Evaluation and assimilation of ATMS data in the ECMWF system

Abstract

This paper reports on the first experiences with Advanced Technology Microwave Sounder (ATMS) data at the European Centre for Medium‐Range Weather Forecasts (ECMWF), both in terms of the contribution to the calibration/validation exercise and in terms of initial assimilation trials. Comparisons in antenna temperature space against short‐term forecasts are used to establish the fidelity of the data. Monitoring of ATMS data against short‐term forecasts shows that the data are generally of good quality, with a noise performance that is well within specification and after appropriate averaging, comparable to or better than that of the Advanced Microwave Sounding Unit‐A (AMSU‐A). Biases vary smoothly with scan positions, even before an appropriate antenna pattern correction has been established, and ATMS looks better than AMSU‐A in this regard. Outer scan positions can be assimilated without restrictions due to biases, and together with the wider swath, this leads to a better coverage from one ATMS compared to one AMSU‐A. There are indications of larger interchannel and spatial error correlations in ATMS data than for AMSU‐A, possibly linked to a weak striping effect. The analysis and forecast impact in initial assimilation trials over two seasons are significantly positive in the short range over the Southern Hemisphere and in the long range over the Northern Hemisphere, with an otherwise overall neutral impact. Experiments in the context of a depleted observing system suggest that ATMS gives a comparable forecast impact to that from a single AMSU‐A/Microwave Humidity Sounder combination.