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Author ORCID Identifier

https://orcid.org/0000-0001-8281-9834

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Electrical and Computer Engineering

Year Degree Awarded

2023

Month Degree Awarded

September

First Advisor

Stephen Frasier

Subject Categories

Electrical and Electronics

Abstract

The Stepped Frequency Microwave Radiometer (SFMR) is a key instrument for estimation of ocean surface wind speed and rain rate in tropical and extra-tropical cyclones research. Through the observed brightness temperature (TB) over a range of six C-band frequencies, the SFMR derives these key parameters used by hurricane specialists to issue watches and warnings. The information gathered with this instru- ment is also pivotal for post-storm studies and satellite calibrations. Currently, the SFMR requires an average time of 5-10 seconds of averaging to cycle through the six di↵erent frequency channels, so in regions with strong wind/rain gradients such as the eye wall of a hurricane, finer scale details can be overlooked. The University of Mas- sachusetts Amherst Microwave Remote Sensing Laboratory (MIRSL) has developed a specialized version of the SFMR called UMass Simultaneous Frequency Microwave Radiometer (USFMR) that operates six frequency channels simultaneously, eliminat- ing the averaging time. This project aims to assess the performance di↵erences of these two sampling methods with data collected in the 2019 hurricane season, where the USFMR was installed alongside the operational SFMR aboard one of the NOAA WP-3D aircrafts. The radiative transfer model (RTM) employed by the stepped frequency microwave radiometer (SFMR) and its application in airborne wintertime observations of high- latitude storms and extra tropical cyclones is considered. It is found that the current RTM, developed and tuned for use in tropical cyclones (TCs), does not adequately model the observed brightness temperatures typically encountered in these cold con- ditions. While the brightness temperatures observed at several frequencies across C-band are lower, they are more spread apart from each other than the TC RTM predicts. This study considers two hypotheses to explain the di↵erences between the measurements and model. One hypothesis assumes the presence of a melting layer be- tween the aircraft and the surface which imparts enhanced attenuation and emission, which would result in enhance spreading of brightness temperatures. The properties of the melting layer scale with rain rate. The other hypothesis is a wind-dependent excess emissivity possibly due to a surface-based layer of mixed-phase droplets lofted from the surface. The latter hypothesis is most consistent with observations when the freezing level, as deduced from the flight-level temperature and an assumed lapse rate, is at or below the surface. It is found that the latter hypothesis appears to represent the observations better than the first, in large part because there is often little to no rain present in the observations. Finally, a scaling of the TC RTM’s wind excess emissivity is found to be required to obtain the best match with available dropsonde observations. An excess emissivity model for winter conditions is provided. This project has been done in collaboration with NOAA/NESDIS/STAR and with funding support from the National Science Foundation through grant AGS-2016809.

DOI

https://doi.org/10.7275/35743704

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