The Micro Rain Radar
The Renaissance Computing Institute (RENCI) deployed
a mobile Micro Rain Radar (MRR) to Winston-Salem, North Carolina during this event. The (MRR)
is a vertically pointing Ku-band radar that is commercially available from METEK Inc.
The output from the MRR can be processed to provide the user with a vertical view of reflectivity and Doppler velocity.
The high temporal and vertical spatial resolution of the MRR along with its unique data makes it an invaluable
tool to view precipitation type changes during winter storms. In addition to the data from the MRR,
RENCI deployed a handful of other meteorological instruments including a disdrometer, temperature, and
wind sensors during this event.
There are two primary facets of MRR output, radar reflectivity in dBZ and the
Doppler velocity. These datasets are displayed with a vertical axis
depicting height in thousands of feet and a horizontal axis depicting time.
The reflectivity data can be used to determine precipitation intensity, when precipitation
will reach the surface, and potential bright banding. In the MRR reflectivity imagery
to the right, black shading represents no reflectivity, blue/purple colors depict light
reflectivity returns or light precipitation, with red/orange colors representing greater reflectivity
returns and typically heavier precipitation or bright banding.
Examining the reflectivity imagery to the right more closely, a very brief period of precipitation reaches the
ground at around 245 am local time. A longer and more sustained period of precipitation
reaches the ground between around 400 am and 530 am. Data from a disdrometer
confirms that precipitation particles were reaching the ground during this period. Additional periods of precipitation, often
very short in duration, continued for the next several hours through 1000 am.
The fall velocity data can be used to determine precipitation type based on the fall velocity of the hydrometeor.
Snowflakes have a much slower fall velocity than sleet or rain (including freezing rain). The location
where a slower fall speed changes into a faster one can be inferred as the melting layer.
The fall velocity imagery from the MRR paints a vivid picture of the precipitation type changes
over Winston-Salem during the morning hours of January 14. Based on the reflectivity imagery shown above and
the disdrometer data, there was a short period of precipitation at 245 am and a longer, more
sustained period of precipitation between 400 am and 530 am. The fall velocity during this period is consistent
and rather slow, generally less than 8 mph, indicating the precipitation was likely falling as snow.
The fall velocities are noticeably higher just before and around 530 am and then again at around 600 am. The
faster fall velocities are indicative of a precipitation type other than snow. The sudden increase
in fall velocities begins just above 2,500 feet and this is indicative of the melting layer. When snow
flakes encounter temperatures above freezing, they do not melt immediately, it typically takes
several hundred feet for the snow flakes to melt. Approximately 15 miles to the east, an
AMDAR aircraft sounding is recorded at GSO at 1137Z or 637 am and
the sounding shows a small warm layer centered at 850 mb with temperatures just above freezing.
Later that morning, the MRR fall velocity data easily shows the melting layer increasing with height. At 730 am
local time the melting layer can be estimated at around 4,000 feet. An hour later, the melting layer had increased
a couple thousand feet with the melting layer estimated to be over 6,000 feet. Note the significant warming indicated
by the AMDAR soundings in the 850 mb to 900 mb layer at GSO between 1137Z or 637 am and
1231Z or 731 am. The mid levels continue to warm markedly
during the late morning and midday hours with the MRR fall velocity data continuing to indicate a rising
melting layer. This is largely confirmed with the 1446Z or 946 am
AMDAR aircraft sounding at GSO.