Monitoring Tropical Cyclone Surface Winds with Satellite Microwave Radiometers

Satellite measurements of ocean surface winds in Tropical Cyclones are beneficial to the operational community as they provide information about storm parameters, such as intensity and size, in remote locations, when in-situ (e.g, dropsondes, drones or airborne sensors) measurements are not available, and consistently during the full lifetime of each storm. However, space-based wind measurements in these extreme environments have historically been challenging because of the presence of rain, which impacts the observed signal and is hard to discern from the wind signal.

In recent years, important advancements have been made using microwave (MW) radiometers to measure extreme winds in storms, beginning with L-band sensors, such as SMAP or SMOS, which retain very good sensitivity to winds (10-70 m/s) without being affected by rain. Other radiometers such as WindSat and the AMSR also allow extreme measurements in rain by taking advantage of specially trained all-weather algorithms, which use a combination of C- and X-band channels to retrieve high winds in storms.

Here we summarize a set of products specifically developed at Remote Sensing Systems (RSS) for Tropical Cyclone conditions. These products, known as the RSS ‘TC-winds’, are available from 2002-present and include measurements made by SMAP, AMSR-2, AMSR-E, and WindSat. The presentation will cover the basic methodology for the SMAP and C/X-band (e.g., AMSR2) all-weather algorithms and a summary of the RSS data products for extreme winds. It will then outline recent validation activity of these surface TC-wind fields, which includes comparisons with surface winds from other observations such as Saildrones, airborne SFMR and Synthetic Aperture Radars (SAR), and with the HWRF model surface winds. We will present updates on recent improvements to the microwave TC-wind algorithms designed to address some unresolved biases due to residual rain contamination.

A benefit of radiometer TC-winds is the ability to detect rapid intensification events, due to their consistency over time and over different sensors. An additional new validation source of the satellite-based high winds is provided by observations using unmanned Saildrones in storms in the Atlantic Ocean. It is important to highlight that due to the lower resolution of these radiometer measurements relative to in-situ or the high resolution HWRF model, the difference in observed scales must be accounted for when comparing them, by resampling the high-resolution observations to satellite scales (25-40 km footprints) or averaging them in time to represent a 10-minute sustained wind, which is closer to what the satellite sensors actually see.

In addition to the surface wind fields, the MW radiometers provide storm parameters such as storm intensity and size in Near-Real-Time, which are routinely used by the operational community. We will present how the satellite-derived 10-minute sustained maximum winds compare with intensity timeseries from Best-Track data and a Satellite consensus dataset (Satcon from CIMSS), and similarly how the size timeseries compare over the lifetime of some storm.