Near-Real Time Wind Speeds from SMAP for Tropical and Extratropical Cyclone Forecasting

The spaceborne SMAP (Soil Moisture Active Passive) L-band radiometer has excellent capabilities to measure high ocean surface wind speeds in strong tropical and extratropical storms (Meissner et al., BAMS 2017). The big strengths of the passive SMAP sensor are very good sensitivity of the received signal to wind-speeds above 12 m/s and very small impact by rain, which constitutes a significant error source for satellite wind measurements at higher frequencies.

The data acquisition and processing for the SMAP sensor was originally designed to provide science quality soil moisture data. Following demands from the forecasting community, who is now widely using both SMAP soil moisture as well as ocean wind speed in their assimilations, NASA has established a near-real (NRT) feed for the SMAP radiometer measurements. This NRT data stream uses pre-computed spacecraft ephemeris data, which require a significantly smaller data volume to be downlinked from the satellite to the ground processing. The downlink constitutes a major bottleneck in the SMAP data processing and is the main driver in the latency at which SMAP products can be provided.

Remote Sensing Systems (RSS) is using the SMAP NRT Level 1 radiometer data to produce ocean surface wind speeds. In order to minimize additional latency, the wind speed retrieval is set up to ingest only static climatology ancillary data. The most important are sea surface salinity and temperature. We are able to provide NRT SMAP storm wind speeds with latency of about 2 hours. The quality of this SMAP NRT winds is only marginally degraded from the final product, which uses actual spacecraft ephemeris measurements and ingests more accurate dynamically measured ancillary data. RSS is extracting several parameters from the SMAP NRT wind fields that are used by the tropical cyclone forecasters: maximum wind speed and the radii of the 17.5, 25 and 33 m/s winds. We are supplying these parameters in NRT to the US Navy and the Joint Typhoon Warning Center, who are ingesting them into their Automated Tropical Cyclone Forecasting systems.

We are in the process of establishing a similar process for extratropical storms. We plan to provide extratropical storm intensity and sizes for the NOAA Ocean Prediction Center for issuing warnings for vessels to avoid areas that are impacted by extreme winds.