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A Characterization of Atmospheric Blocking. Huw C. Davies & Mischa Croci-Maspoli Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland & MeteoSwiss, Zurich, Switzerlan d. OUTLINE. I Spatial Structure - Basis for the characterization II Temporal Features

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slide1
A Characterization

of

Atmospheric Blocking

Huw C. Davies

& Mischa Croci-Maspoli

Institute for Atmospheric and Climate Science,

ETH Zurich, Switzerland

& MeteoSwiss, Zurich, Switzerland

slide2
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

slide3
I:

Spatial Structure

Conventional Perspective

  • Notable features:
  • .
  • surface anticyclone, with
  • ridge aloft & local easterly flow
  • elevated tropopause &
  • jet bifurcation

Tropopause

SLP anomaly & 500hPa pattern

Latitudinal cross-section of height anomaly

slide4
I :

Spatial Structure

An Alternative Characterization

  • Block also evident as :
  • .
  • a negative PV anomaly
  • on upper-level isentropes

- anomaly located beneath

an elevated tropopause

PV=2

latitude [°N]

- contiguous anomalies present at surface and upper-level

slide5
I :

Spatial Structure

Essence of Characterization

ABLOCK constitutes

.

“a LENS of low PV located beneath an elevated tropopause”.

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.

slide6
II: Temporal

Features

Some Salient Features

  • A Block / PV Lens
  • occurs in preferred geographical regions,
  • persists for supra-synoptic time scales, and
  • 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)
slide7
II: Temporal

Features

Credibility of Characterization

(B) Synoptic

Simultaneity

1

2

3

4

5

6

7

8

9

10

  • Lens Climatology
  • Comparable !

T&M

P&H

13%

10%

5%

1%

DJF

TIME

(days)

476 events -> 3.5 per month

slide8
Essence of

a Block

Quasi-stationarity

slide9
III: Dynamics

Questions

Questions prompted by “Lens” characterization of a Block:

  • Origin of the ‘Lens’ (i.e. the negative PV anomaly) ?
  • (B) Dynamics of system’s structuralresilience ?
  • (C) Dynamics of the system’s quasi-stationary ?
  • Establishment of the overall PV pattern ?
  • (- i.e. of the lens plus contiguous features)
slide10
III: Dynamics

(A) Origin of Lens

  • NOTE: Two possible sources for anomalously low PV near tropopause :
  • - advection from low latitudes
  • - convection (- diabatic cross-isentropic flow) from the low troposphere.

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

slide11
III: Dynamics

(A) Origin of Lens

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 temperature

TWO 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.

Verifiying

ECMWF Analysis

Control

Simulation

slide12
III: Dynamics

(B) Resilience

How does a “PV-Lens” retain its coherent structure ?

(i) PV-lens in a horizontal uniformly sheared flow

slide13
III: Dynamics

(C) Quasi-stationarity

  • What keeps a PV Lens quasi-stationary ?

(i) PV-lens in a horizontal uniformly sheared westerly flow

slide14
III: Dynamics

(C) Quasi-stationarity

IMPLICATION: STATIONARITY requires a richer anomalous PV pattern

- isolated LENS does not suffice

High PV

Low PV

Consider the typical instantaneous

PV distribution on an isentropic surface

crossing the tropopause.

North

High PV

Low PV

slide15
Dynamics

An Example of a Block with a di-polar PV configuration

slide16
(C) Quasi-stationary: Schematic of possible

alternative configurations

III: Dynamics

High PV

Low PV

slide17
(C) Alternative quasi-stationary configurations

III: Dynamics

An Example of a Block with a tri-polar PV configuration

slide18
III: Dynamics

(D) Establishment of overall PV-pattern

BREAKING WAVE(s) SCENARIOS

  • TYPE C TYPE A

High PV

Low PV

High PV

Low PV

High PV

Low PV

High PV

Low PV

High PV

Low PV

slide19
III: Dynamics

(D) Establishment of overall PV-pattern

BREAKING WAVE(s) SCENARIOS

High PV

Low PV

High PV

Low PV

slide20
III: Dynamics

(D) Establishment of overall PV-pattern

EXAMPLE OF A BLOCK FORMATION

Breaking wave

(TYPE A)

..

Secluded Lens

Breaking wave

(TYPE C)

PVU

PV on 320K

slide21
III: Dynamics

(D) Establishment of overall PV-pattern

HOVEMOELLER COMPOSITE (centred on Block)

Meridional Velocity from Day-6 to DAY+6

  • ATLANTIC PACIFIC
slide22
III: Dynamics

(D) Establishment of overall PV-pattern

COMPOSITE OF BREAKING WAVES

  • ATLANTIC
  • PACIFIC
  • TYPE A
  • TYPE C
slide23
IV: Related

Phenomena

Forcing, Patterns of Climate Variability (PCV)

and BLOCKS

CONVENTIONAL CAUSAL CHAIN

Forcing PCV Character ofWeather Systems

AN ALTERNATIVE CAUSAL CHAIN

Forcing Weather Systems

PCV

slide24
IV: Related

Phenomena

Forcing, Sudden Stratospheric Warmings

and BLOCKS

Troposphere - Stratosphere Linkage

Baldwin and Dunkerton 2001

slide25
IV: Related

Phenomena

Sudden Stratospheric Warming & BLOCKS

slide26
IV: Related

Phenomena

Sudden Stratospheric Warmings & BLOCKS

SSW

rules OK !! ?

A. Scaife

Blocks

rule OK !! ?

Evolution of mean zonal wind at 600N between 1000 and 0.1 hPa

slide27
IV: Related

Phenomena

PCV, the NAO and BLOCKS

Normalized time-traces of

the Atlantic Blocking Frequency and the NAO - index

for the three winter months

Blocking

Frequency

NAO-

r = -0.65

slide28
IV: Related

Phenomena

The NAO & BLOCKS

Evolution of NAO index during a blocking event

total tracks

random

random

short tracks (< 10 days)

short duration (< 10 days)

long tracks (> 10 days)

long duration (> 10 days)

random

random

slide29
SOME POSSIBLE INFERENCES

What is

a

BLOCK ??

Requisite for representation

of

BLOCKS in models

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