Detecting polar lows and polar mesocyclones using microwave satellite sensors

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.


Left: MODIS/Aqua visible image, 15 March, 2005, 1103 UTC showing polar mesocyclones at 64°N, 4°E, and at 73°N, 8°E. Right: Concurrent brightness temperature difference map, TB(183±1)-TB(183±7), showing signatures of the same two PMCs: small red areas surrounded by blue, marked by the arrows. The large red areas are caused by very dry cold air over land (centre of map: Northern Scandinavia, southern edge of map, 7°–10°E: Southern Norway) and sea ice (northeastern corner of map).
PMC image

Remote sensing of polar mesocyclones and polar lows at the University of Bremen

The group has developed a method for detecting PMCs and PLs from data of the microwave humidity sounders AMSU-B and MHS on operational meteorological satellites since 1999 (Melsheimer et al., 2016) – PMCs are well visible in channel difference maps as small isolated areas of reversed sign (see Figure). 

Inventories of  PMCs over the Norwegian Sea (complete year 2004 and selected months in 2000–2009) compiled with the methods compared well with existing inventories of the area: Almost all polar lows in those inventories were found by the method (details see Melsheimer et al., 2016).

Currently we are working on refining the detection method and making it work semi-automaticcally. Further work is oriented towards detection of PMCs and PLs in the Southern hemisphere.