Exploring the use of CYGNSS for Surface Flux Estimation in Extratropical Cyclones​

Surface latent and sensible heat fluxes play an important role in Earth's climate and weather. These fluxes, driven by strong temperature/moisture gradients and surface winds, can transport heat and moisture between the Earth's surface and planetary boundary layer. Previous studies have shown that surface fluxes are critical in marine extratropical cyclogenesis, intensification, and evolution over the world's oceans. Nevertheless, observing and estimating these surface fluxes continuously over the oceans presents many challenges due to a lack of in-situ observations. While current space borne instruments provide global coverage, current scatterometers cannot accurately observe surface winds in the presence of precipitating clouds, an important component in estimating surface fluxes. Since cyclones feature heavy bands of precipitation, observation gaps may result in inaccurate surface wind estimates within cyclones. The Cyclone Global Navigation Satellite System (CYGNSS), scheduled for launch in October 2016, aims to address this by providing improved surface wind observations within cyclones and under precipitating clouds.

The motivation for the work presented here stems from a case study of a strong marine extratropical cyclone (ETC) that occurred in late November 2006 off the Atlantic Coast of the United States (Crespo and Posselt 2016). The warm front of this cyclone experienced a significant stratiform-to-convective transition in its cloud structure and thermodynamic field. Examination of reanalysis and QuikSCAT surface wind data indicate large values of latent and sensible heat fluxes around the cyclone, correlating with faster surface wind speeds. These likely played a direct role in the cyclone's intensification and warm frontal convective transition.

A portion of the observations from QuikSCAT were not representative of the true surface winds due to the precipitating clouds within the cyclone attenuating the signal. By using simulated orbits and observations, our research will show how CYNGSS would have aided in surface flux estimation thanks to its ability to estimate surface winds in the presence of precipitation and its higher spatial and temporal resolution. Though CYGNSS will be in a tropical orbit (35°N to 35°S), we will demonstrate its potential ability to observe extratropical cyclones forming in the lower midlatitudes and their associated surface fluxes.