Analysis of the potential and limitations of microwave radiometry for the retrieval of sea surface temperature: Definition of MICROWAT, a new mission concept

Abstract

The sensitivity of passive microwave observations to the sea surface temperature (SST) is carefully analyzed, with the objective of designing an optimized satellite instrument, MICROwave Wind And Temperature (MICROWAT), dedicated to an “all‐weather” estimation of the SST at high spatial resolution (15 km). Our study stresses the importance of low‐frequency observations around 6 GHz for accurate SST retrieval. Compared to the 11 GHz channel, the 6 GHz channel provides more sensitivity to the low SSTs and offers lower instrument noise, thanks to possibly broader channel bandwidths. However, it requires much larger antenna size for a given spatial resolution. Two instrument concepts have been suggested, one using a classic real aperture antenna and the other using synthetic interferometric antennas. This first analysis shows that 2‐D interferometric systems would be very complex and would not satisfy the user requirements in terms of SST accuracy. A 1‐D interferometric system could be proposed, but its development requires additional investigation. A dedicated conical scanner onboard a microsatellite with a 6 m antenna and channels at 6.9 and 18.7 GHz (both with V and H polarizations) can provide an SST accuracy of 0.3 K with a 15 km spatial resolution, with today's technology.

Optimal Estimation of Sea Surface Temperature from AMSR-E

“…Sensitivities from 0.39 to 0.65 were reported in Gentemann et al [64] for 0 • C and 30 • C SST, respectively, for an AMSR-E regression type retrieval. In addition, Prigent et al [63] used simulations to derive channel sensitivities ∂ TB/ ∂ SST of~0.3 to 0.6 for the 6 GHz V. These results are in good agreement with the sensitivities obtained here. …”

“…The mean SST sensitivity for all drifter matchups during 2010 is found to be 0.50 with above OE setup and Figure 10a shows the geographical distribution of SST sensitivity. The SST sensitivity is lowest in high latitudes and increases towards the equatorial region, which is consistent with the fact that ∂ TB/ ∂ SST is smaller for cold waters (especially for X-band 10.65 GHz channels) [63]. The equatorial region reveals sensitivities of~0.6 while high latitudes have sensitivities around 0.4.…”

The Role of Advanced Microwave Scanning Radiometer 2 Channels within an Optimal Estimation Scheme for Sea Surface Temperature

“…Prigent et al [26] carried out a similar analysis for a new mission concept, Microwat, simulating retrievals based on AMSR-E channel sensitivities. They retrieved SST and wind speed assuming initial uncertainties on these two quantities of 3.31 K and 1.33 m·s −1 , respectively.…”

“…They also carried out an information content analysis including water vapour content uncertainties of 10% on model levels. To provide comparability, we conduct an analysis below based on this specification using, as did Prigent et al [26], a retrieval vector containing the four variables SST (T s ), the natural logarithm of TCWV (W) and the two wind-speed components (u, v):…”

“…Our aims are to establish the fundamental limits of retrieval uncertainty for AMSR2 SST retrieval in the framework of optimal estimation (OE), and to inform strategies about channel selection for developing a new MW SST product, ultimately intended for joint use with IR products in a climate data record. A previous study with similar objectives [26] neglected the importance of variable cloud liquid water in MW SST retrieval, and did not address itself to the prioritisation of channels, both addressed here.…”