Estimation of past and future
polar low frequency over the
North Atlantic
Matthias Zahn
ESSC-University of Reading
Based on my PhD work at
GKSS research centre and University of Hamburg
Supervised by
Hans von Storch and Stephan Bakan
2
Intro
intense/ strong winds (>13.9 m/s)
severe weather
typically induced by disturbances
in the air flow
~300 km
Sca
n
occur poleward the Polar Fronts in
both hemispheres during winter
din
mesoscale (< 1000 km) sized
maritime storms
avia
Spi
t zber
ge n
Polar lows (mesoscale cyclones)
typically driven by convective
processes
© Dundee Satellite Receiving Station
3
Intro
Long-term climatology
• Dataset of polar low cases
• Comprehensive measurements
required
• long in time
• high in spatial detail
• Homogeneous
• Problem: Such measurements do
usually no exist
4
Intro
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)
Intro
Setup of my PhD work
Part 1: Polar lows in RCMs
Part 2: Detect polar lows automatically
Part 3: Compilation of past polar low climatology
in addition to my PhD work:
Part 4: Projection of future polar low activity
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P1
Ensemble simulations for three
polar low cases in climate mode
NCEP (~200 km)
CLM (~50 km)
Oct. 1993 (Dec. 1993, Jan. 1998)
Initialised: approx. 2 week prior to PL formation
With spectral Nudging (4x) and without (4x)
(v. Storch et al. 2000)
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7
Oct. 1993 case
Sca
ndi
nav
ia
Spi
t zbe r
gen
P1
15.Oct.93, 05:24
© Dundee Satellite Receiving Station
P1
Mean Sea level pressure (hPa) and
10m wind speed
15. Oct. 1993,
6:00
Dundee
15.10.93, 05:24
NCEP
DWD
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P1
Mean Sea level pressure (hPa) and
10m wind speed
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15. Oct. 1993,
6:00
NCEP
CLM01nn
CLM02nn
DWD
CLM03nn
Dundee
15.10.93, 05:24
CLM04nn
P1
Mean Sea level pressure (hPa) and
10m wind speed
10
15. Oct. 1993,
6:00
NCEP
DWD
Dundee
15.10.93, 05:24
CLM01nn
CLM02nn
CLM03nn
CLM04nn
CLM01sn
CLM02sn
CLM03sn
CLM04sn
P1
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2-d digital bandpass filter
Digital
bandpassfilters are
able to
separate
large,
medium and
small scales
in gridded
RCM output
fields
(Feser and
v. Storch,
Example from DWD 15. 10. 1993, 6:00 am
P1
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Bandpass filtered mslp (hPa)
15 Oct. 1993,
6:00
NCEP
DWD
Scales
between
200-600 km
are retained
CLM01nn
CLM02nn
CLM03nn
CLM04nn
CLM01sn
CLM02sn
CLM03sn
CLM04sn
P1
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Dec 1993 case
9 Dec 1993, 0:00
Greenland
Iceland
NCEP
CLM22-nn
Dundee
Weatherchart, DWD
CLM22-sn
9.12.93, 16:00
CLM22-sn, filtered
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Jan 1998 case
18 Jan 1998, 0:00
Berliner
NCEP Weatherchart
Dundee
18.1.98, 4:00
Sc
an
din
av
ia
P1
CLM01-nn
CLM01-sn
CLM01-sn, filtered
P1
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Intermediate results
• In principle, polar lows can be 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 et al, 2008, Tellus
P2
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Setup of the detection algorithm
st
1 : detection of minima in the filtered mslp fields
(< -1hPa)
P2
st
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Setup of the detection algorithm
1 : detection of minima in the filtered mslp fields
(< -1hPa)
2nd : combine detected positions to individual
tracks, distance to next (3h) pos < ~200 km
P2
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Setup of the detection algorithm
st
1 : detection of minima in the filtered mslp fields
(< -1hPa)
2nd : combine detected positions to individual tracks,
distance to next (3h) pos < ~200 km
3rd : 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
Zahn and v. Storch, 2008a, MetZeit
P3
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Set-up of long-term simulation:
NCEP/NCAR reanalysis 1/ CLM 2.4.6
Initialised: 1.1.1948 finishing: 28.2.2006
spectral nudging of scales > 700 km
P3
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Tracks of three Polar Lows
Reproduced and detected even
after a simulation time of 40 years
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Frequency of polar lows
Zahn and v.Storch,
2008b, GRL
PLS: Polar Low Season
(Jul-Jun)
P3
P3
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Frequency of polar lows
Zahn and v.Storch,
2008b, GRL
Mean:56
Min:36
Max:100
σ=13
P3
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Sensitivity of the algorithm
c > 0,9
Number of Polar Lows per „Polar Low Season“, sensitivity
to changing wind speed criterion >10,2| >13,9 | >17,2 m/s
P3
24
Number of polar lows per PLS
Zahn and v.Storch (2008b) (black) and
Zahn and v.Storch (2008b) (black) and
observations (red) of Wilhelmsen (1985) observations (red) of MetNo (Noer, pers
comm)
Similar courses of polar low frequency
PLS: Polar Low Season (Jul-Jun)
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P3
C=0,58
C=0,72
Monthly comparison of
Zahn and v.Storch
(2008b) (in black) with
observed data (in red)
u.r.: Norwegian Met.
Service
l.l.: Blechschmidt (2008)
P3
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Density of polar low genesis
Similar spatial characteristics
Bracegirdle, T. J. and S. L. Gray, 2008
P3
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Intermediate results
Strong inter annual variability
Frequency remains on a similar level
Qualitative similarity with observations/
other studies
Zahn and v.Storch,
2008b, GRL
P4
Polar lows in IPCC-climate change
scenarios
Global data delivered by
IPCC-ECHAM5-MPI-OM experiments:
C20: Control with GHG 1960-1990
- B1,A1B,A2: projected for period 2070-2100
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P4
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Number of polar lows per PLS
Zahn and v.Storch,
2010, Nature
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Evolution of SST and T500
Measure of vertical
stability:
Vdt = SST - T500
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P4
Projected changes in polar low frequency
and vertical atmospheric stability
C20
B1
A1B
A2
Zahn and v.Storch,
2010, Nature
P4
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
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P4
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Final Results
Hindcasts:
Strong inter annual variability
Frequency remained on a similar level
In climate warming scenarios:
Polar lows become less frequent
Their mean origin regions shift farther north
Atmosphere becomes more stable
Zahn and v.Storch,
2010, Nature
P5
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Outlook
●
Two PhD theses are under way at GKSS
drawing my work further:
Fei Chen: North Pacific
– Lan Xia: comparing my algorithm
–
●
Myself: Post-Doc (supervised by Richard Allan
and Lennart Bengtsson) on hydrological cycle:
PREPARE project: Projected Response of
Extreme Precipitation and Atmospheric
Radiative Energy
P6
Thanks a lot for
your attention
Homepage: http://www.nerc-essc.ac.uk/~maz/
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