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

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Presentation on theme: "Huw C. Davies & Mischa Croci-Maspoli Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland & MeteoSwiss, Zurich, Switzerland A Characterization."— Presentation transcript:

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

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 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

4 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

5 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.

6 Some Salient Features II: Temporal 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)

7 (A) Lens Climatology Comparable ! Credibility of Characterization II: Temporal Features (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%

8 Quasi-stationarity Essence of a Block

9 Questions III: Dynamics Questions prompted by Lens characterization of a Block: (A) Origin of the Lens (i.e. the negative PV anomaly) ? (B) Dynamics of systems structural resilience ? (C) Dynamics of the systems quasi-stationary ? (D) Establishment of the overall PV pattern ? (- i.e. of the lens plus contiguous features)

10 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 III: Dynamics 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 Verifiying ECMWF 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 temperature TWO INFERENCES -Block formation sensitive to upstream surface conditions, -THE ULTIMATE TEST of a models cloud dynamics and microphysics is the delivery of correct PV distribution aloft.

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

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

14 IMPLICATION: STATIONARITY requires a richer anomalous PV pattern North High PV Low PV High PV Low PV (C) Quasi-stationarity III: Dynamics Consider the typical instantaneous PV distribution on an isentropic surface crossing the tropopause. - isolated LENS does not suffice

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

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

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

18 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 III: Dynamics High PV Low PV TYPE C TYPE A

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

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

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

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

23 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 IV: Related Phenomena

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

25 Sudden Stratospheric Warming & BLOCKS IV: Related Phenomena

26 Evolution of mean zonal wind at 60 0 N between 1000 and 0.1 hPa Blocks rule OK !! ? SSW rules OK !! ? Sudden Stratospheric Warmings & BLOCKS IV: Related Phenomena A. Scaife

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

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

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


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