Justin Cox, Bobi Fox, Mike Seaman, and Andy Siffert for Meteo 5140/6140
Spring 2000 semester

Introduction:
As a component of Dr. Zipser's Mesoscale Meteorology class, we have analyzed data collected on a wintertime cold frontal squall line that was observed by a number of platforms during the IPEX field program. The front passed during IOP-4 of the field program, on 14 February 2000. The work that has been done for this paper should be viewed as a prelude to a more detailed analysis of the structure and evolution of the convective elements of the system. Included here is a synoptic scale analysis and large-scale overview conducted by Mike Seaman, an analysis of 3 hourly soundings from NWS offices and NSSL mobile labs by Andy Siffert, a surface based analysis using the MesoWest network by Bobi Fox, and a doppler radar analysis of the squall line as it approached the KMTX WSR-88D in Salt Lake City by Justin Cox. In addition, concluding remarks address work that is proposed for the completion of this project, to be continued through the summer of 2000.
 
 

IPEX IOP-4 Synoptic Scale Summary

Hand Analyses performed by Mike Seaman: 14 February: 15Z, 18Z, 21Z        15 February: 00Z, 03Z 
 During the time period between 0Z 14 Feb and 06Z 15 Feb, a rather active subtropical flow with several embedded shortwave features was evident on water vapor imagery moving across the eastern Pacific onto the California coast.  During this time period, a relatively strong extra-tropical cyclone and associated shortwave trough rapidly developed and intensified over the eastern Pacific.  It then moved inland and weakened as it moved through the northern Rockies.  A surface cold front associated with this system moved through the Great Basin and Intermountain West with a squall line developing along portions of the front.

 A jet max moving through the subtropical flow across the eastern Pacific between 0Z 14 Feb and 12Z 14 Feb seemed to be responsible for the formation of this cyclone per water vapor satellite imagery.  This jet max is indeed analyzed over northern California in the 12Z RUC2 analysis.  A vort max associated with this shortwave trough is also analyzed on the 12Z RUC2 moving onshore in northern california, resulting in strong large scale lift out ahead of it across northern California and southwestern Oregon.  During this time period a weak shortwave ridge is analysed in the RUC2 500mb field over the Great Basin and Intermountain West.

 Between 12Z and 18Z 14 Feb this shortwave trough and attendant features moved across northern California.  At 18Z the RUC2 analyzed a vort max located over northern California as well as strong large scale vertical motion.  A 65+ kt 200 mb jet max was located to the south of Lake Tahoe.  The low level circulation center can be seen in visible imagery moving onshore at 17Z along the California/Oregon border.  The center of this circulation is analyzed as a 992mb surface low in the 18Z RUC2.  A band of cloudiness can be seen extending from this low across western Nevada in the 18Z visible imagery.  This band of cloudiness is likely associated with a surface cold front as the RUC2 analyzes a 700mb baroclinic zone and surface pressure trough to the lee of the Sierra.  Over the intermountain west the shortwave ridge amplified somewhat during this time frame in response to the shortwave trough moving onshore.  At 18Z the RUC2 analyzed a 700mb thermal ridge axis and surface pressure trough over central Nevada.  Relatively strong 700mb southwesterly flow (35-60 kts) over the northern Rockies and intermountain west region resulted in strong warm air advection ahead of the thermal ridge.  The result was a widespread 500mb negative omega field over the northern Rockies and Intermountain West, resulting in cloudiness evident in visible imagery.

 Between 18Z 14 Feb and 00Z 15 Feb the shortwave trough moved into the Great Basin as seen on water vapor imagery and in the RUC2 analysis.  During this time the surface cold front moved across Nevada and central Idaho.  Convection can be seen developing along this boundary across south central Idaho and central Nevada.  A vort max moving across northern Nevada and good 700mb warm air advection ahead of the front resulted in strong Q-G forcing over southern Idaho and northern Utah during this time.  A strong negative omega field resulted across this region.  By 00Z the mesonet showed the surface cold front had passed through southern Idaho into western Wyoming, and extended back into extreme northwestern Utah into central Nevada.  The RUC2 analyzed the surface low to be in eastern Montata at this time.  The 700mb baroclinic zone was across south-central Idaho back into eastern Nevada, and a vort max was analyzed near Twin Falls, Idaho. A 55+ kt 200mb jet was located over central Nevada.  Visible satellite imagery at this time showed scattered convection over southeastern Idaho, northern Utah, and eastern Nevada.  The RUC2 analyzed an area of instability with CAPE values of 100-500 j/kg over southern and eastern Idaho between 21Z and 00Z.

 After 00Z 15 Feb the surface cold front pushed through northern Utah with a line of convection along the front.  A vort max moved over northwestern Utah between 00Z and 06Z, and a 55+ kt 200 mb jet spread across central Nevada and northern Utah.  A strong negative omega field remained over northern Utah in response to the vort max.  Between 00z and 03Z the 700mb flow slowly turned from SW to W over northern Utah and increased to 60 kts under strong cold air advection behind the baroclinic zone.  The RUC2 analyzed CAPE of near 500j/kg along the Wasatch front at 02Z, and CAPE of 100-300j/kg over western Wyoming.

Large Scale Convective Line and Front Summary
 In association with the shortwave trough that moved through the northern Rockies, a cold front swept through the region with a line of convection along/ahead of it.  Surface analysis showed the front to be near the northeastern corner of Nevada at 15Z 14 February, from west of Boise to west of Winnemucca, NV by 18Z, from near Twin Falls, ID to near Elko, NV by 21Z, and into northwest Utah across the Great Salt Lake by 00Z 15 February.  The front was through the Wasatch front by 03Z and into eastern Utah by 06Z.  A line of convective precipitation was evident on radar and a band of cloudiness associated with the convection could be tracked across Nevada, southern Idaho, and northern Utah.  The band of precipitation moved faster across southern Idaho than it did across NV/UT due to a 75kt + 500mb jet max moving across southern Idaho.  This is evidenced on  Boise's soundings from this period.  The line was determined to be moving across southern Idaho at an average rate of approximately 50 kts, while it moved across northern NV and northern UT at approximately 40 kts.
 
 

IPEX IOP-4 Mesonet Analysis
Link to page
 
 

IPEX IOP-4 Sounding Data
Background
IOP 4 was conducted to measure an intense cold front passing through northern Utah from 2000z on Feb. 14 to 0400z on Feb. 15.  There were several special soundings taken for this event. Most of the concentration will be on the soundings from northern Utah. There were also soundings taken from a wind profiler.  The locations of the soundings that will be used form a square pattern. The profiler is located at Dugway, which is at 113.00'W, 40 01'N.  The two mobile units, NSSL 4 and NSSL 5, were located on either side of the Great Salt Lake.  NSSL 4 was located on the west side of the lake at 112 56.20'W, 41 02.94'N, and NSSL 5 was located on the east side of the lake in Ogden, Utah, at 112  00.66'W, 41 11.76'N. The third sounding was from the NWS office at the Salt Lake City International Airport at 111 96.22'W, 40 77.26'N. The times of the launches vary from three to 45 minutes apart  from each other.

For future reference, the time of the frontal passage (Fropa) for the three sites varies over an hour.  Fropa at NSSL 4 was at 2357Z Feb. 14. Fropa at NSSL 5 was at 0048Z Feb. 15.  Fropa at  SLC  was at 0115Z Feb. 15. Fropa at the profiler was just after 2300Z. These times will be talked about in more detail later as they are important in determining the flow into and out of the convective part of the front.    See the mesoscale section for more detail in how Fropa was determined. From the soundings it was possible to compute the following parameters:   LFCV -  Level of Free Convection, PWAT - precipitable water, LIFT  -  Lifted Index, CINS  - Convective Inhibition, CAPE - Convective Available Potential Energy  The CAPE computation is done by averaging the lowest 50 mb from the lifted parcel.  I feel this method best represents the case.

The main thing that the soundings are used for in this case is to get a vertical profile of  pre- and post-frontal conditions. The  case has great documentation of this because  there were balloon launches right before and right after the front passed. I will bring these two soundings up in the discussion of the periods. Using this data, conclusions can be  drawn about relative and  the environmental flows.

Analysis
Looking at the first launch of the balloons around 2100Z, it is clear that the soundings represented the  prefrontal environment. There were strong winds from the south-southwest at the surface and veering winds with height, becoming strong out of the southwest above 700 mb. SLC and NSSL 5 show  southerly winds at the surface with winds veering to southwest with height, while NSSL 4 seems to be more southwesterly at all levels. In all three cases the wind speeds increase with height, with the highest wind speed of 65 knots at 300 mb.   All three sites show large dewpoint depressions from the surface to 300 mb, were there was a hint of cirrus clouds. The largest dewpoint depression is  around 500 mb in all three soundings.  All three soundings seem to show a  dry adiabatic layer to about 50 mb above the surface.  NSSL 5 and SLC  show a very small stable layer at the top of the dry adiabatic layer.  The values in the table below seem to be very consistent with each other. The LFCV values are about the same; however, the soundings are dry and at this point there is no lifting mechanism to produce a cloud.  In general, the soundings from this period are stable and would require forced lifting to get something to fire.
 

Location	NSSL 4	NSSL 5	SLC
Time	2/14/00  2106Z	2/14/00  2142Z	2/14/00  2100Z
LIFT degrees	2	2	3
LFCV mb	854	854	-
PWAT mm	9	11	10
CINS j/kg	0	0	0
CAPE j/kg	0	0	0

Looking at the second launch period, just before 00z, it is clear that things in the preceeding three hours moistened up considerably at 500 mb. NSSL 4 lost its GPS signal, so there are only a couple of wind readings. This launch was still very important, because the balloon was launched ten minutes prior to fropa. This is the sounding  that best  represents the air that is flowing into the line of storms. Looking at this sounding it is clear that the air ahead of this system is stable and well mixed at low levels and it needs a good lifting mechanism to produce any precipitation.  The lifting in this case is the front that will be moving into the area. The NSSL 4 sounding almost has an inverted "V" look to it, but the surface dewpoint is a little moist for it to be a classic inverted "V" sounding. A surface parcel from this sounding would rise dry adabatically to about 675 mb.  The surface temperature stayed about the same since the last sounding, while the NSSL 5 warmed about 3° C in the previous three hours. In this sounding there is still a small area of dry air at 500 mb. The winds in this sounding are the same as the 21Z launch. The SLC sounding has also warmed at the surface  and is also starting to moisten up at 500 mb where the dry layer was. The winds back with height, with corresponding cooling from the last sounding. The winds from NSSL 4 and 5 are about the same, veering with height. This sounding seems to be a little bit more unstable. The table below shows a little bit more instability than at 21Z.  However, it should be noted that a minor increase in surface dewpoint will make the sounding more unstable in all three cases. For instance, just by adjusting the dewpoint a couple of degrees at the surface you could end up with a CAPE value of over 100 j/kg.  The PWAT has also risen since the earlier launches. This is the only period which shows any CINS, with a lot more CINS computed at SLC than at NSSL 4 and NSSL 5. But again, this is very dewpoint-dependent.
 

Location	NSSL 4	NSSL 5	SLC
Time	2/14/00  2347Z	2/14/00  2328Z	2/15/00  0000Z
LIFT degrees	0	0	1
LFCV mb	360	410	-
PWAT mm	13	12	9
CINS j/kg	31	81	312
CAPE j/kg	62	32	10

The third launch period was just after 0100Z. This launch only contains soundings from NSSL 4 and 5; however, they are very important. Both are soundings of post-frontal conditions. The NSSL 4 sounding was an hour after Fropa,  and we can see that the sounding is saturated from the surface to the top of the sounding where the balloon popped, probably due to icing.  The winds have shifted to the west and the surface temperature is about 10°C. At about 800 mb there is a small stable layer with winds veering aloft. NSSL 5 is the sounding that would best represent the conditions just after Fropa. The sounding was launched about 15 minutes after Fropa. The surface temperature has cooled off about to  7° C and the winds are light out of the west. There is weak veering and then backing of the winds with height just above the surface. It looks like the balloon that was launched from NSSL 5 penetrates the front at this point.  The sounding is saturated from the surface to the troposphere, with the highest winds above 400 mb. The balloon at NSSL 4 also seems to be penetrating the front because the sounding is isothermal for the first  50 mb. The PWAT has increased since the last launch, which is an important change.
 

Location	NSSL 4	NSSL 5
Time	2/15/00  0118Z	2/15/00  0115Z
LIFT degrees	2	2
LFCV mb	857	855
PWAT mm	15	16
CINS j/kg	0	0
CAPE j/kg	0	0

We now look at the fourth and final period, which was just before 0300z. All three of these soundings are well after Fropa.  NSSL 4 shows that the surface winds are out of the south and that the lower level westerly flow from the surface to about 650 mb is drying out this layer. It appears that the sounding is penetrating the stratiform precipitation region. This is evidenced by the dramatic rise in dewpoint temperatures at about 650 mb.  The most important thing from this sounding is the winds. The winds just below the stratiform region are out of the west, but once you get into this region the winds are out of the southwest.  This will help in determining the relative and environmental winds. NSSL 5 shows much of the same thing that we saw in the sounding from NSSL 4 at 0118z, which showed light west winds and a saturated atmosphere until the balloon iced up. The winds aloft flow out of the southwest.  SLC for this period shows that the sounding is saturated up to 300 mb, at the troposphere. However, this sounding is much different from the NSSL 4 and 5 soundings because the winds at the surface are out of the northwest at the surface, then back  to the west, then veering to the southwest where they are still very strong. The computed data for this period shows that the soundings are becoming more stable as the lifted index rises. NSSL 4 also shows signs of drying, as the PWAT value has dropped.
 

Location	NSSL 4	NSSL 5	SLC
Time	2/15/00  0236Z	2/15/00  0224Z	2/15/00  0300Z
LIFT degrees	2	4	4
LFCV mb	854	786	-
PWAT mm	9	15	15
CINS j/kg	0	2	0
CAPE j/kg	0	0	0

Summarizing what happened over the time period at Dugway, it is clear that much of the same conditions existed there. The profiler gives a great feel of how the winds shifted with height over time. Fropa occurred in the Dugway area just before 2300Z. Unfortunately, we do not have any profiles after 0200Z, but we can get a general idea of what the winds were doing in front of the storm. The winds at this site do not shift from the southwest to the west. Instead, they stay constant out of the southwest, which might suggest what the radar data shows, which is the front is a little slower there than in the north. This would lead to a more constant flow out of the southwest.  In summary, it is clear that the air in front of this system was not that unstable and that it need a lifting mechanism, which in this case would be the approaching front from the north.
 
 
 

IPEX IOP-4 Doppler Radar Analysis from KMTX WSR-88D
In order to examine the structure of this squall line and compare it to current conceptual models in the literature, I have analyzed a cross section perpendicular to the squall line, as well as cross sections that are at slight angles (~15 degrees) to the perpendicular cross section. The purpose of the additional cross sections is to better resolve the flow parallel to the squall line that would not show up in the perpendicular cross section.
There are pitfalls in this type of analysis, such as nonuniform flow along the squall line (any bowing of the line would be problematic), but for this "first look", I will deal with these problems qualitatively. I have selected a time when the leading edge of the intense precipitation is about five nautical miles from the radar. The extent and resolution of the vertical structure of the leading edge is maximized, between the cone of silence close to the radar and the increasing height and width of the radar beam at large distances. It is important to optimize our view of the squall line, especially because of the radar's elevation (2111 m MSL, or ~700m above the surrounding terrain).

In order to combat the poor choice of color table in the WATADS radial velocity display, I generated two sets of velocity figures. The first displays only radial velocities below ~40 kt, the second, only radial velocities above ~40 kt. This is also convenient because according to the progression of the precipitation band as viewed by the radar, the storm motion was about 40 kt through this period. The figures with Vr>40 kt, then, are rear-front storm relative velocities, and similarly, the Vr<40 kt figure represents front-rear storm relative flow. This scheme only works for cross sections that are perpendicular to the squall line, but the same thresholding was used in order to deal with color table ambiguities. Here is the velocity PPI at 0.5 degrees elevation angle. This also has the velocity scale on the bottom, useful when viewing the cross sections.

There are some interesting features in the velocity cross section taken perpendicular to the front. This cross section was taken from 48 nm to the northwest of the radar, to 25 nm to the southeast along the 120-300 degree line. There is a rear-to-front component of the storm relative flow (NW-SE at >40 kt absolute velocity) above 14000' (4.3 km) AGL. There is also a small area of rear-front relative flow near the surface 5 nm upstream of the radar, in the same area as the maximum reflectivity. There may be a rear-front flow at this level extending upstream from this point, but the radar beam misses this area because of its elevation. The echo tops are at 6.4 km AGL, which limits our ability to examine the flow above that height. We do get a good look at the precipitating portion of the anvil that extends ahead of the surface front.

The front-rear component of relative velocity also suggests some important features. Low inbound absolute velocities in the region of maximum reflectivity implies very strong front-rear relative flow. This makes physical sense given the probable location of the gust front and convective updraft. This region of strong front-rear flow rises from the front of the system over the highest reflectivities, but is lower to the rear of the high reflectivities.

To the right of the radar in the cross section, or in front of the squall line, there are weak outbound absolute velocities near the surface, also implying strong front-rear relative flow, while aloft, outbound velocities are closer to 40 kt, meaning weak storm relative flow.
The two cross sections that were taken at 15 degrees to either side of the perpendicular cross section display similar characteristics, but they contain important components of squall-parallel flow. For example, the cross section that was taken along the 135 degree line shows absolute outbound velocities rearward of the squall line, implying either strong front-rear flow or a southwesterly component to the wind. We know that there was such a component, so this will be taken into account in further analyses.
 
 

Summary and Conclusions
In this paper, we have given a general overview of the case in question. In conducting this preliminary study, we have observed some features that beg further examination. For example, a more detailed analysis of the relative flow in two, if not three, dimensions, may reveal some interesting similarities and differences to conceptual models of squall lines that have been published.
Much of the overview work regarding the nature of the large scale flow and dynamics has been done, and the work that remains will be mostly on the convective and mesoscales. This will involve further analysis of the 3 hourly soundings, the MesoWest data, and the radar data.
The addition of data from O.U.'s DOW2 radar will be especially valuable, as it will provide low-level radial velocity and reflectivity data that is missing from the KMTX data. If it is possible to obtain the TDWR data from the Salt Lake City site, that would also enhance our observations of the squall line. One important part of dealing with the radar data will be to interpolate it to a Cartesian coordinate system. Such interpolation would make analysis easier, and would aid comparison between radar platforms such as KMTX and DOW2. Though this was attempted to no avail during the semester, a continued effort with the aid of outside experts will yield valuable results.