Presenter: Jezabel Vilardell Sanchez (University of Massachusetts Amherst)

Description:
The estimation of ocean surface winds and rain retrieved from airborne Stepped Frequency Microwave Radiometer (SFMR) brightness temperatures has proven immensely valuable for the assessment of tropical cyclones. The forward radiative transfer model (RTM) and the associated retrieval algorithm used to derive both surface wind speed and columnar rain rate from the SFMR data, was developed particularly for tropical cyclones and has been further updated and refined over several years, leading to an SFMR performance highly reliable in hurricanes. However, when the RTM and retrieval algorithm are applied to data collected over extra tropical cyclones, particularly high-latitude winter storms, the retrievals often deviate from other ground-truth sources. In several cases, erroneously high rain rates are retrieved. The potential cause of such inaccurate retrievals could be that the RTM makes certain assumptions of the state of the surrounding atmosphere and weather conditions, based on the well known tropical cyclones climate, that are not valid for the extra-tropical environment. Using SFMR data collected during winter flights over high-latitude storms, we have experimented with a modified RTM more consistent with high-latitudes areas marine atmospheric boundary layer (MABL), where the freezing level is below the aircraft. This has the desired effect of reducing the retrieved rain rate. In cases with low rain rate retrievals from the instrument where no rain is reported from other sources, we hypothesize that a shallow atmospheric surface-layer (ASL) with mixed-phase hydrometeors, formed by droplets ejected from the sea surface into the air, driven by the wave height, surface stress and wind speed, may contribute to a precipitation-like signature. In this presentation we describe the modified RTM and present comparisons with other data sources.