Polar mesocyclones and polar lows

A polar mesocyclone (PMC) is a mesoscale cyclone forming in cold air masses in sub-polar and polar areas, poleward of the main polar front. A polar low (PL) is an intense marine polar mesocyclone – the usual criterion is surface wind speed above 15 m/s . In addition, polar lows are often associated with strong precipitation. Polar mesocyclones and polar lows are a significant and relevant meteorological phenomenon as they can seriously affect ship and offshore operations as well as coastal communities by strong wind and snowfall. Moreover, they can significantly contribute to the sensible and latent heat flux and to the moisture transport. Polar mesocyclones have a – probably significant – impact on ocean circulation, in particular, on oceanic deep convection. Their contribution to the general circulation on the Northern and Southern hemisphere is still unclear. Polar mesocyclones are not well represented in coupled climate models as well as weather prediction models, and observational information on the occurrence of polar mesocyclones and polar lows is needed.

Because of the small scale, short lifetime, and the remoteness of the regions where they develop, polar mesocyclones and polar lows are often missed by weather observation networks, are not well represented in synoptic-scale models, and are therefore poorly monitored and hard to predict. Therefore, in order to address the need for more and better data on polar mesocyclones and polar lows, satellite remote sensing is an appropriate tool.

Observing polar mesocyclones and polar lows

Systematic observations and monitoring of polar mesocyclones and polar lows have been possible since satellite remote sensing data were available in sufficient temporal and spatial coverage. Most approaches use visible and infrared (IR) imagery, e.g., from the instruments AVHRR (Advanced Very High Resolution Radiometer), HIRS (High Resolution Infrared Radiation Sounder) or MODIS (Moderate Resolution Imaging Spectroradiometer). On such imagery, PMCs and PLs are visible by their distinct cloud patterns. Additionally, data from active or passive microwave instruments for wind information can be used in order to distinguish PLs from PMCs. Other approaches rely on passive microwave (= microwave radiometer data), e.g., from AMSU-A and B (Advanced Microwave Sounding Unit A and B), or AMSR-E (Advanced Microwave Scanning Radiometer for EOS). These instruments mainly respond to water vapour, cloud ice and atmospheric temperature.

One of the aims of using satellites to observe PMCs and PLs is obviously to compile inventories or climatologies of them, i.e., to monitor the occurrence of them over large areas over extended time periods, and set this into relation with ongoing global change.