Detecting winds aloft from water vapour satellite imagery in the vicinity of storms

2 Introduction The most extensive use of water vapour imagery in recent years has been to identify upper-level wind features such as shortwave troughs and to compare their location and intensity to those produced by numerical forecast models. For example, Weldon and Holmes (1991) compiled a catalogue of water vapour imagery for use in identifying a wide range of upper air features. The identification of these features arises from spatial patterns of brightness in the water vapour images (related to the variation of radiance with moisture content in the upper troposphere). Subjective adjustments to forecasts can be made from observed differences in the location and intensity of troughs, jets, and other features in satellite imagery to those in model analyses and forecasts. In addition to the subjective use of water vapour imagery in forecasting, the imagery has been used quantitatively to provide estimates of the wind through movement of clouds and moisture features between successive images, where direct measures from weather balloons are lacking. Winds were determined by manually tracking clouds between successive images and calculating the wind from the displacement and the time interval between images, in the early years of this type of research (Stewart et al., 1985). The height of the wind estimate was derived from the cloud top temperature and a vertical profile of temperature. This presupposes that the cloud moves with the speed of the wind, an assumption that is not strictly valid. Comparisons with independent balloon measurements suggest an underestimation of wind speed and the need for an empirical adjustment. In recent years, the technique has been automated and is based on cross correlation of the patterns within boxes which are displaced according to the winds from a forecast model 'guess' field (Velden et al., 1997). With this approach, winds cannot only be estimated from tracking clouds, but also from the displacement of small-scale structures present in clear areas of the imagery. These structures occur due to 3 humidity fluctuations in the middle and upper layers of the troposphere. While winds obtained from tracking cloud features in the imagery are prone to greater uncertainty than balloon measurements, they have been used to improve analyses and et al., 2002). In order to explore the use of satellite winds over land, an automated method for calculating water vapour winds, developed at the Cooperative Institute for Meteorological Satellite Studies, CIMSS (Velden et al., 1997), is being applied …