Mesocyclone activity over the Southern Ocean from satellite infrared mosaics for winter 2004
Number of studies demonstrates overview of mesocyclone activity over the Southern
Hemisphere. These studies are based both on automated tracking algorithms and on manual
identification and tracking and show inconsistent assessments. Due to high time consumption manual tracking studies are commonly regional.
To clarify the Southern Hemisphere mesoscale cyclone activity and to provide a reference
dataset that may be used to validate automated tracking schemes in SH we manually tracked all
mesocyclones over Southern ocean for winter of 2004.
In our study we used hemispherical infrared mosaics from Antarctic Meteorological Research
Center, which covers polar region inside 40th degree with 3-hourly temporal and 5-km spatial resolution.
At each time step we visually analyzed position of all centers of mesocyclones according to their cloudiness imprints in IR images, together with their radius according to Julie Harold, cloud form according to the classification of Andrew Carleton and large scale synoptics. We totally tracked 1735 mesocyclones during 500 hours of manual tracking.
Comma clouds occur more frequently than any other type of cloud forms. They typically occur in troughs of synoptic scale cyclones as surrounding. The ratio between comma and spiral
clouds is 1:11, which is consistent with assessments of Cartleton, 1990.
In terms of the spatial distribution of cyclogenesis, cyclolis and track density of cyclones we observe maximum mesoscale cyclogenesis and track density over Drake Passage and northward of Ross sea.
We found that mean diameter of mesocyclone in
our dataset is 301 km, while other studies report from 300 to 1000.
Mean lifetime of our cyclones is 12 hours, they maximally live more than 2 days: they migrate to
distances up to 2000 with mean 15 m/s propagation velocity. These characteristics of
mesocyclones' lifecycle may be used in the future for effective threshold choice in automated
tracking schemes. Next we validated reanalysis over the Southern hemisphere in reproducing
wind speed characteristics in mesoscale cyclones against QuikSCAT Seawinds scatterometer.
We evaluated maximum wind speed in cyclone (in R+50 km zone around its and center) and
its location relative to the southern direction.
Maximum wind speed locates in cyclones mainly on periphery of vortex not only in
scatterometer, but in reanalysis data too, what means that reanalyses reproduce cyclonic
circulations, however the absolute values of wind speed are underestimated. The closest
agreement with scatterometer data demonstrates MERRA2 reanalysis.
Underestimation of maximum wind speed reaches 10 m/s for ERA Interim and JRA 55, and 5
m/s for NCEP CFSR and MERRA 2. Mean wind speed over the cyclone area is underestimated
from 1 to 5 m/s in NCEP CFSR and MERRA2 and Interim and JRA55 reanalysis accordingly.
Using widely accepted threshold of 15 m/s we evaluated number of mesocyclones which
belongs to polar lows class.
We revealed that the 15 m/s wind speed marks 85 percentile of wind speed in Southern
Hemisphere, while maximum in the cyclone area wind speed 15 m/s corresponds
to 50 percentile of empirical distribution. In this sense polar lows are mostly located over the Bellingshausen sea, Drake Passage and southern part of Indian ocean. They mainly occur westward of Drake Passage and than decay on Tierra del Fuego. The second storm track lies in southern Indian Ocean sector where cyclones are moving eastwardly.
72% of all our mesocyclone tracks fall into the polar low class, what is consistent with other
study. In these areas they form from 40 to 100 % of total meosocyclone population.