Mesoscale Atmospheric Processes Seminar: Ian S. Adams

Remote Sensing Division, Naval Research Laboratory


Aerodynamical forces exerted upon precipitating hydrometeors induce preferential orientations with respect to the fall directions and influence the morphologies of the particles. For liquid precipitation, spherical drops are flattened and bowed, with larger drops breaking up due to instability. The largest axes of pristine ice crystals align perpendicularly to the fall directions. Aggregates will also align preferentially, and the complex morphologies of these hydrometeors influence airflow about the particles and, thus, further aggregation or melting. Turbulence can reduce the degree of alignment and may result in random orientation under certain conditions, such as in convection. These preferentially-­‐‑oriented hydrometeors form a dichroic medium, allowing polarimetric remote sensing observations to discern particle shapes and alignments. Moreover, aligned hydrometeors alter the polarization state of radiation emitted and reflected by the surface beneath the precipitating layer. This talk reviews particle orientation and the associated dichroic effects, with a focus on microwave and millimeter wave radiometry. The scattering properties of particles with arbitrary and preferential orientations are discussed. Radiative transfer simulations involving simple particle models illustrate the polarizing effects of horizontally-­‐‑aligned rain and snow. Finally, an analysis of dendrites and plates elucidates shape-­‐‑ and orientation-­‐‑dependent scattering.