Page 33 - Volume 15 Number 2

even if you plan to depart a little after 1330 UTC or 0130 UTC (when they become available), there’s a good chance it may not be very representative of the tops along your route. However, it still may be useful when the conditions within the area are somewhat homogeneous. But you’ll need other sources of data to come to that conclusion.
Forecast Soundings
Even though radiosondes are only launched twice a day at locations that are widely spread throughout the country, forecast models such as the Rapid Update Cycle (RUC) are refreshed hourly and data similar to a RAOB is available with a resolution as low as 20 km (11 nm). I have to emphasize that this is not an observation using sensors like a radiosonde. It is data from a numerical weather prediction model that is based on a plethora of surface-based and upper-air observational data collected hourly. Nevertheless, you can expect more uncertainty and inherent forecast errors in this sounding data.
Every hour, the RUC model is executed on the latest set of observations, which in turn, produces an analysis (initial conditions of the model) and forecasts through 18 hours at one- hour increments. RUC sounding analyses and forecasts are available from NOAA using the RUC sounding Java tool (http://rucsoundings. noaa.gov). This Web site also provides forecast soundings for other numerical weather prediction models to include the North American Mesoscale (NAM) and Global Forecast System (GFS).
Interpreting a forecast sounding is similar to a ROAB, except when determining the tops of cumuliform clouds. Forecast models should never be expected to model every cumulus cloud – even large ones. Expect the forecast sounding to represent the environment that cumulus clouds can develop and grow within. This exercise requires
FEBRUARY 2011
that you understand what is called parcel theory to lift an air parcel and determine if the conditions are favorable for cumulus development how far the parcel could potentially rise within the environment presented in the forecast sounding.
It’s essentially a “what if” game. Often in this situation you can only determine the maximum growth of the clouds at any particular time. When there’s a significant amount of instability present aloft, the answer
might be growth to FL450 – not an answer that is always very helpful.
VAD Wind Profile
The Velocity Azimuth Display (VAD) Wind Profile (VWP) can sometimes be used to determine the height and thickness of stratiform cloud layers. Similar to echo tops and composite reflectivity, VWP are also generated by the same NWS WSR-88D NEXRAD Doppler radars. It is displayed as a time-height
BLR Aero- space Half Page 4/C Ad
­TWIN & TURBINE • ­31