
General Circulation Models (GCM) are used. Climate models are complex computer
models to simulate past and present climate conditions and to calculate projections
of the future climate with different assumptions of social and political development.
The individual subsystems of the climate are considered in separate models which
are coupled to each other and in which atmosphere and ocean are the most important components of the climate system. Important is the conservation of momentum,
mass and energy. Climate models based on the simplified Navier-Stokes equations,
these are non-linear, partial differential equations.
A three-dimensional grid is placed over the earth and divides it in individual grid
cells, these cells represent the horizontal resolution of the models. By numerical solution of the Navier-Stokes equations the physical conditions of the individual grid
cells are calculated as well as the exchange of energy and mass between the cells
from time step to time step. Climate models that do these calculations for the entire
planet are called General Circulation Models (GCM’s). In the IPCC report 2007
the most global models used a horizontal resolution of about 200 km×200 km.
Additionally, climate models are divided into different vertical levels to investigate
the thermodynamic variables at different heights. Due to the rough resolution, smallscale processes can not be directly represented, like a thunderstorm wich extends up
to 30 km or a tornado with an extension between 60 meters and 2 kilometers. Therefore small-scale phenomena have to be approximated by parametrization, this means
that the effect of this processes of the large-scale calculations must be estimated for
deep convection.
For this work models from the fifth phase of the Coupled Model Intercomparison
13
CHAPTER 3. DATA AND METHODS
Project phase are used, which was created in September 2008 at a meeting of 20
climate modeling groups from around the world [Taylor et al., 2012]. It is a new
set of different models with the aim to consider the simulated past climate and the
future climate, its change and impacts of this change in the different models and
the comparison between them. The past is simulated from 1850 to 2005 and the
calculation of the climate change are performed on two time scales. The first is
the short-term scale, which shows the climate until 2035 and the second time scale
extends to 2100 and moreover to look at the longer-term changes in climate [Taylor
et al., 2012].
The advantages over the previous CMIP are, that more models are available, some of
them with higher resolutions, and more model diagnostics for the better comparison
of the models.
For the calculation of the future climate with models from CMIP5 the Representative Concentration Pathways (RCP’s) are used. The RCP’s are four emission
scenarios, which are defined by the radiative forcing or the carbon dioxide content
and the content of other important trace gases. With the different emission scenarios
the influence of different policy measures and social developments should examined.
For the past, from 1860 to 2005, the course of the radiative forcing corresponds to
the observed past. The trends for the future up to 2100 depend on the evolution
of the population and on climate politics. Trace gas concentrations are calculated
depending on the future development with carbon cycle models [van Vuuren, 2011].
These concentrations are presented, as the radiative forcing, in figure 3.1 from 2006
up to 2100.
14
Figure 3.1: Represantive Concentration Pathways. source: van Vuuren (2011)
The curve of the RCP 2.6 is slowly rising until a peak of 3
a decrease is observed until 2.6
W
m2
W
m2
in 2040, thereafter
in 2100 through rapid implementation of the
policy measures to counteract the climate change. The RCP 4.5 shows an increase of
radiative forcing to 2050 to a value of about 4.5
W
,
m2
after 2050 it shows a stabilization
owing to changes in policy and population. An increase up to 6
W
m2
in the year 2100
are found in RCP 6. For this work the pathway with a rising up to 8.5
W
m2
in 2100
is important, because it is the most realistic path for greenhouse gas concentrations
according in the absence of substantial emission reductions.
In this study data from the MPI-ESM-LR, the GFDL-CM3 and the HadGEM2CC of the CMIP5 are used. In the Max-Planck Institute Earth System Model
(MPI-ESM) ocean and land surface are coupled by the exchange of energy, water,
momentum and important trace gases such as carbon dioxide [Giorgetta, 2012]. The
MPI has a horizontal resolution of 200 km (1.8759◦ ) and 47 vertical levels. For these
studies, interpolated data are used, the variables are analyzed for pressure levels.
15
CHAPTER 3. DATA AND METHODS
The Geophysical Fluid Dynamics Laboratory Coupled Model 3 (GFDL-CM3) is a
coupled model of atmosphere, ocean and sea ice, which includes interaction between
aerosol and clouds and between chemistry and climate. The troposphere and stratosphere are coupled in it. The horizontal resolution varies from 163 km to 231 km
[Donner et al., 2011].
The Hadley Centre Global Environment Model version 2 Carbon Cycle (HadGEM2CC) is a global climate model with couplings between troposphere, stratosphere,
land surface, hydrology, ocean and sea ice and under involvement of aerosols. The
horizontal resolution over land is 1.8759◦ ×1.259◦ , in mid-latitudes this is a grid box
having a size of about 140 kilometers [Martin et al., 2011].
To characterize the large-scale environment of thunderstorms and tornadoes in
the models, five parameters are used. This includes CAPE, the vertical wind shear
between 1000 hPa and 700 hPa (VWS), LCL, CIN and the moisture near the earth
surface, which were described in chapter 1. Further empirical variables are used
like the product between VWS and CAPE (Brooks et al., 2003) as well as the
days on which the indices exceed or fall below a prescribed threshold. The input
parameters for the MPI-ESM-LR are the four dimensional variables: temperature
(ta), specific humidity (hus) and the wind vectors (ua, va), which depend on time,
height, latitude and longitude. Furthermore, atmospheric temperature two meters
above ground (tas) and the surface pressure (psl ) are used, depending on time,
latitude and longitude. The input variables of the GFDL-CM3 and the HadGEM2CC agree with the ones of the MPI, but the surface pressure is not available, instead
the soil moisture (huss) is present.
An important prerequisite for the formation of thunderstorms and tornadoes
is the atmospheric moisture near the surface, it is averaged over the lowest three
pressure levels. In order to investigate the vertical wind shear, at first the magnitude
of wind shear is calculated from the horizontal components (ua, va)
~v =
p
u2a + va2 .
16
(3.1)
The vertical wind shear between the two lowest pressure levels of the models above
the ground is then given by the following equation:
V W S = ~vLevel 2 − ~vLevel 0 .
(3.2)
The height above sea level is needed for the calculation of CAPE and results using
the barometric formula, derived from the hydrostatic equation (eq. 3.3))
dp
= −% g
dz
(3.3)
, where g is the gravitational acceleration, dp and dz the changes in pressure and
height and % the density of air. The density % is replaced by using the ideal gas
equation
ta −→ % =
p = % Rgas
p
Rgas ta
(3.4)
and it results in the following equation for the change in height:
dz =
Rgas ta
dp.
p
(3.5)
The virtual temperature (Tv0 ) for the ground and the other pressure levels (Tv ) is
required to compute CAPE, which is obtained from the temperature (ta) and the
specific humidity (hus)
Tv = ta + (1 + 0.61 · hus).
(3.6)
To calculate CAPE with NCAR Commend Language (NCL), which is a programming language, the wrfcape2d function is used, for that CAPE is computed with the
maximum equivalent potential temperature. This is the temperature, which would
take the air humidity when the water vapor contained therein completely condensed
at constant pressure and the released heat of condensation is exclusively supplied to
the moist air. Then the air parcel is brought dry adiabatic to 1000 hPa.
Z z
θe,air parcel − θe,surrounding
CAP E =
dx
θe,air parcel
zLFC
(3.7)
with the equivalent potential temperature:
θe = Te
17
p0
.
p
(3.8)
CHAPTER 3. DATA AND METHODS
The equivalent temperature is computed by
Te = ta + 2.5 · hus.
(3.9)
θe,air parcel is the equivalent potential temperature for the air parcel and θe,surrounding
the temperature of the surrounding of the air parcel, it is integrated from the Level
of Free Convection (LFC) to the heighest model level.
The used function gives the LCL and the CIN, too. The input is computed from
1970 to 1989 for the history and from 2050 to 2069 for the RCP 8.5. The equation
is the same for CIN and CAPE except for the integration limits. CIN is in contrast
to CAPE integrated from the ground to the Level of Free Convection. Thus, CIN
is dependent on the specific humidity like CAPE.
18
4 Results
4.1 Results of the MPI-ESM-LR
The changes of specific humidity, CAPE, LCL, CIN and VWS are investigated for
the United States of America between the past and the future climate. The history
period is determined over 1970 - 1989 (HP) and the future period from 2050 to 2069
(FP). Probability Distribution Functions (PDF) are used to investigate the changes
in likelihoods of certain values of the different parameters averaged over the time and
the whole USA. Additionally, contour plots of the USA are considered to examine
the changes in mean values of the parameters averaged over the past and the future
period and the alteration in the number of days on which the quantities exceed or
fall below a empirical threshold. Only land points were considered in the analysis,
while ocean points were cut out. For CAPE a threshold of 1000
for the vertical wind shear 30
m
s
J
kg
was chosen and
by considering all values of it. The threshold of
LCL was set to 1200 m, values below it favor the formation of a tornado.
First, the difference between the future period and the past period of the five
described parameters is considered for the MPI-ESM-LR (Fig. 4.1). CAPE, CIN,
LCL, vertical wind shear and specific humidity are averaged over the time periods.
In general an increase in specific humidity, CAPE and CIN as well as a decrease in
the LCL and the vertical wind shear almost for the entire USA is observable. Furthermore, the growth of CAPE and the decline of LCL go along with the increase in
the humidity. The rise of CIN also follows the specific humidity for the southeastern
U.S.. In the following, the used parameters are examined more closely.
19
CHAPTER 4. RESULTS
Figure 4.1: Difference (FP - HP) in mean values between the FP and the HP for CAPE,
VWS, specific humidity, CIN and LCL
20
4.1. RESULTS OF THE MPI-ESM-LR
Figure 4.2: PDF of Specific humidity for the HP (left) and the FP (right)
Figure 4.3: Mean specific humidity for the HP (left) and the difference (FP-HP) (right)
If the past and the future periods are compared, it is clear, that the probability
of the moisture is shifted to higher values up to 0.006
kg
kg
(Fig. 4.2). For larger
values, the percentage remains approximately constant and is subject only to minor
fluctuations. Between the values 0.006
kg
kg
and 0.01
kg
kg
the likelihood decreased
for the future period and shifts to higher values above 0.01
kg
.
kg
A look at the
contourplot with time averaged humidity values in figure 4.3 confirms this increase.
The strongest changes are found near the Gulf of Mexico and along the Gulf Stream,
which flows along the east coast of the United States, where the specific humidity is
increased from 0.008
kg
kg
to 0.0094
kg
.
kg
A strong growth extends from the Gulf to the
north west to Oklahoma and to the north to the Great Lakes. It is known, that the
air temperatue will still increase in the future. This is accompanied by an increase
21
CHAPTER 4. RESULTS
in moisture, because the Sea Surface Temperature will be larger in the future and
thus larger amounts of water can evaporate. Additionally the air will be warmer
and can therefore absorb more moisture. As a result, the content of water vapor in
the air will increase. The humidity is the impulsion of a thunderstorm and supports
its formation.
Figure 4.4: PDF of CAPE for all ranges (top) and for values of CAPE > 1000
J
kg
(bottom) for the HP (left) and the FP (right)
The two pictures in the top of figure 4.4 show the PDF of CAPE and the ones below
show the probability of the values of the CAPE too, but only for values greater
22
4.1. RESULTS OF THE MPI-ESM-LR
than 1000
J
.
kg
For all CAPE values a shift of the height of the bins to the right is
recognized, that means a shift to larger values of CAPE for the future period. This
becomes more clear if we look at the values larger than 1000
J
.
kg
For all values of
CAPE an percentage increase is present and extreme values of CAPE larger than
4000
J
kg
are achieved.
Figure 4.5: Mean CAPE for the HP (left) and the difference (FP-HP) in mean CAPE
(right)
Figure 4.6: Days per year on which the mean CAPE > 1000
J
kg
for the HP (left) and
the difference (FP-HP) (right)
To examine the mean values of CAPE the contourplots in figure 4.5 are used. On
the left side of the figure CAPE is shown for the history period and on the right
side the change between the two periods. CAPE increases for the entire USA up
to about 280
J
.
kg
The strongest growth is expected to the Gulf of Mexico, North
Texas and Oklahoma, this is the southern part of the Tornado Alley, and along the
Gulf Stream. For the other parts of the Tornado Alley there is an excessively strong
increase of more than 100
J
.
kg
23
CHAPTER 4. RESULTS
Also the number of days per year on which CAPE is larger than 1000
an increase (Fig. 4.6). The history shows that the threshold of 1000
J
kg
J
kg
results in
is exceeded
by about 7 days at the Gulf of Mexico and the number increases in the future period
by 2 days. The growth from 2 days to about 2.5 - 3 days in the southern part of the
Tornado Alley has to be mentioned, too. The higher values of CAPE in the future
and the increase of days per year on which the threshold of 1000
J
kg
is exceeded are
associated with the growth of specific humidity. First, the averaged values of CAPE
(Fig. 4.5) and moisture (Fig. 4.3) are compared. It is notable that the largest
changes in values of CAPE correlate to the biggest increase in specific humidity.
The results agree with those of Trapp et al. (2007). It shows the direct relationship
between specific humidity and CAPE, which is justified in the Clausius Clapeyron
equation. This also corresponds to the anticipation according to the CAPE formula
(eq. 3.7), that means if the equivalent temperature increases due the growth of the
specific humidity, CAPE also increases. In the PDF functions in the figures 4.2 and
4.4 the relationship between moisture and CAPE is clearly, too. The probability for
higher values of CAPE increase, as well as the high specific humidity values. When
the humidity rises above 0.006
kg
,
kg
the probability stays comparative constant, but
small changes are also evident for the larger values. The values above 0.006
kg
kg
shift
to higher values of specific humidity, which results in higher values of CAPE. This
is consistent with the function of the correlation between CAPE > 1000
specific humidity (Fig. 4.7).
24
J
kg
and the
4.1. RESULTS OF THE MPI-ESM-LR
Figure 4.7: Connection of specific humidity (y-axis) and CAPE (x-axis)
The probability of CAPE is increasing with increasing likelihood of specific humidity
between values of moisture from around 0.03
kg
kg
to 0.05
kg
.
kg
The largest increase in
CAPE goes hand in hand with the growth of specific humidity between 0.009
and 0.14
kg
.
kg
kg
kg
This correlates very well with the PDF of the specific humidity for the
future period.
How strong the CAPE grows, is decisively influenced by additionally factors like the
vertical temperature gradient of the surrounding of an air parcel. The warmer the
air parcel is, compared with the proximity of it, the more CAPE may increase.
High values of CAPE are not enough to say severe thunderstorms can form. Other
factors must be considered such as CIN, which indicates how much energy has to
be overcome to trigger CAPE and thus a thunderstorm.
25
CHAPTER 4. RESULTS
Figure 4.8: PDF of CIN for the HP (left) and the FP (right)
Figure 4.9: Mean CIN for the HP (left) and the difference (FP-HP) in mean CIN (right)
Figure 4.10: Days on which the mean CIN < 15
J
kg
for the HP (left) and the difference
(FP-HP) (right)
For CIN the largest likelihood is for small values, between 0 and 25
J
kg
(Fig. 4.8).
This is conducive to thunderstorm formation, because too large values of CIN would
26
4.1. RESULTS OF THE MPI-ESM-LR
prevent the convection. In comparison between the past run and the RCP szenario,
a shift of the probability to larger values of CIN is recognized. The values of CIN
for the United States range from 0 to over 100
J
kg
(Fig. 4.9). The CIN in the future
is higher than in the past almost everywhere in the USA. The increase is up to 40
J
.
kg
The days on which CIN is smaller than 15
J
kg
decrease for the entire USA, most
near the Gulf of Mexico and the Tornado Alley (Fig. 4.10). A connection between
CIN, CAPE and specific humidity can be supposed, especially recognizable in the
surroundings on the coast of the Gulf of Mexico. The Convective Inhibition is like a
lid on a boiling pot, it prevents very sudden convection to reduce the CAPE. CAPE
and CIN are interdependent. If there is no CIN, the CAPE would be reduced directly
in only a weak convection. Where a strong lid is broken severe thunderstorms in
connection with tornadoes may form. That means a higher CIN can help the CAPE
to reach higher values by preventing immediate convection, but CIN has to be small
enough, that the LFC can be achieved and so severe thunderstorms can form. If the
CIN is too large, nevertheless, strong thunderstorms in connection with tornadoes
can form by raising on a hill or mountain, by fronts or at convergence lines.
27
CHAPTER 4. RESULTS
Figure 4.11: PDF of LCL for all ranges (top) and for values of LCL < 1200 m (bottom)
for the HP (left) and the FP (right)
In general the probability of small values of the LCL increases for the future period,
this also applies to the values smaller than 1200 m (Fig. 4.11). This matchs to the
mean values of LCL in figure 4.12, LCL decreases near the Gulf of Mexico and for
the southern Tornado Alley associated with the increase in specific humidity. As
already mentioned, LCL is the height, where the temperature of the air parcel is
equal to the dew point. The dew point depends on the moisture and the vertical
temperature gradient. The more humidity, the earlier the temperature is equal to
28
4.1. RESULTS OF THE MPI-ESM-LR
the dew point and the cloud formation may begin at lower heights.
Figure 4.12: Mean LCL for the HP (left) and the difference (FP-HP) in mean LCL
(right)
Figure 4.13: Days on which is LCL < 1200 m for the HP (left) and the difference
(FP-HP) (right)
The days on which LCL is smaller than 1200 m are increasing for the future period
almost for the entire eastern half of the United states (Fig. 4.13) with the strongest
increase for regions near the Gulf of Mexico and north of it as well as on the Atlantic
coast up to half a day. For the almost entire western half of the United states the
days, on which is LCL smaller than 1200 m decrease up to 1 day. The changes in
LCL heights go along with the increase in specific humidity, especially for the eastern
part of the USA. The dependence from LCL of the specific humidity is shown in
figure 4.14. The largest probability of high LCL values is connected to small values
of the specific humidity. With increasing humidity, above a value of 0.006
height of the LCL is decreasing, which favor the formation of tornadoes.
29
kg
,
kg
the
CHAPTER 4. RESULTS
Figure 4.14: Connection of specific humidity (y-axis) and LCL (y-axis) for the FP
Figure 4.15: PDF of VWS for the HP (left) and the FP (right)
30
4.1. RESULTS OF THE MPI-ESM-LR
Figure 4.16: Mean VWS for the HP (left) and the difference (FP-HP) in mean shear
(right)
Figure 4.17: Days on which VWS > 30
m
s
for the HP (left) and the difference (FP-HP)
(right)
For a organized thunderstorm and tornadoes a positive vertical wind shear is needed.
The probability of the vertical wind shear growth for smaller values and negative
values (Fig. 4.15). This correlates with majority of the mean values in figure 4.16,
because the wind shear decrease for large parts of the eastern USA and also for parts
in the west. In the major land areas for thunderstorms, at the Gulf of Mexico and
in the Tornado Alley, the vertical wind shear decreases by 1
2
m
s
and 20
m
.
s
m
s
from values between
The decrease in the future period can be expected by using the
thermal wind relation (eq. 2.1).
The days on which the vertical wind shear is greater than 30
m
s
remains virtually
constant for the future period. For some parts of the USA an small increase are
found like for most of the western half of the U.S., small decreases are found for a
large part in the Eastern USA.
31
CHAPTER 4. RESULTS
The decrease of the mean values of vertical wind shear acts hindrance on the formation of organized thunderstorms and tornadoes, but the increase of CAPE and
the decline of the LCL indicate an increase in the number of tornadoes and severe
thunderstorms. To examine, which parameter is predominant, the product of the
mean values of CAPE and vertical wind shear is considered based on Trapp (2007).
This composite parameter shows an increase for almost the entire USA in the future,
that means that CAPE is predominant compared with the shear (Fig. 4.18).
Figure 4.18: Product of mean CAPE and mean VWS for the HP (left) and the FP
(right)
4.2 Model Comparison
The MPI-ESM-LR is now compared with the GFDL-CM3 and the HadGEM3-CC.
For the comparison of the models I focused on the indices CAPE and CIN, because
the LCL and the vertical wind shear have similar values for the history and the
future period as well as the changes in their values between the two periods.
32
4.2. MODEL COMPARISON
Figure 4.19: MPI mean CAPE for the HP (left) and the difference (FP-HF) in mean
CAPE (right)
Figure 4.20: GFDL mean CAPE for the HP (left) and the difference (FP-HF) in mean
CAPE (right)
Figure 4.21: HadGEM mean CAPE for the HP (left) and the difference (FP-HF) in
mean CAPE (right)
The mean values in CAPE of the MPI are presented here again in figure 4.19 to
compare it with the ones of the GFDL (Fig. 4.20) and the HadGEM (Fig. 4.21).
While the values of CAPE in the MPI range up to 1000
the GFDL reaches maximum values of about 640
33
J
.
kg
J
kg
for the history period,
The highest CAPE for the
CHAPTER 4. RESULTS
HadGEM is about 400
J
.
kg
The CAPE values in the GFDL yield high values for the
entire Tornado Alley up to 400
J
.
kg
The biggest changes in CAPE values for the MPI
and the HadGEM are obtained in the proximity of the Gulf of Mexico, the southern
part of the Tornado Alley and along the coast near the Gulf Stream. The changes
are up to 300
about 250
J
.
kg
J
kg
for the MPI and a little less for the HadGEM with an increase of
For the GFDL results large changes for the same regions as in the
other models, too, up to around 300
J
,
kg
but the strongest one in this model are
found for the Tornado Alley with changes up to approximately 500
J
.
kg
Figure 4.22: MPI mean CIN for the HP (left) and the difference (FP-HF) in mean CIN
(right)
Figure 4.23: GFDL mean CIN for the HP (left) and the difference (FP-HF) in mean
CIN (right)
34
4.2. MODEL COMPARISON
Figure 4.24: HadGEM mean CIN for the HP (left) and the difference (FP-HF) in mean
CIN (right)
The Convective Inhibition is near the Gulf of Mexico greatest both in the MPI and
the HadGEM, but the values are very different. The CIN in the model of the Max
Planck -Institute takes values of about 120
J
kg
for this region and the one of the
HadGEM are much smaller wih maximum values of about 35
J
.
kg
It increase for
both models in the future. In the HadGEM the increase is greatest for the areas
near the Gulf, the Gulf Stream and for North Texas and Oklahoma. Consequently,
the increase follows the rise of the specific humidity. The GFDL is different to the
other models. In the history period the CIN is largest over the entire Tornado Alley
with values up to 120
J
.
kg
The change in the future is also greatest for this region
and increases by approximately 60
J
.
kg
The growth of CAPE and CIN go along with the increase in specific humidity. In
order to find the reason of the strongest increase for the GFDL in the Tornado
Alley the specific humidity of the GFDL is considered in more detail below. The
increase in specific humidity looks the same for the MPI and the HadGEM, the
largest growth is on the coasts of the Gulf Stream and the Gulf of Mexico. For the
other parts of the USA the values of the increase for the future period are equal,
but not so for the GFDL. The specific humidity is highest for the coast on the Gulf
of Mexico with values up to 0.008
specific humidity between 0.04
kg
kg
kg
,
kg
but also the Tornado Alley has high values of
and 0.06
kg
.
kg
The largest increase for the future
was found for the Gulf of Mexico and the Gulf Stream, but also for large parts of
the eastern U.S. and the Tornado Alley. So the increase of CAPE and CIN of the
35
CHAPTER 4. RESULTS
GFDL goes along the growth of specific humidity (Fig. 4.25).
Figure 4.25: GFDL mean specific humidity for the HP (left) and the difference (FP-HF)
in mean specific humidity (right)
Figure 4.26: MPI days on which CAPE > 1000
J
kg
for the HP (left) and the difference
(FP-HP) (right)
Figure 4.27: GFDL days on which CAPE > 1000
(FP-HP) (right)
36
J
kg
for the HP (left) and the difference
4.2. MODEL COMPARISON
Figure 4.28: HadGEM days on which CAPE > 1000
J
kg
for the HP (left) and the
difference (FP-HP) (right)
The strongest increase in days per year, on which CAPE is greater than 1000
J
kg
results in all three models for the regions near the Gulf of Mexico. For the MPI
the days with a CAPE above 1000
J
kg
reaches up to 8.5 days, this is one and a half
day more than in the history period. There is also an small increase in days in the
Tornado Alley. In the GFDL the greatest growth is from 3 days up to around 5
days in the future period near the Gulf as well as in Florida and the Tornado Alley
and west of it. The maximum value in the HadGEM in the history is 4 days for the
Gulf coast and increase by 3 days in the future period.
Figure 4.29: MPI days on which CIN < 15
(FP-HP) (right)
37
J
kg
for the HP (left) and the difference
CHAPTER 4. RESULTS
Figure 4.30: GFDL days on which CIN < 15
J
kg
for the HP (left) and the difference
(FP-HP) (right)
Figure 4.31: HadGEM days on which CIN > 15
J
kg
for the HP (left) and the difference
(FP-HP) (right)
The biggest decrease of days on which CIN is smaller than 15
J
kg
agrees closely with
the greatest changes in CAPE. The strongest decrease is in all three models in the
states near the Gulf of Mexico (Texas, Louisiana, Mississippi, Alabama, Florida)
and for parts of the Tornado Alley and in the west of it. This changes correlate with
the changes in specific humidity, too.
A summary of the three models: MPI-ESM-LR, GFDL-CM3 and HadGEM2-CC
is shown in figure 4.31. Picture (a) depicts the days per year on which the parameters
exceed or fall below the speified thresholds for the history period. Picture (b) shows
the increase or decrease of these days in the future period.
CAPE exceed the threshold of 1000
J
kg
on 6 days per year for the MPI, 1 per year
for the GFDL and 5 for the HadGEM. For the future period, the number of days
increase for all models, by around 3 days in the GFDL and the HadGEM and by
38
4.2. MODEL COMPARISON
(a)
(b)
Figure 4.32: Days per year on which CAPE > 1000
and VWS > 30
m
s
J
kg ,
CIN < 15
J
kg ,
LCL < 1200 m
for the HP (a) and the difference in days per year (FP-HP) (b)
almost 2 days in the MPI. CIN is smaller than 15
J
kg
on 13 days per year in the
GFDL and the MPI and on 12 days in the HadGEM. The days, on which CIN falls
below the threshold of 15
J
,
kg
decrease for the future period in all three models by
1.5 - 2 days. In the history period is LCL smaller than the threshold of 1200 m on
14 days per year in the GFDL and the HadGEM and on 13 days in the MPI. For
all models, the number of days is increasingly, in the GFDL it growth by 2 days, in
the MPI by 1 day and in the HadGEM it growth by half a day. The vertical wind
shear exceeds the threshold of 30
J
kg
by around half a day per year in the models.
The days on which the values of the shear are larger than the threshold increase by
around half a day.
In summary, it can be said that the probability of days with severe thunderstorms
and tornadoes increases in the future in all three models.
39
CHAPTER 4. RESULTS
40
5 Summary and Discussion
The aim of this study was to investigate the potential changes in number and intensity of thunderstorms and tornadoes due to the climate change. This was examined
for the United States of America with parameters, that reflect the large-scale environment of such storms well, because the resolution of global climate models are to
rough to consider thunderstorms and its by-products directly. Three models were
used of the fifth phase of the Coupled Model Intercomparison Project (CMIP5)
to compare the results. For these models five parameters were computed, which
contribute to the formation of thunderstorms and tornadoes. As one parameter
the specific humidity is considered, because it provides the drive of thunderstorms.
To examine the CAPE is important to estimate the strength of the storm, which
could form. CAPE is not always translated into convection, so the CIN must be
considered. The vertical wind shear provides clues about whether there can form
organized storms and tornadoes. In addition the LCL is considered to estimate the
risk for the formation of tornadoes. The changes of these parameter were compared
between the historical period, from 1970 until 1989 and the future period from 2050
to 2069. To simulate the future climate the RCP 8.5 was used.
CAPE increases for the entire USA as expected and it is associated with the increase in moisture. The strongest changes in CAPE are observed near the Gulf
of Mexico, the Gulf Stream and the southern Tornado Alley for the MPI and the
HadGEM, but for the GFDL the strongest growth results for the entire Tornado
Alley. This is connected to the high entry of moisture for the Tornado Alley in this
model. CIN is also increasingly throughout the U.S., but this is not a reason for
less convection. On the contrary, it can cause the growth of CAPE by preventing
immediate convection. CAPE can rise more and more and if the LFC is reached,
41
CHAPTER 5. SUMMARY AND DISCUSSION
severe thunderstorms especially supercells in connection with tornadoes can occur.
CIN can thus lead to an increase in severe storms, but the number of less strong
thunderstorms may decrease.
The increase in CAPE counteracts the decrease in vertical wind shear for large parts
of the USA in all three models, which is caused by a lower meridional temperature
gradient trough the climate change. The increased CAPE speaks for an increase in
number of severe thunderstorms and a growth in its intensity and the reduced wind
shear for a loss. However, the increase of CAPE more than compensates the decrease in shear (see Chapter 3). Nevertheless, consequently an increase is expected
in the number and intensity of severe storms. This are similar results as in Trapp
et al. (2007).
The lowering of the LCL for the area of the USA where CAPE is increased, increases
the likelihood of tornado formation, because it is easier for a tornado to form if the
way to the ground is smaller.
The difference in the mean values of CAPE in the three models may result of the
moment, when the convection is initiated in the model. If this is late, a higher
CAPE has built up and the convection is more violent.
The values of the examined parameters depend on the model resolution. Models with
a higher resolution reproduce the values of the indices more realistic, this could also
be a reason for the highest values of CAPE and CIN for the Tornado Alley in the
GFDL.
As expected the results of this study speaks for an increase in number of severe
thunderstorms and tornadoes for the southern Tornado Alley as an increase in its
strength in the MPI and the HadGEM. For the GFDL the strongest increase is given
for the states Kansas and Nebraska, which belongs to the Tornado Alley, too. An
increase in severe thunderstorms also in connection of tornadoes are expected for the
states around the Gulf of Mexico as well as for coastel areas along the Gulf Stream,
because the increase in CAPE and specific humidity and through the lowering of
the LCL. This means a higher risk for many cities like New York City and Dallas,
42
which was also found in the study of Trapp et al. (2009).
Finally, it should be said that this study is limited by the number of models and the
resolution of it. For future investigations more models have to be involved to have
more reliability and it must be mentioned if the used parameters are sufficiently or
whether additional factors influence especially the tornado genesis, which has not
yet been clarified in detail. For the formation of a tornado for example the orography have to be involved as well as small-scale changes in the parameters, that means
higher resolution models have to be used.
43
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47
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in
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den
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Erklärung
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