Detectability of Polar Mesocyclones and Polar Lows in Data From Space-Borne Microwave Humidity Sounders

Polar lows are small intense low-pressure systems over polar seas associated with strong winds and heavy snowfall. Because of small scale and short lifetime, they are often missed by weather observation networks. As they can significantly affect ships and coastal communities and can have a large impact on lateral heat and moisture transport, better observation of polar lows with satellites is worthwhile. Here, we assess the detectability of polar lows using data from the microwave humidity sounders advanced microwave sounding unit-B (AMSU-B) and microwave humidity sounder (MHS) on the polar-orbiting satellites of NOAA. It turns out that cloud ice and frozen precipitation in polar lows cause strong scattering of upwelling atmospheric emission in the three water vapor channels around 183.31 GHz. As a result, most polar lows can be detected in brightness temperature differences of the water vapor channels as small areas of reversed sign. Out of 110 confirmed polar lows in the Nordic Seas, 85% could be clearly detected with AMSU-B. Looking for all polar-low-like signatures in AMSU-B/MHS data, we found about twice as many cases as the most thorough remote sensing inventory of polar lows published so far. The main reason is that in contrast to that inventory, we did not impose a minimum wind criterion which would need wind data from other sources. However, using the AMSU-B/MHS brightness temperature differences can be a quick tool to detect possible polar lows in near real time and this can equally well be applied to similar microwaves sounders operational on current and future satellites.