DEPARTMENT OF GEOSCIENCES Spring 2010
San Francisco State University NAME________________________
Metr 201 Monteverdi Quiz #3
(Test will be collected at 10:40 AM)
1. Definitions. Provide a one sentence definition (if the term is an abbreviation, first “unabbreviate” it and then define it). (5 pts each for a total of 30 pts).
Convective Available Potential Energy – a measure of the total potential buoyancy acceleration experienced by an air parcel rising from the LFC to the EL.
Directly related to the “positive” area on a Skew T/Log P diagram bounded by the parcel’s ascent curve on the right and the sounding (ELR)on the left, from the LFC (CCL) to the EL.
(b) Equilibrium Level
The location at the top of the positive area on a sounding at which the air parcel becomes the same temperature as its surroundings and above which it becomes colder than it surroundings.
Alternatively, it is the elevation that separates the region of the atmosphere in which the buoyancy acceleration is positive (beneath the level) from that in which the buoyancy acceleration is negative (above the level).
(e) Lifted Index
The difference (in C or K) between the temperature of a lifted
parcel and the surrounding atmosphere at the 500 mb level
(negative for unstable conditions).
(d) Convective Temperature
The temperature to which the surface temperature must be warmed in the afternoon in order to both eliminate the “loaded gun” sounding inversion, and to make the sounding nearly absolutely unstable.
Lifting Condensation Level: The elevation (in millibars or feet/km) to which an air parcel must be lofted in order for its saturation mixing ratio (determined by its actual temperature and the saturation mixing ratio line upon which it resides) is decreased to the actual mixing ratio (determined by the saturation mixing ratio line passing through the surface dew point). At this elevation or level, the relative humidity of the ascending parcel will be 100% and a cloud base will be found
Level of Free Convection: The elevation (in millibars or feet/km) to which an air parcel must be lofted above which its temperature will be warmer than that of the air surrounding it at the same elevation. Above this elevation the parcel no longer needs to be forced upwards, as it will freely convect (be spontaneously buoyant).
2. Units. Provide the units (metric) used conventionally for the following (3 pts each for a total of 12 pts)
buoyancy acceleration ______ m s-2 (or cm s-2) ______________
lifted index oK or oC
CAPE Joules kg-1 or m2s-2
CIN Joules kg-1 or m2s-2
3. The sounding for Fort Worth, TX (KFWD) for 0000 UTC 4 May 1999 is shown in Fig 1. (30 pts total)
(a) Determine the elevation (mb) of the LCL, LFC, and EL and the value of LI for the sounding. (Show the ascent curve, and make sure to plot the positions of the levels on the ascent curve) (28 pts)
(b) Shade in the portion of the modified sounding that has positive buoyancy (CAPE) in red. (2 pts)
4. The sealevel pressure map for 2200 3 May 1999 is given as Fig. 2. The area experiencing dew point temperatures exceeding 60oF is shaded green. The CAPE/CIN field for 2200 UTC 3 May 1999 is given in Fig. 3. Compare the pattern shown with the dew point field depicted in Fig. 2. (28 pts total)
(a) Briefly explain why the CAPE field is consistent with the dew point field (14 pts);
The dew point field gives an estimate of the mixing ratio field at surface pressure. Mixing ratios determine the amount of latent heat released in an ascending air parcel. For a given temperature, the latent heat released accounts for why an air parcel lifted to, say, 500 mb is considerably warmer than it would be if lifted dry adiabatically. Thus, for a given Environmental Lapse Rate, the higher the surface dew point temperature that more warm a force lifted air parcel would be relative to the surrounding air at the same elevation. Hence, the surface dew point field is directly related to the surface based CAPE field.
By the way, the argument that many of you used that the higher the dew point the greater the chances that saturation would occur (and, hence, a rising parcel would cool wet adiabatically) can be used in the context of the above charts ONLY if the surface temperatures were everywhere the same. That is an unrealistic assumption. That argument only works when looking at one sounding for one place at one time (and increasing the surface relative humidity). See this graphic.
(b) Describe why the fact that thunderstorms were erupting over a broad area of southern Kansas, Oklahoma and north Texas would be expected given the information depicted in both Figs. 1 and 3. (Careful, it’s not just that CAPE is present) (14 pts)
Both figures either suggest or depict large values of CAPE, both in the Fort Worth sounding, and geographically over the southern Great Plains. But this would be not enough to expect a widespread outbreak of thunderstorms if large amounts of CIN were present in all the soundings in the area.
But, as can be seen from the Fort Worth sounding and the geographic distribution shown in Fig. 3, it is clear that the CAPE is occurring in an environment in which there is virtually no CIN. Hence, the soundings in the area are “uncapped” (nearly absolutely unstable) and thunderstorms would be expected to occur over the area.
Figure 1: KFWD Sounding, 0000 UTC 4 May 1999
Figure 2: Sealevel pressure map, 2200 UTC 3 May 1999. Boundaries are indicated as Lines 1, 2 and 3, and area of surface dew point exceeding 60oF is shaded in green.
Figure 3: Distribution of CAPE (red) and CIN (blue), 2200 UTC 3 May 1999.