Calibration and Applications of Pressure Data from Mobile Platforms

Measurements of atmospheric pressure by mesoscale transects of vehicle platforms such as the National Severe Storms Lab (NSSL) Mobile Mesonets have previously been collected in various targeted field campaigns. The challenges involved were specifically documented in the very different environments of tornadogenesis (Markowski et al. 2002) and orographic foehn winds (Raab and Mayr 2008). In recent years the Jackson State University Mobile Meteorology Unit (MMU) has been developed with broad ranging applications in mind. The inclusion of barometric pressure was originally expected only to be used for calculation of potential temperature over transects with significant elevation change. As such, careful positioning of a pressure port intake to reduce effects of flow distortion around the vehicle was not considered necessary. However previous studies have determined a dynamic change of measured pressure due to vehicle motion relative to the air that varies quadratically with speed, in agreement with theoretical expectations. This quadratic relationship has been examined for the MMU under a wide variety of conditions, both with and without use of a pressure port. In order to consider least squares regression of this relationship, it was necessary to also have accurate speed and elevation data. Since even quite small elevation changes can produce measurable pressure changes, it was considered necessary to reduce pressures in each transect to the mean elevation using the methodology of Markowski et al. (2002). In most cases this required a combination of digital elevation model (DEM) and geographic positioning system (GPS) data to have sufficiently accurate elevations matched to the locations of the pressure measurements. Since the MMU does not measure wind velocity and most cases were for light winds, only the speed relative to ground from the GPS was used. Types of cases to be discussed include transects from about 10 to 200 km in length: approximately uniform conditions in flat terrain; crossing of significant orographic barriers (> 500 m); cold fronts; drylines; a mesohigh associated with a strong thunderstorm. The impacts for determination of mesoscale pressure gradients, potential temperature, and other derived quantities will be evaluated.