Potential of 229-GHz Channel for Ice-Cloud Screening

The purpose of this paper is to evaluate the potential of a 229-GHz channel as part of a humidity sounder to improve the sounder´s cloud-screening capability. The humidity-sounding channels centered around the 183.31-GHz water-vapor line are affected by ice-cloud scattering. Usually, in the operational assimilation of humidity data, the problem of undetected clouds is overcome by setting high observation errors for humidity channels. However, high observation errors diminish the impact of good clear-sky data also, thus, leading to a smaller impact from humidity sounders in weather forecasts. It is expected that a channel at 229 GHz can offer a better cloud-detection capability due to its higher sensitivity to ice scattering compared with the existing channels. Here, a simple cloud-screening method is devised based on the difference between observed and background brightness temperatures. This method was tested for 229 GHz and for an existing channel at 166 GHz, and the results are compared. The new method using 229 GHz is also compared with an existing cloud-screening test, namely, the cirrus-cost test which uses the 183-GHz channels. The study demonstrates that a channel at 229 GHz has a better skill at detecting cirrus clouds which could be interpreted as an error in the background humidity profile. Therefore, addition of a 229-GHz channel can allow more weight to be given to the 183-GHz channels in data assimilation thus significantly increasing their impact on weather forecasting.