Passive microwave observations of mesoscale convective systems over the tropical Pacific Ocean

Abstract

This paper presents high resolution passive microwave measurements obtained in the western Pacific warm pool region. These measurements represent the first comprehensive observations of convection over the tropical oceans, and were obtained from the Advanced Microwave Precipitation Radiometer (AMPR) aboard the NASA ER2 during the Tropical Ocean and Global Atmosphere Coupled-Ocean Atmosphere Response Experiment (TOGA-COARE). The AMPR measures linearly polarized radiation at 10.7, 19.35, 37. 1, and 85.5 Ghz. Nadir brightness temperature scatterplots suggest that the three lower frequencies respond primarily to emission/absorption processes. Strong ice scattering is relatively rare, as absolute magnitudes of the ice scattering signature do not approach those measured in strong convection over land. This is a parently related to the reported weaker updraft velocities, which would create and suspend relatively smaller and/or fewer ice particles in the upper cloud. Observations within stratiform regions suggest that-220 K is the minimum 85.5 GHz brightness temperature associated with ice scattering in regions of stratiform precipitation. Detailed observations of nadir brightness temperature traces through several strong convective systems are examined. In particular, traces through a hurricane eyewall and a squall line illustrate the ability of the AMPR data to reveal the tilt of convective systems away from the vertical. It is suggested that this observed tilt of convective lines is responsible, in part, for the observed poor degree of consistency between the 10.7 and 85.5 GHz channels. The passive microwave signatures of heavily raining shallow cloud and stratiform precipitation also appear to contribute to this poor degree of consistency. The AMPR data are averaged to a 24 km resolution, in order to simulate a satellite footprint of that scale. Brightness temperature relationships become more linear, though the scatter is not significantly reduced. The effects of nonhomogeneous beamfilling are obvious. A description of brightness temperature variability within the simulated satellite footprint is also presented. Similar descriptions could be used to develop a beamfilling correction to increase the accuracy of microwave rainrate retrievals over the tropical oceans.