The case of polar lows Hans von Storch 13 and Matthias Zahn 2 1. Institute of Coastal Research,...
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Transcript of The case of polar lows Hans von Storch 13 and Matthias Zahn 2 1. Institute of Coastal Research,...
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.
© Dundee Satellite Receiving Station
~300 km
Scan
din
avia
Spitz-
bergen
Polar lows
mesoscale (< 1000 km) sized maritime storms
●intense/ strong winds (>13.9 m/s) severe weather
●occur poleward the Polar Fronts in both hemispheres during winter
●typically induced by disturbances in the air flow
●typically driven by convective processes
●Here: only Northern North Atlantic
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
Long-term climatologyand future perspectives
Global data(~200 km)
RCM(~50 km)
dynamical downscaling
Set-up of multi-decadal simulation
NCEP/NCAR reanalysis 1/ CLM 2.4.6
Initialised: 1.1.1948 finishing: 28.2.2006
spectral nudging of scales > 700 km
Dec 1993 case
Weatherchart, DWD
CLM22-snCLM22-nn
NCEP
CLM22-sn, filtered
Dundee9.12.93, 16:00
Iceland
Greenland
Jan 1998 case
CLM01-sn, filtered
Berliner Weatherchart
CLM01-sn
Dundee18.1.98, 4:00
CLM01-nn
NCEP
• 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
Polar low simulation and detection
1st : detection of minima in the filtered mslp fields (< -1hPa)2nd : combine detected positions to individual tracks, distance to next (3h) position < 200 km3rd : Either checking further constraints along the tracks:
Detection algorithm
• 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,
=13σMean:56
Max:100
Min:36
Annual frequency of past polar lows
Zahn and von Storch, 2008,PLS: Polar Low Season (Jul-Jun)
Downscaling vs. “obs”
C=0,72
NCEP-based downscaling (black) and observations (red) of MetNo (Noer, pers comm)Monthly comparison of NCEP-
downscaling (in black) with analysed observed data (in red; Blechschmidt, 2008)
Bracegirdle, T. J. and S. L. Gray, 2008
Density of polar low genesis
Genesis in NCEP downscaling RCM simulation
Past occurrence – summary
● Northern North Atlantic
● Polar Lows are simulated by 50 km grid resolution model (not shown)
● Strong inter annual variability
● Frequency remains on a similar level – no systematic trend
● Qualitative similarity with observations in terms of inter-annual, intra-annual 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 C20-experiments: (“control” with GHG 1960-1990) and B1,A1B,A2-scenarios for period 2070-2100.
Dynamically downscaled using CLM.
Zahn and von Storch, 2010
Number of polar lows per PLS
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
A2
C20
A1BB1
Zahn and von Storch, 2010
Differences of the area and time-averaged ice-free SST and T500-hPa over the maritime northern North Atlantic as proxy for frequency of favourable polar low conditions (CMIP3/IPCC AR4)
● 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
Projected occurrence – summary