Extrapolation of Sea Level Pressure by Reconnaissance Aircraft: In Search of an Improved Methodology

In-Situ observations are fundamental to analyzing, forecasting, and researching tropical cyclones.  The National Oceanic and Atmospheric Administration’s Aircraft Operations Center at MacDill Air Force Base in Tampa. FL is often tasked with operational and research hurricane flights.   GPS dropwindsondes are deployed into storms to collect atmospheric vertical profile data including temperature, dewpoint, pressure, wind speed, and wind direction.  Some of the most challenging and important sonde observations are made at the center of storms providing data such as lowest central sea level pressure.  Occasionally, a center sonde fails to measure the minimum central pressure.  Sea level pressure can be extrapolated using flight level observations and used as a substitute for sonde observations.  Although not perfect, the extrapolated sea level pressure equations often produce results within a few millibars of the  minimum sea level pressure measured by a sonde. 

Extrapolated sea level pressure equations at the center of tropical cyclones show a low bias compared to sonde measurements.  The errors in the extrapolation are partly due to use of the standard lapse rate of -6.5 degrees Celsius per kilometer.  The warm core of a tropical cyclone often has lapse rates between -3.0 to -1.0 degrees C per kilometer. The measured lapse rate can be calculated using the flight level temperature, subtracting the surface temperature measured by a sonde, and then dividing  the aircraft geopotential altitude by the difference.  This derived lapse rate can then be substituted into the extrapolated sea level pressure equations in place of the standard lapse rate.  The cases for this study are limited to the center of tropical cyclones where valid sonde data are available.  Data for these cases were extracted by temporally matching  data from Vortex Data Messages, TEMPDROP messages, and HDOBs. The initial results of substituting the measured lapse rate for standard lapse rate in the extrapolated sea level pressure equations has revealed some key trends demonstrating promise for  improving extrapolated sea level pressure calculations.  The initial results of using the calculated lapse rate suggest improvements in the accuracy of extrapolated sea level pressure compared to lowest central sea level pressure as measured by sonde along with increased of precision in extrapolated values are possible.