Airborne Measurement of Cloud Perturbation Pressures Using Differential Global Positioning System (GPS)

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The pressure gradient force is the dominant term in the equation of motion that describes the forcing of atmospheric motions. To understand the dynamics of atmospheric flows, it is necessary to know the horizontal pressure gradient force (PGF). For the past two decades, airborne platforms have been used in attempt to discern variations in the horizontal pressure field over a variety of scales to understand the dynamics of mesoscale to synoptic-scale motion fields. Serious problems have been encountered when conducting flight legs over irregular terrain using altimetry-based methods that have prevented the PGF application from widespread use. Recently it has been shown that accurate detection of the horizontal pressure gradient force from airborne platforms over mesoscale to microscale distances is possible by using differential GPS. In this technique, GPS position measurements by a dual-frequency receiver onboard an aircraft are corrected using GPS measurements from a fixed and precisely surveyed base station. By flying on an isobaric surface using the autopilot, the slope of the isobaric surface, which is a measure of the PGF, can be determined. This technique enables for the first time a means to depict forcing of microscale to mesoscale atmospheric circulations over complex terrain by airborne platforms. Intellectual Merit: The primary goal of this research is to examine the potential of differential GPS position measurements from the University of Wyoming King Air (UWKA) platform to determine pressure perturbations associated with convective clouds over the high western plains of the U.S. Horizontal pressure perturbations are fundamental to studies of cloud dynamics and critical to an understanding of convection. Pressure perturbations result from the atmospheric mass adjustment that accompanies convective motions as buoyant air parcels rise. Knowledge of the cloud pressure perturbations will permit an understanding of the spatial extent and magnitude of the forcing of the local environment by convective motions as well as a means to validate high resolution numerical modeling experiments. A field experiment will be conducted during August 2008 based out of Laramie WY to evaluate the horizontal pressure perturbations that result from convective clouds that range in size up to the cumulus congestus stage. The University of Wyoming Cloud Radar will be deployed on the UWKA to provide additional information regarding the motion field within convective clouds. Horizontal pressure perturbation measurements will be made in directions along and normal to the mean wind beginning at cloud base. Subsequent passes will be made at higher levels within the cloud to provide information on the vertical structure of the horizontal pressure perturbations Broader Impacts: If such measurements are shown to be successful, GPS detection of the cloud-induced horizontal pressure perturbations will become a routine measurement on airborne platforms and become standard for the UWKA and of use to other investigators within the atmospheric science community. This represents a significant enhancement for future studies of cloud physics and dynamics since the fundamental forcing term for horizontal motions can be directly measured. Successful application of this GPS technology will also enable a host of new measurement opportunities, including studies of entrainment in clouds, refinements in cloud motion field and the evolution in dynamics associated with cloud-scale circulations.