Key parameters
MSLP
The polar low is usually seen as a closed low of some 5 to 20 hPa lower center pressure than the surroundings. In some cases, if the large scale gradient is strong, the low may only be an open trough in the MSLP. The cold air outbreak is also best seen in the MSLP, as it gives information on the origin and the flow of the air mass. For the air mass to be sufficiently cooled, it needs to spend some time over cold surface, hence a rather long trajectory above the cold surface is needed.
T500 and sea surface temperature (SST)
The deep convection associated with a polar low is to a large extent fuelled by the heat absorbed from the underlying sea surface, which is directly dependent on difference between the air temperature at the surface and the SST and maintained at a high level where the air is advected over areas with a steep positive gradient in the SST. Polar lows tend to form when the static instability reaches above 500 hPa, i.e in areas with relatively low values of T500. The difference between the SST and the T500 has proven to be a crude but most effective indicator for deep static instability over sea surface, and corresponds well with areas with either polar lows or deep convection. When taken vertically within a gridpoint, 40°C is a good threshold value for the triggering of Polar Low development. Historically, when the forecasted location of the cold cores were less precise, the maximum value within the cold air outbreak as a whole was used, and a value of 43 to 44°C is frequently referred to as a threshold value. Studies at MET-Norway have shown that polar lows form in about 25% of the times when this threshold is exceeded.
Z500 and positive relative vorticity advection (PVA)
An upper trough, often seen in the Z500 is often superimposed over the area of an initial Polar Low. In direct shear situations the trough is slightly behind the developing low. In a reversed shear case the upper low is slightly ahead. Statistical analysis shows the highest value of PVA normally occurs in the developing phase and gradually decreases in the following phases. Finally in the decaying phase the value is almost zero or may even be negative. Generally, deep convection tends to develop over several hours or days as it absorbs heat from the underlying rather cool surface. It is therefore more efficient for polar low formation with a shallow upper trough that is slow moving and has low values of PVA than a fast moving trough with high PVA. The latter case only acts over a brief time, and will therefore only give a marginal enhancement of the instability in the air column.
10m wind
A part of the definition of polar lows is a requirement of 10m wind of 'near gale' Beaufort 7 (> 13,8 m/s) or higher. This is a weak criterion since many similar phenomena give higher winds than this, so near gale force winds does not necessarily make a polar low. Nevertheless the 10m wind is an important aspect of the assessment of polar lows. The strongest winds are normally to be found at the position where the relative motion of the polar low is in the same direction as the actual wind, in other words on the western side of the low.
Cloudiness or 1h precipitation
The convection in a polar low is oriented as cloud bands, spiralling out from the center in a typical cyclone shape, seen in both satellite imagery and radar. The precipitation produced by the convection shows up similarly and gives an easily recognizable pattern in prognostic fields for e.g. 1hr precipitation.
Equivalent potential temperature θe
In most cases a Polar Low develops on a secondary shallow baroclinic zone in a marine cold air outbreak. This zone can be best visualised using the equivalent potential temperature θe contours at 850 hPa, showing the fronts and the gradients between warm and humid or cold and dry air masses.
Vertical Motion (Omega) 850 hPa
Because of unstable conditions and upper level forcing mechanisms, vigorous ascending motion could develop. As a result of a low tropopause, the ascending motion is present in a relatively shallow layer, making omega at 850 hPa more indicative than omega at 500 hPa.
During the development phase the enhanced cloud band is characterized by a negative omega with a relative minimum at the position of the developing Polar Low. This minimum expands and finally, in the mature stage, a cut-off minimum is present surrounded by a band of descending air with positive omega values.
Other indexes
The K-index is mostly used for summer convection, but can also be used for winter conditions, but then with slightly lower values, typically 15 to 20.
The Showalter Index (SI) and the Lifted Index (LI) take the difference between T500 and the parcel temperature at 850 and 925 hPa respectively. The Total-Totals (TTI) is similar to the SI, but considers also moisture at 850 hPa: TTI = (T850-T500)+(Td850-T500). However none of these consider the important aspect of transfer of heat from the sea surface as indicated by the SST, and are generally less used than the SST-T500 described above in the context of polar lows.
The Boyden index is given by BI = DZ(1000-700) + Td850 - 200, where the first part is the thickness below 700 hPa and the last part is a scaling to give the BI values between 90 and 100. The Boyden index is useful for considering convection, but it gives no information about conditions above 700 hPa. Hence it tends to give a rather flat field in areas with deep convection where the tops are typically well above this level.