Eine Langzeitklimatologie
nordatlantischer Polar Lows
Matthias Zahn
Arbeit im SFB 512 seit Mai 2009
basierend auf Doktorarbeit (EXTROP)
betreut durch
Hans v.Storch und Stephan Bakan
GKSS Research Centre, Institute for Coastal Research Germany
2
Spi
tzber
gen
Polar Lows (mesoscale cyclones)
mesoscale (< 1000 km) sized
maritime storms
typically driven by convective
processes
din
typically induced by disturbances
in the air flow
~300 km
Sca
n
occur poleward the Polar Fronts in
both hemispheres during winter
avia
intense/ strong winds (>13.9 m/s)
severe weather
© Dundee Satellite Receiving Station
3
Further examples of Polar Lows
Further reading:
Rasmussen and Turner, 2003:
Polar Lows: Mesoscale Weather Systems
in the Polar Regions
20.12.2002, 2:00
04.03.2008, 11:35
11.03.08, 15:25
16.1.1995, 9:00
IPY-Thorpex field campaign:http://www.ipy-thorpex.com/ , images from http://www.sat.dundee.ac.uk/
Kolstad, E. W. & T. J. Bracegirdle & I. A. Seierstad: Marine cold-air outbreaks in the North Atlantic: temporal distribution and associations with
large-scale atmospheric circulation. Climate Dynamics, published online 19 June, 2008. DOI:10.1007/s00382-008-0431-5
4
Long-term climatology
• Dataset of Polar Low cases
• Comprehensive measurements are
required to compile such a dataset
• long in time
• high in spatial detail
• Homogeneous
• Problem: Such measurements do
usually no exist
5
Solution
Use of numerical models in
combination with existing
measurements to reconstruct
past state of the atmosphere:
Reanalysis
(~200 km)
=> “dynamical downscaling“
=> detection procedure
applied to output fields
RCM
(~50 km)
6
Setup of my PhD work
●
●
●
Part 1: Can RCMs reproduce polar lows ?
Part 2: How to detect polar lows
automatically in RCM output data
Part 3: Compilation of polar low climatology
for past 60 years
+ Part 4: Projection of polar low activity in
future
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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8
Oct. 1993 case
Sca
ndi
nav
ia
Spi
tzbe r
ge n
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
9
P1
Mean Sea level pressure (hPa) and
10m wind speed
10
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
11
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
e.g.: DWD 15. 10. 1993, 6:00 Uhr
P1
13
Bandpass filtered mslp (hPa)
15 Oct. 1993,
6:00
NCEP
NCEP
DWD
DWD
Scales
between
200-600 km
are retained
CLM01nn
CLM01nn
CLM02nn
CLM03nn
CLM04nn
CLM01sn
CLM02sn
CLM03sn
CLM04sn
P1
14
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
15
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
16
Intermediate results
• In principle, Polar Lows can be reproduced
with CLM run in climate mode
• Though, there may be deviations in location
and amount of pressure minima
• Without nudging the large scale, the formation
of Polar Lows is subject to 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 all locations with a minimum in
the filtered mslp field < -1hPa
P2
18
Setup of the detection algorithm
st
1 : detection of all locations with a minimum in the
filtered mslp field < -1hPa
2nd : combine detected positions to individual
tracks, distance to next (3h) pos < ~200 km
P2
19
Setup of the detection algorithm
st
1 : detection of all locations with a minimum in the
filtered mslp field < -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
20
Set-up of long-term simulation:
CLM 2.4.6
initialised: 1.1.1948
finished : 28.2.2006
driven by NCEP/NCAR reanalysis 1
spectral nudging of scales > 700 km
together with the algorithm enables a long-term
climatology of Polar Lows
P3
21
Tracks of three Polar Lows
Reproduced and detected even
after a simulation time of 40 years
P3
22
Frequency of Polar Lows
Zahn and v.Storch,
2008b, GRL
P3
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Frequency of Polar Lows
Zahn and v.Storch,
2008b, GRL
Mean:56
Min:36
Max:100
σ=13
P3
24
Sensitivität des Algorithmus
c > 0,9
Number of Polar Lows per „Polar Low Saison“, sensitivity
to changing wind speed criterion >10,2| >13,9 | >17,2 m/s
P3
25
Climatological comparison
Number of polar lows per PLS. Zahn and v.Storch (2008b) (black)
and observations (red) of MetNo (Noer, pers.comm)
P3
26
Climatological comparison
Number of polar lows per PLS. Zahn and v.Storch (2008b) (black)
and observations (red) of Wilhelmsen (1985)
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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
28
Density of Polar Low genesis
Bracegirdle, T. J. and S. L. Gray, 2006
Number of Polar Lows in various
subregions
Subregions, for which the number of detected polar lows were counted (R1-R14).
29
P3
Canonical Correlation Analyis
(CCA)
Method to study the correlation between two
(or more) random vectors/ multidimensional
sets of variables, e.g. X and Y
Here:
X: number of Polar Lows per PLS in various
subregions
Y: gridded mean MSLP fields per PLS
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P3
31
Relation to large scale flow pattern
1. Muster, c=0,6
Southward mean flow: more polar lows and vice versa
P3
32
Intermediate results
Strong inter annual variability of polar low
occurrences
Polar low frequency remains on a similar
level
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: for projected period 2070-2100
33
P4
Projected cumulative frequency of
Polar Lows in IPCC-scenarios
34
P4
Polar Lows and projected vertical
stability
35
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
36
P4
37
Final Results
Hindcasts:
Strong inter annual variability of polar low
occurrences
Number of polar lows remains on a similar
level
Scenarios:
Under climate warming conditions, polar lows
become less frequent
Their mean origin region shifts farther north
P5
38
Outlook
●
Joint work with higher resolved data (ERAint)
K. Hodges (Reading, UK)
●
●
Application of the approach
(downscaling/detection) to the North Pacific
(GKSS, C. Fei)
Investigation of the tracking algorithm
(GKSS, L. Xia)
[ Use the CCA results to assess Polar Low behaviour on timescales beyond NCEP/NCAR (MSLP fields of Trenberth) ]
P5
Vielen Dank fuer Ihre
Aufmerksamkeit
Homepage: http://coast.gkss.de/staff/zahn/
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Two dimensional bandpass filter
isotropic filters are able
to separate large,
medium and small
spatial scales in a
limited (regional)
gridded field.
Feser, F., and H. von Storch, 2005: Spatial two-dimensional discrete filters for limited area model evaluation purposes. Mon. Wea Rev. 133,
1774-1786
41
Tracks of three cases
Oct
Oct 1993
1993
Dec
Dec 1993
1993
Jan
Jan 1998
1998
Tracks reproduced
and detected even
after a simulation time
of several decades
42
Algorithm's sensitivity
To varied
ws criteria
To varied
dtz criteria
C > 0.9