The Influence of Frequency Shifts in Microwave Sounder Channels on NWP Analyses and Forecasts

Abstract

he sensitivity of numerical weather prediction (NWP) analysis and forecast accuracies with respect to frequency shifts in microwave passbands is quantified through a series of observing system experiments using the ECMWF Integrated Forecast System. First, a parameterization is developed to describe the form and magnitude of the brightness temperature errors arising from frequency shifts in Advanced Microwave Sounding Unit-A (AMSU-A) channels 4?10 and Microwave Humidity Sounder (MHS) channels 3?5. Observing system experiments are then run in which realistic synthetic brightness temperature errors are added to AMSU-A observations for various assumptions about the magnitude of a frequency shift, using the parameterization derived previously. A large negative impact on forecast quality is found when a 20-MHz frequency shift is introduced in experiments using a static bias-correction scheme. Although the degradation in forecast scores is reduced by using a variational bias-correction scheme, it remains around 7%?14% (relative) in RMS 6-h forecast errors for temperature and geopotential. Frequency shifts of 5 MHz or greater give rise to a measurable degradation of the forecast even when the variational correction scheme is used. Only low-frequency shifts (of ~1.5 MHz) are found to have a neutral impact on forecasts. Hence, the value of 1.5 MHz can be regarded as an upper limit below which frequency shifts do not degrade forecasts for the key tropospheric and lower-stratospheric temperature sounding channels in a microwave sounding mission. Calculations show that frequency shift is less problematic for 183-GHz humidity sounding channels due to the symmetric positioning of passbands relative to the 183-GHz absorption line. For these channels a passband center frequency stability of 10 MHz is adequate.