Download ATOC-541

ATOC-541
Thursday, 19 February 2004
Lab 4: Fronts and frontogenesis
(Due Monday, March 8, 2004)
In this lab, we will analyse the fronts that form in association with two cyclones that affected North
American weather patterns during the period 16-20 February 2004.
Data, scripts, and other information for this lab can be accessed from your lab account at:
/disk4f/web/ATOC-541/lab04, http://zephyr.meteo.McGill.CA/wxlab/ATOC-541/lab04
The following directories contain information as indicated below:
- animation: SLP and 1000-500 thickness animations
- frontal_analysis: manual CMC and NWS frontal analyses
- question01: data files and scripts for question 1
- question02: data files and scripts for question 2
- question03: data files and scripts for question 3
- question04: data files and scripts for bonus questions
The animation loop of SLP and 1000-500 thickness shows one cyclone (an "Alberta clipper") forming over
Western Canada along an arctic baroclinic zone; a more intense system forms off the coast of the
southeastern U.S. along a western Atlantic baroclinic zone parallel eastern coast of the U.S. before bringing
strong winds and varied precipitation to Atlantic Canada. Both these cyclones are associated with frontal
formation along their respective baroclinic zones.
1. The GEMPAK subroutine SNCROSS can be used to produce isentropic cross sections of raw rawinsonde
data. The advantage of this type of analysis is that it uses data unaffected by filtering processes. The
disadvantage is that both spatial and temporal resolution are limited. In question 1, you will use
SNCROSS to analyse the effect of a cold front that forms and passes over the southeastern U.S. in
association with the coastal cyclone described above.
The following files are needed:
animation/avn.anim.slp.html
20040216_upa.gem
snmap.exe
2004021612Z_snmap_850.ps
snmap.exe
animation of case
rawinsonde data for indicated date
script to produce plot of available data and stations
map plot of raw station data for indicated date
script to produce map plot of raw station data
2004021612Z_xeast.ps
xeast.nts
snmap.exe
xeast.nts
sncross_4pan.exe
cross section of eastern North America for indicated date
holds settings for producing cross sections in x-windows
script to produce plot of raw station data
holds settings for producing cross sections in x-windows
script producing 4-panel plot or sounding cross sections
The 850 mb map of height, temperature, and rawinsonde data for 12Z 16 Feb 2004 in
2004021612Z_snmap_850.ps shows a strong baroclinic zone just north of Florida in association with a
cold pool over PIT (Pittsburgh, PA) and GSO (Greensboro, NC). The isentropic cross section shows that
this is part of a dome of cold air centred near Hudson's Bay and Northern Quebec (near station WZC)
that extends down to the Appalachians. Another "dome" of cold air can be seen aloft at 250 mb near the
stations PIT (Pittsburgh, PA) and BUF (Buffalo, NY). Enhanced baroclinic zones reside to the south of
these systems. As a result, an enhanced subtropical jet of 150 Knots can be found just above 250 mb at
station GSO.
SNCROSS can be accessed A) by typing in "sncross" on your workstation; or B) by typing "gempak" (to
start the gempak interface), "2" (to access programs that use raw sounding data), and "4" to enter
SNCROSS.
a) Produce 850 mb maps of North America similar to 2004021612Z_snmap_850.ps for 12Z on the 17,
18, and 19 Feb using snmap.exe. Identify the 850 mb cold front (if present) for the coastal system.
You may use surface analyses in the directory "frontal_analysis" to aid you in locating the front.
Remember that cold fronts tend to "tilt" northward with height.
b) Produce cross sections of winds and potential temperature similar to 2004021612Z_xeast.ps for the
dates above. You may use xeast.nts to set SNCROSS for xw plots, or produce printable plots using
sncross_4pan.exe.
c) Briefly describe the evolution of this cold front in the following terms for the above dates.
i) The southward surge of the cold surface air in relation to the passing Atlantic cyclone.
ii) The vertical extent and tilt of the front.
iii) The latitude, intensity of the low-level cold front (below 700 mb) and upper level front.
iv) The affect on the latitude, intensity, and vertical extent of the subtropical jet.
2. From a dynamic point of view, a front is defined as a wave that propagates to regions of frontogenesis
from regions of frontolysis (i.e. negative frontogenesis). A stationary front does not move, and will form
or be strengthened along a zone of stationary frontogenesis.
a) Use the AVN surface analysis and the script "fronto_sfc.exe" to plot maps of raw surface
observations as well as contours of 1000mb height, temperature, and frontogenesis for the following
times:
00Z and 12Z 17 Jan, 00Z and 12Z 18 Jan, and 00Z and 12Z 19 Jan.
Analyse the fronts associated with the evolving Alberta clipper. Note that that the GEMPAK
function gfunc = frnt(thta,obs) uses potential temperature and real winds as input, and a scaled,
dimensionless value of frontogenesis as output. It does not include the effects of diabatic heating or
friction.
b) Briefly describe the evolution of the warm front associated with the system. Describe its strength and
location in relation to the surface cyclone for various times, and it's propagation with respect to zones
of frontogenesis.
3. The terms "overrunning" or "isentropic lift" are often used to describe the flow across warm fronts as
warm air is advected into a region. Air from the warm sector of a cyclone is said to overrun the cold as it
ascends in association with warm advection. This process can be analysed from an isentropic point of
view: because potential temperature must be conserved in unsaturated processes, air parcels follow
isentropic surfaces and the tilt in isentropes along warm fronts is said to provide a "lift". Therefore,
isentropic cross sections across warm fronts, when taken along axes that are parallel to prevailing winds,
can be useful in locating both the height and geographic location of ascent associated with strong thermal
gradients.
a) You will need to choose stations using surface maps generated by "snmap.exe" to alter the SNCROSS
variable CXSTNS and produce frontal cross sections. Circle the stations you use to create your cross
sections.
b) For 00Z 18 February 2004, use SNCROSS to create a cross section of the warm front associated with
the "Alberta clipper".
c) For 12Z 19 February 2004, use SNCROSS to create a cross section of the warm front associated with
the Atlantic cyclone.
d) In each of these cases, use the cross sections to describe the height and location where the isentropic
lift associated with the warm front appears to be strongest. Briefly explain your reasoning.
Bonus question: To be announced. Check the course web page regularly for bonus question.