1. A Characterization of Atmospheric Blocking Huw C. Davies
& Mischa Croci-Maspoli
Institute for Atmospheric and Climate Science,
ETH Zurich, Switzerland
& MeteoSwiss, Zurich, Switzerland

2. 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

3. Conventional Perspective
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

4. An Alternative Characterization
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

5. Essence of Characterization
Spatial Structure
Essence of Characterization
A BLOCK 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.

6. Some Salient Features II: Temporal 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)

7. Credibility of Characterization
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

8. Essence of
a Block
Quasi-stationarity

9. Questions III: Dynamics
Questions prompted by “Lens” characterization of a Block:
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)

10. (A) Origin of Lens III: Dynamics
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

11. (A) Origin of Lens III: Dynamics TWO INFERENCES
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

12. (B) Resilience III: Dynamics
How does a “PV-Lens” retain its coherent structure ?
(i) PV-lens in a horizontal uniformly sheared flow

13. (C) Quasi-stationarity
III: Dynamics
(C) Quasi-stationarity
What keeps a PV Lens quasi-stationary ?
(i) PV-lens in a horizontal uniformly sheared westerly flow

14. (C) Quasi-stationarity
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

15. Dynamics
An Example of a Block with a di-polar PV configuration

16. (C) Quasi-stationary: Schematic of possible
alternative configurations
III: Dynamics
High PV
Low PV

17. (C) Alternative quasi-stationary configurations
III: Dynamics
An Example of a Block with a tri-polar PV configuration

18. (D) Establishment of overall PV-pattern
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

19. (D) Establishment of overall PV-pattern
III: Dynamics
(D) Establishment of overall PV-pattern
BREAKING WAVE(s) SCENARIOS
High PV
Low PV
High PV
Low PV

20. (D) Establishment of overall PV-pattern
III: Dynamics
(D) Establishment of overall PV-pattern
EXAMPLE OF A BLOCK FORMATION
Breaking wave
(TYPE A) ..
Secluded Lens
(TYPE C)
PV on 320K
PVU

21. (D) Establishment of overall PV-pattern
III: Dynamics
(D) Establishment of overall PV-pattern
HOVEMOELLER COMPOSITE (centred on Block)
Meridional Velocity from Day-6 to DAY+6
ATLANTIC PACIFIC

22. (D) Establishment of overall PV-pattern
III: Dynamics
(D) Establishment of overall PV-pattern
COMPOSITE OF BREAKING WAVES
ATLANTIC
PACIFIC
TYPE A
TYPE C

23. Forcing, Patterns of Climate Variability (PCV) and BLOCKS
IV: Related
Phenomena
Forcing, Patterns of Climate Variability (PCV)
and BLOCKS
CONVENTIONAL CAUSAL CHAIN
Forcing PCV Character of Weather Systems
AN ALTERNATIVE CAUSAL CHAIN
Forcing Weather Systems
PCV

24. Troposphere - Stratosphere Linkage
IV: Related
Phenomena
Forcing, Sudden Stratospheric Warmings
and BLOCKS
Troposphere - Stratosphere Linkage
Baldwin and Dunkerton 2001

25. Sudden Stratospheric Warming & BLOCKS
IV: Related
Phenomena
Sudden Stratospheric Warming & BLOCKS

26. Sudden Stratospheric Warmings & BLOCKS
IV: Related
Phenomena
Sudden Stratospheric Warmings & BLOCKS
SSW
rules OK !! ?
A. Scaife
Blocks
rule OK !! ?
Evolution of mean zonal wind at 600N between and 0.1 hPa

27. PCV, the NAO and BLOCKS IV: Related Phenomena NAO -
Normalized time-traces of
the Atlantic Blocking Frequency and the NAO - index
for the three winter months
Blocking
Frequency
NAO -
r = -0.65

28. The NAO & BLOCKS IV: Related Phenomena
Evolution of NAO index during a blocking event
total tracks
short tracks (< 10 days)
short duration (< 10 days)
random
random
long duration (> 10 days)
long tracks (> 10 days)
random
random

29. SOME POSSIBLE INFERENCES
What is a
BLOCK ??
Requisite for representation of
BLOCKS in models