A Revised Bourgouin Precipitation-Type Algorithm
(262)
Kevin Birk, National
Weather Service Chicago, Romeoville, IL,
IL
Eric Lenning, National
Weather Service Chicago
Abstract:
Top-down precipitation-type
forecast methods often use the 0°C threshold to distinguish warm melting
layers from cold refreezing layers. The National Weather Service uses a version
of the top-down technique in its ForecastBuilder tool and its National Blend of
Models. When this version finds a warm layer aloft, melting potential is
computed from the layer's maximum temperature (or wet-bulb). It assumes the
layer depth and maximum temperature are proportional. It also assumes little
melting occurs at temperatures <1°C, total melting occurs at
temperatures >3°C, and partial melting occurs otherwise. It is
understood that these assumptions are not valid in isothermal layers. If such a
layer is present but undiagnosed, this version of the top-down technique
generally predicts ice pellets even though freezing rain becomes much more
likely. Given this shortcoming, a comprehensive method that accounts for both
the depth and magnitude of warm and cold layers is desired for improved gridded
precipitation-type forecasts.
The Bourgouin technique is a
top-down method that calculates melting and refreezing energies (J/KG) in a
sounding. These energies are proportional to the area of warm and cold layers
and have no problem accounting for isothermal layers. Despite being developed
from a small dataset, the Bourgouin technique is widely utilized in
post-processing of NWP output. It also provides precipitation-type nomograms in
the popular Bufkit program for analyzing NWP forecast soundings. However,
recent analysis with a larger dataset suggests revisions are needed to the
original technique. This is especially true for the function used for
discriminating freezing rain versus ice pellets. This presentation describes the
development of these new thresholds. It also suggests additional revisions to
the original technique intended to make this an even more viable and valuable
tool for gridded precipitation-type forecasts.