The case of polar lows
Hans von Storch13 and Matthias Zahn2
1. Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Germany.
2. Environmental Systems Science Center, University of Reading, Reading, UK
3. KlimaCampus, University of Hamburg, Hamburg, Germany.
Polar lows
intense/ strong winds (>13.9 m/s)
severe weather
●
Spitzbergen
mesoscale (< 1000 km) sized
maritime storms
occur poleward the Polar Fronts in
both hemispheres during winter
●
typically induced by disturbances in
the air flow
●
Here: only Northern North Atlantic
Scand
in
typically driven by convective
processes
●
~300 km
avia
●
Long-term climatology
and future perspectives
For the recent past
• Dataset of polar low cases
• Comprehensive measurements required
• long in time
• high in spatial detail
• Homogeneous
• Problem: Such analyses do usually no exist
Solution:
Use of a numerical model (Regional Climate Model,
RCM) in combination with existing global data to
reconstruct past/project future state of the atmosphere
dynamical
downscaling
Global data
(~200 km)
RCM
(~50 km)
Set-up of multi-decadal simulation
CEP/NCAR reanalysis 1/ CLM 2.4.6
nitialised: 1.1.1948 finishing: 28.2.2006
Dec 1993 case
Greenland
Iceland
NCEP
CLM22-nn
Dundee
Weatherchart, DWD
CLM22-sn
9.12.93, 16:00
CLM22-sn, filtered
Jan 1998 case
NCEP
CLM01-nn
Berliner
Weatherchart
CLM01-sn
Dundee
18.1.98, 4:00
CLM01-sn, filtered
Polar low simulation and detection
• In principle, polar lows are reproduced with CLM run in climate
mode
• Deviations in detail (e.g. location and amount of pressure minima)
• Spectral nudging inhibits considerable ensemble variability
• A digital filter could be useful for an automatic detection
• Zahn, M., H. von Storch, and S. Bakan, 2008: Climate mode simulation
of North Atlantic Polar Lows in a limited area model, Tellus A, DOI:
10.1111/j.1600-0870.2008.00330.x
Detection algorithm
1st : detection of minima in the filtered mslp fields (< -1hPa)
2nd : combine detected positions to individual tracks, distance to next (3h)
position < 200 km
3rd : Either checking further constraints along the tracks:
• strength of the minimum ( ≤ −2hPa once along the track)
• wind speed ( ≥ 13.9 m/s once along the track)
• air-sea temperature difference ( SST − T500hPa ≥ 43K)
• north south direction of the track
• limits to allowable adjacent grid boxes
or: strength of the minimum in the bandpass filtered mslp field
decreases below −6hPa once
Annual frequency of past polar lows
PLS: Polar Low Season (Jul-Jun)
Zahn and von Storch, 2008,
Annual frequency of past polar lows
Max:100
Mean:56
σ=13
Min:36
PLS: Polar Low Season (Jul-Jun)
Zahn and von Storch, 2008,
Downscaling vs. “obs”
C=0,72
Monthly comparison of NCEPdownscaling (in black) with analysed
observed data (in red; Blechschmidt,
2008)
NCEP-based downscaling
(black) and observations (red)
of MetNo (Noer, pers comm)
Density of polar low genesis
Genesis in NCEP downscaling RCM simulation
Bracegirdle, T. J. and S. L. Gray, 2008
Past occurrence – summary
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Northern North Atlantic
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Polar Lows are simulated by 50 km grid resolution model (not shown)
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Strong inter annual variability
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Frequency remains on a similar level – no systematic trend
Qualitative similarity with observations in terms of inter-annual, intraannual variations and spatial distribution of genesis / other studies
●
Zahn, M., and H. von Storch, 2008:
A longterm climatology of North Atlantic Polar Lows.
Geophys. Res. Lett., 35, L22702, doi:10.1029/2008GL035769
●
Polar lows in
IPCC-climate change scenarios
Global scenario data generated by IPCC-ECHAM5-MPI-OM in C20experiments: (“control” with GHG 1960-1990) and B1,A1B,A2scenarios for period 2070-2100.
Dynamically downscaled using CLM.
Number of polar lows per PLS
Zahn and von Storch, 2010
Spatial density distribution:
northward shift of genesis region
C20, mean lat = 64,9°N
B1, mean lat = 66,8°N
A1B, mean lat = 66,8°N
A2, mean lat = 67,3°N
Projected changes in polar low frequency
and vertical atmospheric stability
Differences of the area
and time-averaged icefree SST and T500-hPa
over the maritime
northern North Atlantic
as proxy for frequency
of favourable polar low
conditions
(CMIP3/IPCC AR4)
C20
B1
A1B
A2 Zahn and von Storch, 2010
Projected occurrence
– summary
Polar lows become less frequent in the Northern North Atlantic as
supported by
●
regional modelling (downscaling) control and scenario simulations
with one GCM, and by
●
analysis of vertical stability in a large set of (CMPI3) global climate
simulations; all simulations show a increase in projected stability.
●
●
The genesis regions shift northward.
Zahn, M., and H. von Storch, 2010: Decreased frequency of North
Atlantic polar lows associated to future climate warming, nature 467,
309-312
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