Estimating Downdraft Strength

If applied correctly, downdraft CAPE (DCAPE) can be a useful tool to forecast potential downdraft strength. DCAPE is best utilized in weakly sheared environments where pulse and multicellular convection will dominate. To compute DCAPE, we'll first look at this qualitatively using a Skew-T (Figure 1). First, we need to find a representative mid-level wet bulb potential temperature, which will indicate where the downdraft may possibly originate. In this example, we assume the downdraft initiates around 575 hPa. However, be cognizant of the fact that most downdrafts initiate over a layer rather than a specific level. The updraft also has a wet bulb potential temperature and is found by lifting the surface parcel to its LCL. The downdraft parcel will be somewhere between the the mid-level wet bulb potential temperature and the updraft wet bulb potential temperature. Since the downdraft parcel follows a saturation adiabat, we assume the downdraft is completely saturated. The downdraft parcel is slightly warmer than the environmental temperature between 580-370 hPa. From the surface to 580 hPa, the downdraft parcel is colder than the environmental temperature, which results in a negatively buoyant layer. The level where the downdraft parcel becomes colder than the environmental temperature is called the level of free sink (LFS) and is similar to the LFC for updrafts. Therefore, DCAPE is an integration of the layer between the environmental temperature and the downdraft temperature from the surface to the LFS.

Figure 1. Skew-T illustrating DCAPE.

In order to determine the potential gust speed of a downdraft, we must calculate DCAPE quantitatively:

DCAPE = 1/2 * g *((Te - Tpd)/Te) * delta z

The terms of the DCAPE equation are:

g - gravity
Te - environmental surface temperature (K)
Tpd - expected downdraft surface temperature (K)
delta z - depth of negatively buoyant air (m)

For this particular case, the environmental surface temperature was 311 K, the expected downdraft surface temperature was 291 K, and the depth of the negatively buoyant air was 3,500 m.

DCAPE = 1/2 * 9.81 *((311-291)/311) * 3500 m
DCAPE = 1,104 m^2/s^2 or 1,104 J/kg

In order to calculate the maximum theoretical updraft velocity (Wmax), we use the equation Wmax =
square root (2*CAPE). We can actually substitute DCAPE for CAPE to calculate the maximum theoretical
downdraft velocity. Therefore, Wmax = square root (2*DCAPE)

Wmax = square root (2*1,104)
DCAPE = 47 m/s or 105 mph

The maximum theoretical downdraft velocity can often be overestimated by almost 50%. The calculated value overestimates the potential downdraft velocity since DCAPE assumes that the downdraft is fully saturated as it descends. In actuality, the downdraft likely warms somewhere between the dry adiabatic and saturated adiabatic lapse rates since the dry subcloud air enhances evaporation, which decreases both DCAPE and the downdraft velocity. However, significant precipitation loading (> 60 dBZ reflectivity core) is not accounted for in DCAPE but can lead to stronger downdrafts than DCAPE suggests. In this particular case, the thunderstorm was too close to the radar to adequately sample whether or not precipitation loading would have contributed to the downdraft (not shown). However, judging by the sounding and the moderately unstable air mass, it's possible that precipitation loading could have played a role.

What happened on this day? A microburst produced a gust of 81 mph at Amarillo just a few hours before this sounding was released.

July 22 Amarillo Microburst

A microburst occurred during the middle afternoon on July 22 and produced an 81 mph wind gust at Rick Husband International Airport, which tied the highest gust ever recorded at Amarillo. Instead of writing a blog post, I've decided to link to a web story our office created.

Microburst Produces 81 mph Wind Gust at Amarillo -- WFO Amarillo

Massive Heat Bubble

On the evening of July 17, an expansive 500 mb subtropical upper-level anticyclone was centered near the Kansas and Nebraska border and nearly stretched from border to border in the center part of the United States (Figure 1). The strength and placement of this feature is unusual, and in fact, standard deviations of +2.5 were common near the centroid of the anticyclone (Figure 2). When strong anticyclones like this develop, they are accompanied by unseasonably warm temperatures during the day and at night along with high moisture content. The combination of these factors creates a potentially dangerous situation because the heat can add serious stress on the human body. Accordingly, several heat-related watches, warnings, and advisories were in effect for 16 states (Figure 3) last evening. What compounds matters is that a lot of the heat is expected to be concentrated in urban areas, including Chicago, Milwaukee, and Minneapolis where many people do not have air conditioning or cannot afford to run the air conditioner.

Figure 1. 18 July 2011 00Z 500 mb height analysis

Figure 2. 18 July 2011 00Z 500 mb height standardized anomalies.


Figure 3. Watch, warnings, and advisories for July 17.

It surprises many people to learn that heat kills more people on average that tornadoes and lightning combined. Granted, these statistics are slightly skewed by the 1995 heat wave in the Midwest that killed hundreds of people, but it proves that heat is a very underrated killer.

Kunkel, K. E., S. A. Changnon, B. C. Reinke, and R. W. Arritt, 1996: The July 1995 heat wave in the Midwest: A climatic perspective and critical weather factors. Bull. Amer. Meteor. Soc., 77, 1507–1518.