Observation of polar lows by the Advanced Microwave Sounding Unit: potential and limitations

The potential of the Advanced Microwave Sounding Unit (AMSU) observations for the depiction and tracking of intense high-latitude mesoscale maritime weather systems, called polar lows, is explored. Since a variety of mechanisms are important for their development and maintenance, this investigation is based on three polar low cases of different types. The AMSU-B channels at 183 GHz are able to locate convective polar lows (PL) even in their incipient stage, at a time when there is considerable uncertainty as to the nature of the cloud structures seen in the visible or infrared imagery. This detection is based on temperature depression due to scattering by hydrometeors, as confirmed by comparison with radar data. These same channels will, however, fail to unambiguously detect weakly convective and mainly baroclinic PL. The AMSU-A channels help documenting the large-scale environment in which PL develop. Channel 5 clearly shows the cold air outbreaks associated with these developments, whereas the difference between channels 7 and 5 can be used to detect and locate positive upper-level potential vorticity anomalies. Because of the high temporal availability of AMSU observations and despite some limitations pointed out in this study, these results are relevant for PL forecasting and monitoring.