Measurement of Cloud Perturbation Pressures Using an Instrumented Aircraft

Abstract

ertical accelerations during the early stages of convective cloud formation are often the result of buoyancy and the perturbation vertical pressure gradient forces. Convection modifies the local pressure field surrounding the cloud. Measurement of the cloud perturbation pressure field is challenging over distance scales on the order of the convective elements, since the signals are often small and the turbulent environment complicates the measurement of static pressure. A technique is described that enables detection of the horizontal pressure perturbations associated with evolving convective clouds using global positioning system measurements on an airborne platform. Differential kinematic processing of data from dual-frequency, carrier-phase-tracking GPS receivers on research aircraft with static base station receivers enables the three-dimensional aircraft position to be resolved within decimeters. Vertical positioning and precise measurement of static pressure allow horizontal pressure perturbations to be determined to an accuracy of roughly 10 Pa. Errors in the static pressure measurement, rather than the GPS-derived altitude, are the largest source of error. A field experiment was conducted in May?June 2008 to demonstrate measurement of perturbations in the horizontal pressure field associated with summertime cumulus congestus clouds over the high plains. Observations of growing convective clouds show negative pressure perturbations on the order of 100 Pa near cloud base linked to updraft regions. Growing cumulus show a high degree of variability between subsequent passes that demonstrate that the horizontal pressure fields evolve rapidly along with attendant vertical circulations and cloud microphysical characteristics.