Climatology of polar lows in the Nordic and Barents seas over 1995-2008 based on satellite data

Polar lows and their general characteristics

Polar lows are short-living intense mesoscale maritime

atmospheric low pressure weather systems, observed

over high latitudes, both in Arctic and Antarctic, during

wintertime

Short lifetime: from several hours to 2 days (average

15÷20 hours)

Small size: 100÷1000 km

High surface wind speed: > 15 m/s (some time > 30 m/s)

Typically marine phenomenon: polar lows rapidly break

down over land and ice cover

Areas of occurrence of polar lowsin the Arctic

Labrador sea

Arctic polar lows are significantly more intensive than Antarctic ones due to large fluxes of heat and moisture

Most intensive Arctic polar lows are called “Arctic hurricanes”

Nordic and Barents seas is one of the main genesis areas for polar lows in the Arctic

Existing polar low climatologies

Year Authors and Paper Period Type of data Area Method

2011 G. Noer, Ø. Saetra, T. Lien, Y. Gusdal (2011). A climatological study of polar lows in the Nordic Seas. Quarterly Journal of the Royal Meteorological Society, 137(660), 1762–1772

2000-2009 Visible imagery Nordic Seas Subjective analysis

2008 Zahn, M., and H. von Storch (2008). A longterm climatology of North Atlantic polar lows. Geophys. Res. Lett., 35, L22702, doi:10.1029/2008GL035769

1948 - 2006 Model: NCEP/NCAR re-analyses data and CLM data

Sub-Arctic region of the North Atlantic

Objective analysis of pressure fields using digital filter

2008 Blechschmidt, A.-M. (2008). A 2-year climatology of polar low events over the Nordic Seas from satellite remote sensing. Geophys. Res. Lett., 35, L09815, doi:10.1029/2008GL033706

2004 2005‑ Combined use of thermal infrared AVHRR imagery and SSM/I derived wind speeds from HOAPS

Nordic Seas Subjective analysis

2008 Thomas J. Bracegirdle*, Suzanne L. Gray. (2008). An objective climatology of the dynamical forcing of polar lows in the Nordic seas. Int. J. of Climatol., 14(28), 1903-1919

January 2000 to April 2004

Cyclone Database developed by Hewson objectively identified from the UK Met Office global operational model

Norwegian and Barents seas

Objective analysis

2006 Kolstad, E.W. (2006). A new climatology of favorable conditions for reverse-shear polar lows. Tellus, 58A, 344–354

1948 - 2005 Model: ERA-40 reanalysis data

Latitudes over 60°N

Objective analysis

1999 Harold, J.M., Bigg, G.R. and Turner, J. (1999). Mesocyclone activities over the north-east Atlantic. Part 1: vortex distribution and variability. Int. J. Climatol. ,19, 1187–1204

October 1993-September 1995

Infrared AVHRR North-East Atlantic and Nordic Seas

Eye inspection of AVHRR images

1985 Wilhelmsen, K. 1985. Climatological study of gale-producing polar lows near Norway. Tellus, 37A, 451–459

1972-1977 Weather maps Norwegian and Barents seas

Scrutinizing weather maps and documenting mesoscale cyclones with gale-force wind and horizontal size 100-500 km

Shortcomings of existing climatologies

Often polar lows are not detected on the surface weather analysis maps

Mesoscale cyclones/polar lows are under-represented in current reanalysis datasets (Condron et al., 2006):

o Only up to 80 % of cyclones larger than 500 km can be detected in mean sea level (MSL) pressure

o Only up to 40 % of cyclones larger than 250km can be detected in MSL pressure

o Only 20 % of cyclones larger than 100 km can be detected in MSL pressure

Modal size of AVHRR-derived mesoscale cyclones/polar lows is 100-150 km (Harold et al., 1999)

New approach for polar low monitoring and climatology creating

Approach:

o Retrieval of atmospheric columnar water vapor (CWV) fields from satellite passive microwave data (e.g., SSM/I and AMSR-E)

o Revealing vortex structures in these fields

o Identification of these structures with polar lows

o Verification of identity of each revealed vortex structure with polar low using other satellite data (e.g., AVHRR imagery and data on sea surface wind speed derived from SSM/I or radar scatterometers)

o Polar low parameters (life time, size, location, moving speed) estimation and trajectory tracking

31 January 200811:14 UTC

AMSR-E

Advantages:

o independence on day time

o independence on clouds

o regularity and high temporal resolution in polar regions

o existing of 35 + -year record of continuous satellite passive

microwave observations, since 1979 till now and beyond

Retrieved parameters:

o sea surface wind speed

o atmospheric columnar water vapor

o total cloud liquid water content

Bobylev et al., IEEE

TGRS, 2011

NN-based polar regional algorithms for CWV retrieval from SSM/I and AMSR-E

Bobylev et al., IEEE

TGRS, 2010

Based on comparison of SSM/I and AMSR-E retrievals with polar island station radiosonde data from Jan Mayen, Bjornoya and Torshavn (http://weather.uwyo.edu/)

NN-configurations Validation

Accuracy of NN polar regional algorithms: comparison with Wentz global algorithm

Accuracy of polar NN-algorithm is 40% higher than that of Wentz global operational algorithm

Comparison of NN-based polar regional algorithm for SSM/I CWV retrieval with widely used Wentz global operational algorithm based on Torshavn radiosonde measurement data σ = 1.34 kg/m2 σ = 1.90 kg/m2

NN polar regional

algorithm

Kg/m

2

SSM/I1 July 2005, 7:44

Wentz global

operational algorithm

CWV in polar lows might be just

2÷3 kg/m2 higher than in

surrounding areas and polar low

detection thus becomes

impossible if retrieval error is close

to these values.

Polar low detection and tracking by satellite microwave radiometers

Q,

kg

/m2

31 January2:10 UTC

AMSR-E

31 January9:35 UTC

AMSR-E

31 January14:30 UTC

SSM/I

31 January11:14 UTC

AMSR-E

31 January3:50 UTC

AMSR-E

31 January12:47 UTC

SSM/I

Polar low trajectory

31 January7:58 UTC

SSM/I

75N

Polar low over Norwegian Sea on 30-31 January 2008 tracked by SSM/I and AMSR-E

Uncertainties:o number of different definitions of term “polar low” used by various researcherso specific features of detection schemes applied for catching polar lows

Definition of polar lows for our climatology:o mesoscale cyclones with horizontal size ≤ 1,000 km and surface wind speed ≥ 15 m/s

o such climatology might include the following types of polar lows (following classification by Turner and Bracegirdle):

• reverse shear systems• trough systems• boundary layer fronts• cold lows• comma clouds• baroclinic-wave forward shear• orographic polar lows

Creating polar low climatology: definitions

Verification of polar low detection

Polar low in Barents Sea on 19 January 2000

CWV field from SSM/I data Wind speed field from SSM/I data(RSS)

AVHRR thermal infrared imagery

Polar low climatology creation

Portal, SSM/I F13, TB for all channels and all circuits

Columnar water vapor retrieval

with enhanced resolution - 12.5 km

NN-algorithm

Detection of vortex structures in water

vapor fields

Verification of polar low detection

Wind speed > 15 m/s:• 1995-2011 – RSS, SSM/I• 2002-2011 - QuickScat

Cloud structure typical for mesocyclones:NOAA AVHRR

Detected polar low for climatology

Estimation of polar low parameters from

passive microwave and other available

data

Polar low climatology for

Nordic and Barents seas

over 1995-2008Generation of

columnar water vapor fields for

Nordic and Barents seas

Polar low climatology over the Nordic and Barents seas for 1995-2008

Spatial distribution of detected polar lows

Year

Num

ber

of

cycl

ones

Number of polar lows per year (637 total)

Polar low size distribution

Num

ber

of

cycl

ones

Cyclone diameter (km)

Polar low life-time distribution

Num

ber

of

cycl

ones

Cyclone life-time (hr)