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