Observing Non-Convective Low Level Wind Shear
By January 31st, it was apparent to forecasters at the Raleigh forecast
office that a hybrid damming event was likely and non convective low
level wind shear would be a concern. Model forecasts were essential
in helping meteorologists determine the severity of the anticipated LLWS.
Although model forecasts are critical in forecasting LLWS in advance,
it is just as important to have tools able to detect the onset and evolution
of LLWS. On the morning of February 1st, forecasters had several methods
of observing LLWS conditions including upper air RAOB soundings, AMDAR Aircraft Soundings, WSR-88D VAD Wind Profiles,
Other Wind Profilers, Pilot Reports (PIREPS), and the Graphical Airman’s Meteorological Advisory (G-AIRMET).
Upper Air RAOB Soundings
RAOB is an acronym for "rawinsonde observation" which are observations made by a rawinsonde
unit attached to a weather balloon. RAOB Soundings are typically conducted every 12 hours
at 00Z and 12Z although they may be done more frequently for significant events. The observations provide
details on the thermal, moisture, pressure and wind fields in the vertical atmosphere.
The Skew-t plot below from Greensboro depicts the 12Z, February 1, 2008 RAOB which shows a
significant near surface based inversion with a tremendous amount of shear, both from
a veering wind (directional shear) and increasing velocities with height (speed shear).
The wind, temperature, moisture, and pressure data can be interpolated to
provide a detailed estimate of the data on small vertical intervals. The
chart below displays the wind speed and direction in the fourth column with the
height in feet above ground level (AGL) at 100 foot intervals.
AMDAR Aircraft Soundings
AMDAR is an acronym for Aircraft Meteorological DAat and Reporting (AMDAR) which is an international
effort within the World Meteorological Organization to coordinate the collection of
environmental observations from commercial aircraft. In the United States, we often refer to the
Meteorological Data Collection and Reporting System (MDCRS) which is a private/public partnership
facilitating the collection of atmospheric measurements from commercial aircraft to improve aviation safety.
AMDAR is very useful for short term forecasting situations where conditions are changing rapidly and in
particular for aviation forecasting. Regarding winter weather events,
AMDAR data can provide forecasters with the height of the freezing level, the presence of elevated warm layers,
indications of thermal advection and dry layers. All of these are necessary for
accurate precipitation type forecasts. The availability of this upper air data at times
and locations where RAOB data may be lacking is invaluable.
During the 30 hour period from 12Z on 01/31 through 22Z on 02/01 there were over 25
AMDAR soundings available at RDU.
The image below contains a loop of AMDAR soundings at RDU during the event. There are 19 soundings in
the loop that run from 1301Z on Thursday, January 31 through 2130Z on Friday, February 1, 2008.
A Java Loop
of AMDAR soundings from 1301Z on 01/31 through 2130Z on 02/01 that can be stopped, controlled and zoomed is
also available.
The aircraft soundings in this case show a dramatic increase in southwest winds
just above the surface starting with the 09Z sounding. As precipitation fell through
the surface cold dome, a very pronounced temperature inversion became established
from the surface to near 900 mb. This inversion separated the light northeast surface
winds with the strong southwest winds at the top of the inversion, resulting in a
classic low level wind shear event.
The table below was created based on the text output of selected AMDAR
soundings. The aircraft sensor reports multiple observations during the course of
its ascent or descent and each observation is included in the text output. In the
table, one can see how the wind shear dramatically increased by 12Z.
Data from the selected AMDAR soundings used in the table below...
0231Z
Graphic or Text |
0918Z
Graphic or Text |
1142Z
Graphic or Text |
1357Z
Graphic or Text |
1450Z
Graphic or Text
It should be noted that the aircraft soundings report heights based on a standard
atmosphere and are not adjusted to local barometric pressure like a cockpit altimeter
would be. Therefore, there is some variability in the heights reported by the AMDAR
observation and have to be converted based on local barometric pressure readings.
WSR-88D VAD Wind Profile
The National Weather Service Doppler radar has the capability of detecting both
precipitation and wind speeds, based on how fast the precipitation picked up on
radar is moving toward or away from the radar beam. The software on the WSR-88D
can calculate wind direction and speed at various levels in the
atmosphere and produce a Velocity Azimuth Display (VAD) Wind Profile (VWP)
product. The radar produces both a text product and a
graphical product which helps forecasters determine the estimated
winds speeds in the lowest 2,000 feet.
The image below (click to enlarge) is from the KRAX WSR-88D near Raleigh, NC at 1058Z on
Friday, February 1, 2008. The increasing amounts of low level wind shear can be seen at 1058Z with a
northeast wind at around 15 knots observed at 1,000 feet, a southerly wind
at 25 knots at 2,000 feet and a south-southwesterly wind at 40 knots at 3,000 feet. While the
temporal resolution of the VWP product is great (it is available with each volume scan, typically
around every 5 minutes in precipitation mode), the vertical resolution is limited to every 1,000 feet.
A Java Loop
of Velocity Azimuth Display (VAD) Wind Profile (VWP) products from 0303Z through 1502Z
on Friday, February 1, 2008 is also available.
Other Wind Profiler
A wind profiler is a type of weather observing equipment that uses radar or
sound waves (SODAR) to detect the wind speed and direction at various elevations
above the ground.
The North Carolina Department of Environment and Natural Resources (DENR), Division of
Air Quality (DAQ) has a wind profiler located near Raleigh, NC. The purpose of this
equipment is to measure winds from the surface up to approximately 16,000 ft
MSL. The output is very much like the WSR-88D VAD Wind Profile. This tool can
be extremely useful to forecasters as it provides very good vertical
resolution. In addition, on clear weather days when the WSR-88d radar wind profile will not
be as accurate due to lack of radar returns needed to estimate the winds, the profiler is
sensitive enough to provide good data. Finally, the wind profilers can provide detailed
data in locations that are far away from the radar.
The two images below (click to enlarge) are from the DAQ wind profiler data near Raleigh on February 1, 2008.
The data is from around 06Z to around 15Z with the vertical height up to around 10,000 feet
in the first image and up to around 3,000 feet in the second image. The rapid increase in wind shear can
be seen from around 07Z to 11Z as the near surface flow remains east to northeasterly
at 10 to 15 knots while the flow at around 2,000 feet rapidly increases and veers to
south-southwesterly at around 45 knots.
Data from various wind profilers across the country can be accessed via the
Cooperative Agency Profilers (CAP) project web site produced by GSD (formerly FSL).
The web site collects wind profiler data from various participating agencies,
apply GSD's data quality control algorithms, and make these value-added data
available on the web and to the National Weather Service.
More information on the CAP project is available at
http://madis-fsl.org/cap/index.jsp.
Real time profiler data is available via the Graphical Data Display page which contains
time-series displays of Wind Speed and Direction, Spectral Moment Displays (Signal Power,
Signal to Noise Ratio, Radial Velocity, and Velocity Variance), and RASS Temperature Displays.
Real time data from the Raleigh profiler is available via
this link.
Pilot Reports (PIREPS)
A pilot report or PIREP is a report of actual weather conditions encountered
by an aircraft in flight. This information is usually relayed by radio to the
nearest ground station. The message is then encoded and relayed to weather
offices and air traffic service units.
Because reports are made by pilots, they may occur at any time and may contain
a variety of information. Common to all reports should be location, time,
altitude (MSL), type of aircraft, and at least one weather element:
- Sky cover (MSL)
- Flight visibility and weather (Statute miles)
- Temperature (C)
- Wind (knots)
- Turbulence
- Icing
- Remarks
An example pilot report from the event is shown below.
If wind shear is reported in the PIREP that indicates a gain or loss of 20 knots,
then LLWS must be included in the terminal forecast.
PIREPS can be accessed on the internet via the Aviation Weather Center (AWC) web
page at
http://adds.aviationweather.gov/pireps/.
Real time PIREPS during the most recent 12 hours and within 200 miles of KRDU are available via
this link.
Graphical Airman’s Meteorological Advisory (G-AIRMET)
The Aviation Weather Center, based out of Kansas City, MO, now offers a new
suite of graphical guidance. The Graphical Airmet (G-Airmet) produces a 12 hour
outlook highlighting the potential for numerous aviation hazards, including
the possibility of non convective low level wind shear. The G-Airmet can be accessed
on the internet by going to http://aviationweather.gov/testbed/gfa/
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