The Assimilation of Observations from the Advanced Microwave Sounding Unit over Sea Ice in the French Global Numerical Weather Prediction System

Abstract

The aim of this study is to test the feasibility of assimilating microwave observations from the Advanced Microwave Sounding Units (AMSU-A and AMSU-B) through the implementation of an appropriate parameterization of sea ice emissivity. AMSU observations are relevant to the description of air temperature and humidity, and their assimilation into numerical weather prediction (NWP) helps better constrain models in regions where very few observations are assimilated. A sea ice emissivity model suitable for AMSU-A and AMSU-B data is described in this paper and its impact is studied through two assimilation experiments run during the period of the Arctic winter. The first experiment is representative of the operational version of the M et eo-France NWP model whereas the second simulation uses the sea ice emissivity parameterization and assimilates a selection of AMSU channels above polar regions. The assimilation of AMSU observations over sea ice is shown to have a significant effect on atmospheric analyses (in particular those of temperature and humidity). The effect on temperature induces a warming in the lower troposphere, especially around 850 hPa. This leads to an increase in the Arctic inversion strength over the ice cap by almost 2 K. An improvement in medium-range forecasts is also noticed when the NWP model assimilates AMSU observations over sea ice.

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

“…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, ; Baordo and Geer, ), following the methods of Karbou et al ., (; ), or values from the FASTEM model are used over the ocean (Liu et al . ; Bormann et al .…”

Observation impact over the southern polar area during the Concordiasi field campaign

“…This is also an area where the satellite radiance usage is generally poor due to the difficulty in using radiances over sea‐ice, as can be seen in Figure (f). However, some developments have recently taken place in Météo‐France to better characterize the microwave sea‐ice emissivity in order to be able to assimilate Advanced Microwave Sounding Unit B/Microwave Humidity Sounder (AMSU‐B/MHS) data on this surface (Karbou et al, ), and the coverage is then improved for this particular centre, as we can see by comparing coverage of AMSU‐B/MHS between NRL in Figure (f) and Météo‐France in Figure .…”

THz spectroscopy of the atmosphere for climatology and meteorology applications

“…Moreover, among the whole set of available channels only about 100 are currently assimilated in NWP models, for both practical and scientific reasons [9]. For water vapour retrieval, the AMSU-B/MHS instruments are widely used in satellite data assimilation systems for numerical weather prediction, even though the limited number of spectral channels provides very poor vertical resolution [10][11][12][13]. A recent European Space Agency (ESA) funded study by Prigent et al [14][15][16] compared the predicted retrieval performance of a future hyperspectral microwave instrument to the predicted performance of the microwave (MW) instruments selected to fly on board the next generation of European operational meteorological satellites (MetOp-SG).…”