huw c. davies & mischa croci-maspoli institute for atmospheric and climate science, eth zurich,...
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Huw C. Davies& Mischa Croci-Maspoli
Institute for Atmospheric and Climate Science,
ETH Zurich, Switzerland
& MeteoSwiss, Zurich, Switzerland
A Characterization of
Atmospheric Blocking
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OUTLINE
I Spatial Structure- Basis for the characterization
II Temporal Features - Credibility of the characterization
III Dynamics- Utility of the characterization
.
via consideration of :- block origin & resilience, quasi-stationarity & formation
IV Relationship with other Phenomena
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SLP anomaly & 500hPa pattern
Notable features:.
- surface anticyclone, with- ridge aloft & local easterly flow
- elevated tropopause & jet bifurcation
Latitudinal cross-section of height anomaly
Tropopause
Conventional Perspective I: Spatial Structure
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PV=2
latitude [°N]
Block also evident as :.
- a negative PV anomaly on upper-level isentropes
- anomaly located beneath an elevated tropopause
- contiguous anomalies present at surface and upper-level
An Alternative Characterization I : Spatial Structure
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A BLOCK constitutes .
“a LENS of low PV located beneath an elevated tropopause”.
Essence of Characterization I :Spatial Structure
Develop an "identification and tracking" tool that can catalogue every block (sic. negative PV lens) in terms of its:- amplitude, location, structure, movement and duration.
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Some Salient Features
A Block / PV Lens
(a) occurs in preferred geographical regions,
(b) persists for supra-synoptic time scales, and
(c) during its mature phase does NOT undergo significant :.
- change of shape despite being subject to large-scale deformation
(sic. a structurally resilient system).
- translation despite its location within band of zonal mean westerlies
(sic. a quasi-stationary system)
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(A) Lens Climatology
Comparable !
Credibility of Characterization
(B) Synoptic Simultaneity
TIME(days)
1
2
3
4
5
6
7
8
9
10
T&M P&H
DJF
476 events -> 3.5 per month
13%
10%
5%
1%
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Quasi-stationarity
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Questions
Questions prompted by “Lens” characterization of a Block:
(A) Origin of the ‘Lens’ (i.e. the negative PV anomaly) ?
(B) Dynamics of system’s structural resilience ?
(C) Dynamics of the system’s quasi-stationary ?
• Establishment of the overall PV pattern ? (- i.e. of the lens plus contiguous
features)
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NOTE: Two possible sources for anomalously low PV near tropopause : - advection from low latitudes - convection (- diabatic cross-isentropic flow) from the low
troposphere.
(A) Origin of Lens
ASSESS relative contribution by - examining backward trajectories from the ‘Lens’
Indication that two major sources contribute to the ‘Lens’- tropopause-level air from far-upstream, and- low level moist air-stream ascending after passing over warm SST anomaly
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(A) Origin of Lens
VerifiyingECMWF Analysis Control
Simulation
QUERY :Is the LENS formation influenced by ascent of the coherent moist airstream ?
NUMERICAL EXPERIMENT :Modify nature of airstream by changing the positive upstream anomalies in SST and land surface temperatureTWO INFERENCES
- Block formation sensitive to upstream surface conditions, - THE ULTIMATE TEST of a model’s cloud dynamics and microphysics is the delivery of ‘correct’ PV distribution aloft.
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How does a “PV-Lens” retain its coherent structure ?
(i) PV-lens in a horizontal uniformly sheared flow
(B) Resilience
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(C) Quasi-stationarity
What keeps a PV Lens quasi-stationary ?
(i) PV-lens in a horizontal uniformly sheared westerly flow
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IMPLICATION: STATIONARITY requires a richer anomalous PV pattern
North
High PV
Low PV
High PV
Low PV
(C) Quasi-stationarity
Consider the typical instantaneous PV distribution on an isentropic surfacecrossing the tropopause.
- isolated LENS does not suffice
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An Example of a Block with a di-polar PV configuration
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High PV
Low PV
(C) Quasi-stationary: Schematic of possible
alternative configurations
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(C) Alternative quasi-stationary configurations
An Example of a Block with a tri-polar PV configuration
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High PV
Low PV
High PV
Low PV
High PV
Low PV
High PV
Low PV
(D) Establishment of overall PV-pattern
BREAKING WAVE(s) SCENARIOS
High PV
Low PV
TYPE C TYPE A
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High PV
Low PV
High PV
Low PV
(D) Establishment of overall PV-pattern
BREAKING WAVE(s) SCENARIOS
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(D) Establishment of overall PV-pattern
EXAMPLE OF A BLOCK FORMATION
PVUPV on 320K
Breaking wave (TYPE A)
..
Secluded Lens
Breaking wave (TYPE C)
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(D) Establishment of overall PV-pattern
HOVEMOELLER COMPOSITE (centred on Block)
Meridional Velocity from Day-6 to DAY+6
ATLANTIC PACIFIC
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(D) Establishment of overall PV-pattern
COMPOSITE OF BREAKING WAVES
ATLANTIC
PACIFIC
TYPE A TYPE C
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Forcing PCV Character of Weather Systems
CONVENTIONAL CAUSAL CHAIN
Forcing Weather Systems
PCV
AN ALTERNATIVE CAUSAL CHAIN
Forcing, Patterns of Climate Variability (PCV)
and BLOCKS
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Forcing, Sudden Stratospheric Warmings
and BLOCKS
Troposphere - Stratosphere LinkageBaldwin and Dunkerton 2001
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Sudden Stratospheric Warming & BLOCKS
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Evolution of mean zonal wind at 600N between 1000 and 0.1 hPa
Blocks rule OK !! ?
SSWrules OK !! ?
Sudden Stratospheric Warmings & BLOCKS
A. Scaife
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PCV, the NAO and BLOCKS
r = -0.65
Blocking Frequency
NAO -
Normalized time-traces of the Atlantic Blocking Frequency and the NAO - index for the three winter months
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Evolution of NAO index during a blocking event
The NAO & BLOCKS
total tracks
short tracks (< 10 days) short duration (< 10 days)
long tracks (> 10 days)long duration (> 10 days)
randomrandom
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SOME POSSIBLE INFERENCES
What is a BLOCK ??
Requisite for representation of BLOCKS in models