New Ocean Observations Capabilities Provided by CMIS

Abstract

The configuration of the Conical Microwave Imaging Sounder (CMIS) includes polarimetric channels at 10.7, 18.7 and 36.5 GHz in addition to the vertical/horizontal polarizations at 6.9, 10.7,18.7, 23.8 and 36.5 GHz. This offers the capability of a simultaneous measurement of sea surface temperature (SST), sea surface wind speed (SSWS) and sea surface wind direction (SSWD). SST is measured at an 86 by 52 km resolution utilizing all 6.9-36.5 GHz frequencies. SSWS and SSWD direction are measured at a 56 by 35 km resolution utilizing the frequencies between 10.7-36.5 GHz. In addition, a high resolution (20 km) SSWS is retrieved utilizing the frequencies between 18.7-36.5 GHz. The current NPOESS requirements for the RMS precision are 0.5 K or less for SST, 1.4 m/s or less for SSWS and 20 deg or less for SSWD. We discuss the basic structure of the algorithms of the NPOESS CMIS Ocean Algorithm Suite for retrieving SST, SSWS and SSWD. They are based on a physical radiative transfer model (RTM). CMIS measured brightness temperatures over the ocean are Monte Carlo (MC) simulated from the RTM using geophysical parameters from a numerical weather prediction model together with the CMIS swath geometry and including sensor errors. The simulated data sets are used for training and testing the ocean retrieval algorithms. We give performance estimates for the precision of the CMIS retrieved SST, SSWS and SSWD based on the results MC simulations. We have also tested the CMIS Ocean Suite algorithms using real measurements from the WindSat instrument, which has a very similar channel configuration than CMIS and comparing the results with in situ measurements. Finally, we will discuss limitations of the capability of polarimetric radiometers such as CMIS and WindSat for retrieving SSWD at low wind speeds below 5 m/s due to the weakness of the signal.