A. Midland TX Sounding, 1200 UTC 4/13/99
The sounding for Midland TX for 1200 UTC 4/13/99 is a good example of a conditionally unstable sounding. The sounding is deceptive and the indices calculated from it should be used with caution.
For example, the difference in temperature between the environment and the surface lifted parcel at 500 mb (Te -Tp)500 known as the Lifted Index (LI). Tables that relate LI to thunderstorm risk show that an LI of -4 or less are associated with soundings capable of supporting severe thunderstorm development.
This may be true if the instability in the sounding "is realized." In the case of the Midland sounding above, the LI -4.4 merely expresses a potential for instability which would NOT be realized given the need for force-lifting to the 600 mb level (nearly 10000 feet). Typical mechanisms for force-lifting of air parcels do not contribute more than around 50-100 mb of lift.
B. Midland Sounding Altered for Afternoon Heating
The Midland sounding can be altered to estimate how much surface warming would be needed to eliminate the stable layer near the ground. The assumptions in this method are that surface heating will affect the ground temperature the most, with decreasing effects upwards.
The "convective temperature" is that temperature to which the surface parcel must be warmed in order to eliminate the elevated inversion that occurs typically in the so-called "Loaded Gun" sounding. There are two procedures that must be followed in order to determine this. The first procedure is followed if the saturation mixing ratio that passes through the surface dew point intersects the actual environmental lapse rate (temperature sounding) at or above the top of the inversion layer, and the second is followed if the saturation mixing ratio that passes through the surface dew point temperature passes beneath the inversion layer.
|Procedure 1: The method involves finding the saturation mixing ratio line that intersects the morning surface dewpoint temperature. Highlight that line and draw it upward until it intersects the main sounding. This intersection is known as the Convective Condensation Level (CCL). As you will see, the CCL is sort of a combined LCL and LFC.|
Procedure 2: But what if the saturation mixing ratio line passing through the surface dew point temperature does not intersect the sounding above the inversion layer? See box to right.
Continuation of Procedure 1: From the CCL, a line should be drawn to the surface pressure at Midland. The intersection of this line with the surface gives the so-called Convective Temperature, the temperature to which the surface air must be warmed in order to eliminate the surface stable layer. In this case, the Convective Temperature is 30C.
Next, one accepts this sounding temporarily as a reasonable estimate of the one which may occur by late afternoon. We now may use parcel theory once again.
Force lift a surface air parcel upwards and you will see that it becomes saturated at the CCL. Also, at the CCL it becomes warmer than its surroundings.
Continuing the drawing, we can develop a new CAPE area and a new EL. The region between the ascending parcel curve BENEATH the CCL and the original sounding is analagous to CINH, since it represents the amount of energy ( or work to be done) to be expended in order to heat the bottom of the sounding.
In this case, the CAPE has nearly doubled to over 2500 J/kg. If the CAPE>CINH the sounding is said to be potentially unstable and thunderstorms can be expected. The LI has decreased from -4.4 to -8 Also, in this case, insertion of the CAPE into the formula for vertical velocity at the EL yields a value >60 m/s, or over 120 mph, for the strength of the convective updraft.
Such buoyancy and updraft strength can support GIANT hail (>2" in diameter). In addition, the wind profile (note the wind profile on the right which shows a wind VEERING with height and increasing in speed very rapidly with height in the lowest part of the troposphere) is favorable for storm rotation (supercell formation). The relationship of the wind profile to rotating thunderstorms is a topic for future discussion.
Relate the Area Forecast Discussion (AFD) from Midland (script "disc KMAF" in our lab), to the discussion above.
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MIDLAND TX
930 AM CDT TUE APR 13 1999
LATEST VIS LOOP/SFC OBS SHOW PAC FRONT JUST ENTERING CWA AND GOOD
HEATING AHEAD OF FRONT...FOR A BUSY DAY AHEAD. DEWPOINTS AHEAD OF FRONT
LOOK MORE LIKE E TX COASTAL DEWPOINTS ON A DRY DAY...W/MOST IN THE
60S. 12Z KMAF RADAT WAS SATURATED FM H8-SFC AND VERY
UNSTABLE...W/LI OF -8...CAPE OF 2800 J/KG...AND ZILCH CIN. EVEN
MEAN PARCEL LIFTING YEILDS -6 LI/S AND LITTLE CAPE REDUCTION. IN
EITHER SCENARIO...HODOGRAPHS FAVOR ROTATION. SPARKS ALREADY FLYING
N OR CWA ALONG FRONT...AND EXPECT CONVECTION TO EITHER TRAIN S ALONG
FRONT...OR ALL AT ONCE AS FRONT MEETS DRYLINE HEAD-ON. ONLY CHANGES
ARE UPDATES FOR CLOUD COVER FOR TXZ058-059-067...AND MENTION OF
POSSIBLE SEVERE THERE. WZIS.