Huw C. Davies& Mischa Croci-Maspoli

Institute for Atmospheric and Climate Science,

ETH Zurich, Switzerland

& MeteoSwiss, Zurich, Switzerland

A Characterization of

Atmospheric Blocking

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

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

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

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.

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)

(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%

Quasi-stationarity

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)

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

(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.

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

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

(B) Resilience

(C) Quasi-stationarity

What keeps a PV Lens quasi-stationary ?

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

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

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

High PV

Low PV

(C) Quasi-stationary: Schematic of possible

alternative configurations

(C) Alternative quasi-stationary configurations

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

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

High PV

Low PV

High PV

Low PV

(D) Establishment of overall PV-pattern

BREAKING WAVE(s) SCENARIOS

(D) Establishment of overall PV-pattern

EXAMPLE OF A BLOCK FORMATION

PVUPV on 320K

Breaking wave (TYPE A)

..

Secluded Lens

Breaking wave (TYPE C)

(D) Establishment of overall PV-pattern

HOVEMOELLER COMPOSITE (centred on Block)

Meridional Velocity from Day-6 to DAY+6

ATLANTIC PACIFIC

(D) Establishment of overall PV-pattern

COMPOSITE OF BREAKING WAVES

ATLANTIC

PACIFIC

TYPE A TYPE C

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

Forcing, Sudden Stratospheric Warmings

and BLOCKS

Troposphere - Stratosphere LinkageBaldwin and Dunkerton 2001

Sudden Stratospheric Warming & BLOCKS

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

Blocks rule OK !! ?

SSWrules OK !! ?

Sudden Stratospheric Warmings & BLOCKS

A. Scaife

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

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

SOME POSSIBLE INFERENCES

What is a BLOCK ??

Requisite for representation of BLOCKS in models