Damaging Downburst Prediction and Detection Algorithm (DDPDA)
Algorithm Components
Environmental Data
No environmental data are used in the algorithm at the moment, but at least
some preliminary uses of these data are promised to the OSF this year.
Some methods of predicting the maximum gusts in a given environment
are:
-
WINDEX (McCann 1994) -- predicted maximum gusts based on moisture
content, lapse rate, and height of the melting level (empirical)
-
DCAPE -- downdraft CAPE used to calculate theoretical maximum downdraft
speed
-
Atkins and Wakimoto (1991) -- 20 K difference in eqivalent potential temperature
from the surface to the height of minimum theta-E implies an enhanced probability
of microbursts
Another method of predicting maximum gusts which incorporates both radar
and environmental data is given by Stewart (1995), although the environmental
data are used simply as a threshold to determine which gust predictions
are valid. We have promised the OSF to test this technique.
Roberts and Wilson (1989) suggest the following radar signatures which
relate to the environment:
| Evaporative Cooling |
-
Radial convergence below cloud base (esp. when Z decreases significantly
towards the ground
-
Radial convergence above cloud base, pariticularly when associated with
a Z notch and/or a very dry environmental layer
|
| Melting Cooling |
-
Radial convergence just below the 0 degree C level
|
| Precipitation Drag |
-
Radial convergence within a descending reflectivity core > 50 dBZ
|
| Vertical Pressure Gradients |
-
Azimuthal radial velocity couplet indicative of rotation, especially when
vorticity increases with decreasing height
|
Separating these signatures in the broad mid-altitude convergence field
may prove to be problematic given the vertical resolution of the
radar beam. However, they may be useful to determine which
levels to examine the convergence field.
[Of course, none of this will help if there is
a pre-existing stable layer than cannot be taken into account...]
[Try looking at convergence at the height of the
theta-E minima]
[A higher core will likely have more melting and
sublimation]
[There are conflicting theories on the effects
of precipitation drag]
[WINDEX/DCAPE/etc may be useful as a filter to
remove false alarms]
Next page
Table of contents