Understanding SFMR Measurement Errors Within High Wind Speed and Rain Gradients

For the past two decades the Stepped Frequency Microwave Radiometer (SFMR) has provided surface wind speed and mean columnar rain rate estimates in near real time (NRT) in support of tropical cyclone (TC) forecasting and warning functions. While the validity of the maximum wind speed estimates within rain columns has come under increased scrutiny over the past several years, it is only since coincident surface wind and rain observations from new instruments on the NOAA P-3 airplanes, such as the Imaging Wind and Rain Airborne Profiler (IWRAP) and the Ka-/Ku-band Interferometric Altimeter (KaIA), that the extent of these errors has become clearer. Figure 1 shows the geometry of these instruments.

IWRAP is a C- and Ku-band downward looking conically-scanning profiling Doppler radar/scatterometer system. It measures the reflectivity and Doppler velocity profiles between the aircraft and the surface with 30 m range gate resolution at pulse-repetition frequencies from 10–20 kHz. IWRAP has been installed on a NOAA P-3 as a research instrument every hurricane season since 2002, and starting in 2021, NRT 3D winds have been generated on the P-3. KaIA was installed alongside IWRAP in 2019 and has flown every hurricane season since providing NRT SWH retrievals. For the 2021 hurricane season, the collocated Ku-band channel was added to KaIA.

The IWRAP 3D winds are retrieved every 150 m along the aircraft track. The surface winds are separately retrieved utilizing scatterometry techniques also at a 150 m along track spacing. While the scatterometery winds can be retrieved along the entire fight track, the 3D winds are only available when sufficient precipitation is present in the observation column. Additionally, the IWRAP reflectivity observations provide enough information to retrieve an average rain rate assuming a Z-R relationship. The IWRAP measurement geometry complements that of SFMR, providing us with a unique opportunity to compare rain rate estimates from two instruments that are observing almost identical parts of the rain column. IWRAP also takes measurements at two different incidence angles at each frequency. This configuration provides us an opportunity to better understand which measurements are the most ideal within TCs to observe the physics of the marine boundary layer.

In this presentation we will show how IWRAP rain rate and wind speed estimates compare to SFMR in tropical cyclones and focus on the conditions in which a high wind or rain gradient is observed. We also separately analyze the inbound/outbound tracks to evaluate the impact of the frequency stepping measurement process that the SFMR uses.