climatology of polar lows in the nordic and barents seas over 1995- 2008 based on satellite data
TRANSCRIPT
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)