Download Atmospheric Sciences 101, Summer 2003 Homework #6 Solutions

Atmospheric Sciences 101, Summer 2003
Homework #6 Solutions
1. Fronts and middle latitude cyclones
a. Why do cumuliform clouds typically form at a cold front?
Ahead of the cold front the air is usually relatively warm and moist and thus often
conditionally unstable. Due to friction caused by the cold air mass moving along the
surface, the edge of the air mass (the cold front) slopes rather steeply, and this causes the
warm air to be forced upward. This push is often enough to get convective cumuliform
clouds to form at the front.
b. What is the primary energy source for middle latitude cyclones?
The primary energy source is the horizontal temperature difference due to warmer air on
one side of the system and colder air on the other side. The mid-lat cyclone takes utilizes
this energy when the warmer, less dense air flows sloping upward and the colder, more
dense air flows sloping downward turning the potential energy of the air masses into
kinetic energy of motion.
c. What physically is an occlusion? What does it mean for a middle latitude cyclone when an
occlusion occurs?
An occlusion, or occluded front, occurs when around a middle latitude cyclone the cold
front catches up to the warm front forcing the warm air up off of the surface over the
colder air masses around the system. When a middle latitude cyclone becomes occluded it
will begin to dissipate and eventually die off because it is cut off from the supply of warm
air that helps provide the horizontal temperature contrast that energizes the system.
2. Forecasting
a. In order to forecast future weather you must know the current weather, which requires
observations. The two methods of observing the current state of the atmosphere were
discussed in class – in situ measurements and remote sensing. In what way are in situ
measurements superior to remote sensing? Conversely, in what way is remote sensing
superior to in situ measurements?
In situ measurements are more accurate since they are direct. Remote sensing allows for
greater spatial and temporal resolution as more measurements are made.
b. Even if you could have ideal observations, those must be input into a computer model to
produce a forecast. Unfortunately computer models are not perfect. What are two distinct
shortcomings involving weather models that hinder forecasting?
Examples include:
• Observations must be adjusted to fit onto the grid the models use for calculations
• Non-global models have boundaries that must be accounted for some way
• Models are limited in their spatial and temporal resolution
• Models must parameterize small scale phenomena and features that are too small to
be explicitly modeled
c. There is a limit on how far out in time weather forecasts can be made because the atmosphere
is “chaotic”. Explain why the chaotic nature of the atmosphere limits predictive ability.
Since the atmosphere is chaotic small disturbances in the weather can amplify and grow
with time. The same is true in computer weather models, as small errors because the input
data is not perfect grow with time and come to dominate the forecast out into the future.
No matter how good the model is, the tiny initial imperfections cannot be eliminated from
the observations and the input of them into the model, thus forecasts looking ahead more
than about two weeks will not be very accurate unless by luck.
3. Thunderstorms and tornadoes
a. Why does an ordinary single cell thunderstorm last for such a short period of time?
The updraft that produces the storm is cut off by the downdraft of cold air caused by
entrainment of drier air and evaporation. This occurs because there is little vertical wind
shear, and as a result the downdraft occurs in the same place as the updraft.
b. How does low level vertical wind shear (wind changing with height) affect the gust front and
cold pool such that severe thunderstorms live longer than single cell thunderstorms?
Low-level wind shear has the net effect of pushing the convective thunderstorm tower over
the edge of the cold pool where the tower can be fueled by warm, moist air being forced up
by the edge of the gust front. The gust front and cold pool along the surface are pushed
along more slowly than the convective tower above. Thus the tower ends up being
positioned over the edge of the cold pool and can continue to be receive warm, moist air
that is forced to rise by the moving gust front and thus persists longer than a single cell.
c. Thunderstorms are observed most commonly in the United States along the Gulf coast and
specifically in Florida. Why is hail rarely observed in that area?
Conditions are generally less favorable over Florida for more severe thunderstorms that
produce hail. Vertical wind shear is typically relatively weak, the jet stream is rarely over
the state, and there is less likely to be much cold air aloft. Also, since the air is warmer and
moister through a deeper layer, if they do form hailstones are more likely to melt before
reaching the surface.
d. What are the atmospheric conditions most favorable for tornadoes to occur?
The following conditions are conducive for tornado formation:
• Strong winds aloft, typically from the polar jet and just upstream of a trough axis
• Cold, dry air aloft and warm, moist air at the surface to set up a strong conditional
instability
• A shallow temperature inversion over the warm, moist air so that convection does
not occur continuously but rather in a large burst that causes a severe thunderstorm
• Strong vertical wind shear to lead to the rotation of the storm and eventually the
tornado
4. Tropical cyclones
a. Why can tropical cyclones not form at the Equator (0° latitude)?
To develop the necessary rotation as the air converges around the low pressure center,
some Coriolis force is required, but there Coriolis force is zero at the Equator.
b. Why are ocean waters in middle latitudes (generally poleward of 30° latitude) not favorable
for tropical cyclone formation?
The ocean surface waters are not warm enough in general there to provide enough energy
to support tropical storm growth and development.
c. What conditions aloft may keep tropical cyclones from forming even if surface conditions
are optimal?
Associated with subtropical high pressure centers there may be subsidence (sinking air),
which results in warming, perhaps to the point of creating an inversion that suppresses
convection keeping clouds from developing vertically. Also, tropical cyclones cannot form
where horizontal upper-level winds are very strong because such winds disperse the heat
that drives the storm and keep the convection from organizing into a coherent system.
d. Suppose a hurricane with maximum sustained storm-relative winds near the eye of 100 mph
is moving onshore due northward at a speed of 15 mph directly over Mobile, Alabama on the
Gulf coast. Would the winds observed at a surface weather station as the eye of the storm
moved onshore be faster at Biloxi, Mississippi or Pensacola, Florida? Explain your choice.
The observed winds at the surface would be faster at Pensacola, which is to the east of the
eye of this northward moving hurricane. Winds around the hurricane (since it is in the
Northern Hemisphere) circulate counterclockwise, meaning they are southerly (from south
to north) on the east side of the cyclone and northerly (from north to south) on the west
side of the cyclone. The storm-relative winds of 100 mph are added to the 15 mph due to
the northward movement. On the east side the winds are in the same direction as the
movement so their observed speed would be 100+15=115 mph, while on the west side the
winds are opposite the movement so their observed speed would be 100-15=85 mph.