Assimilation of SSMIS humidity‐sounding channels in all‐sky conditions over land using a dynamic emissivity retrieval

Abstract

The extension of all‐sky assimilation of Special Sensor Microwave Imager/Sounder (SSMIS) humidity‐sounding channels to land surfaces is investigated in this article. The scattering index, which is able to discriminate between cloudy and precipitating regions over land, is used as a predictor to develop a ‘symmetric’ model for observation error. This formulation is able to increase the observation error in those scenes that are more difficult to model because of radiative transfer and ‘mislocation’ errors. The implementation of an instantaneous emissivity retrieval from SSMIS observations is also presented. In clear‐sky scenes, emissivity retrievals appear better at capturing daily differences in surface conditions compared with emissivity atlas values. In the presence of clouds, retrievals show different behaviour. In the lower microwave frequencies (less than 50 GHz), emissivity estimates appear nearly as reliable as those in clear skies, but at higher frequencies, as the magnitude of scattering increases, so does the error in the retrieval and the resultant emissivity estimate can be unphysically low or high. However, the retrieval still appears feasible at high frequencies in light cloud situations; the number of retrievals discarded due to this kind of problem is around 10%. In these cases, an estimate from an emissivity atlas can be substituted instead. Together, the new observation‐error model and the instantaneous emissivity retrievals were adopted for the assimilation of SSMIS 183 GHz channels over land in all‐sky conditions. Assimilation experiments showed that the assimilation system is not degraded and the improvements in analysis and forecast scores are about the same as those obtained by the equivalent clear‐sky approach. The developments described in this study were an essential first step to create framework to allow the all‐sky assimilation over land of other microwave humidity sounders: this started operationally at the European Centre for Medium‐Range Weather Forecasts (ECMWF) in 2015, covering both SSMIS and four Microwave Humidity Sounder (MHS) instruments.

Use and impact of Arctic observations in the ECMWF Numerical Weather Prediction system

“…(), Geer et al . (), Geer and Bauer () and Baordo and Geer (), which were developed initially for the assimilation of microwave imagers and were more recently applied to the assimilation of humidity sounding channels (Geer et al . ).…”

“…). The use of microwave sounding data over the Arctic has changed greatly over the last 10 years, with the majority of data now assimilated over all surface types including ocean, snow‐covered land, snow‐free land and sea ice (e.g., Di Tomaso and Bormann, ; Baordo and Geer, ). The current usage of microwave sounding channels in the Arctic is summarised in Table .…”

“…For tropospheric sounding channels sensitive to the surface, the surface emission is estimated prior to assimilation using the model skin temperature and an estimate of surface emissivity. Constant values are used for emissivity of infrared instruments and either a dynamic retrieval is used for microwave observations over land and sea ice (Di Tomaso and Bormann, 2012;Baordo and Geer, 2016), following the methods of Karbou et al (2005;, or values from the FASTEM model are used over the ocean (Liu et al, 2011;Bormann et al, 2012;Kazumori and English, 2015). A skin temperature sink variable is then used during the assimilation of clear-sky observations to account for uncertainties in the model skin temperature.…”

The growing impact of satellite observations sensitive to humidity, cloud and precipitation

“…The impact of 8% in 2012 demonstrates the benefit of the all‐sky assimilation of microwave imagers that had recently been introduced (Bauer et al , ; Geer et al , ; Geer and Bauer, ). Since then increases have come by applying all‐sky assimilation to microwave humidity sounding channels, for the SSMIS instrument (Geer, ; Baordo and Geer, ) and later MHS (Geer et al , 2014), along with the activation of ATMS (Bormann et al , , in clear skies only to date; the Appendix gives satellite and sensor acronyms). Further benefits have come from the all‐sky assimilation of new microwave imagers AMSR2 and GMI (Kazumori et al , ; Lean et al , ) and the novel sounder MWHS‐2 (Lawrence et al , ).…”

Satellite-Based Atmospheric Infrared Sounder Development and Applications

“…In a similar vein, has introduced an operational analysis scheme using cloud-affected radiances to good effect by extending the 4DVAR analysis control to include parameters that describe the cloud conditions and simultaneously to estimate these parameters together with temperature and humidity. Direct assimilation of all-sky radiances is also being studied, with progress being realized for microwave humidity sounders (Baordo and Geer 2016;Guerbette et al 2016). Lavanant et al (2011) compared the results from different retrieval schemes for cloud parameters, finding consistency in those schemes.…”