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.