Mesoscale Atmospheric Systems

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1 Mesoscale Atmospheric Systems Introduction Spring Semester 2018 Heini Wernli

2 Outline of lecture course See also 20 Feb Introduction & Precipitation measurements 27 Feb Convection 06 Mar Surface fronts 13 Mar Surface / Upper-level fronts 20 Mar Upper-level fronts 27 Mar Frontal instability 03 Apr Easter holidays 10 Apr Surface weather extremes I 17 Apr Surface weather extremes II 24 Apr Ocean evaporation 01 May Labor Day 08 May Stable water isotopes 15 May Radar I (Urs Germann) 22 May Radar II (Urs Germann) 29 May Stratosphere-troposphere exchange (Michael Sprenger) 20 Feb 2018 H. Wernli 2

3 Prerequisites Large-scale atmospheric dynamics (Q-vectors, PV concept, baroclinic instability) Atmospheric physics (thermodynamics) These topics are essential for this lecture course! 20 Feb 2018 H. Wernli 3

4 The examination Oral exam in Prüfungssession 30 Min. with Heini Wernli Essential: basic physical understanding interpretation of diagrams (and equations) precise scientific language not so much about learning things by heart 20 Feb 2018 H. Wernli 4

5 Overarching Questions I: What is the mesoscale? II: Which are mesoscale systems? III: What is distinctive about / What characterizes mesoscale systems? IV: Why are there mesoscale systems? V: How are they observed? VI: Are they important? 20 Feb 2018 H. Wernli 5

6 What is the Mesoscale? TENTATIVE DEFINITIONS First appearance of the term mesoscale : It is anticipated that radar will provide useful information concerning the structure and behavior of that portion of the atmosphere which is not covered by either micro- or synoptic-meteorological studies. We have already observed with radar that precipitation formations which are undoubtedly of significance occur on a scale too gross to be observed from a single station, yet too small to appear even on sectional synoptic charts. Phenomena of this size might well be designated as mesometeorological. Extract from Radar Storm Observations in Compendium of Meteorology (Ligda 1951) Glossary Definition (- in fact, this is a non-definition!) Mesometeorology - that portion of meteorology concerned with the study of atmospheric phenomena on a scale larger than that of micrometeorology, but smaller than the cyclonic scale. 20 Feb 2018 H. Wernli 6

7 A Preliminary Definition of Mesoscale Systems The foregoing is essentially a descriptive definition of mesoscale systems. It prompts two diametrically different sub-questions : SUB-QUESTION I: What systems possess these space-time scales? SUB-QUESTION II: Is there an alternative physical and/or dynamically based definition that would have a broader generality? 20 Feb 2018 H. Wernli 7

8 Characteristic space-time scales of atmospheric phenomena 20 Feb 2018 H. Wernli 8

9 Which are the Mesoscale Systems? Refined sub-division Wide range (see next slides) The challenge 20 Feb 2018 H. Wernli 9

10 Internal Thermal Transition Zones Sub-synoptic tropopause-fold Meso-a scale elongated cold front Meso-b scale sea-breeze front Meso-g scale gust front 20 Feb 2018 H. Wernli 10

11 Destructive wind systems Sub-synoptic wind storm Lothar Meso-b scale hurricane annular wind maximum Meso-g scale thunderstorm gust front 20 Feb 2018 H. Wernli 11

12 What is distinctive about and what characterizes mesoscale systems? The nature of mesoscale systems is complex diverse intermediate 20 Feb 2018 H. Wernli 12

13 Illustration of complexity Consider the formation & evolution of one of the simplest meso-scale systems - the sea-breeze FORCING differential response to solar heating of land & sea, influenced by large-scale setting MANIFESTED in near surface & boundary layer via thermal diffusion (molecular & turbulent) RESULTING IN differential heating, horizontal pressure gradient, surface flow with ascent at interface, possible condensation & cloud activity, inland penetration modified by friction, three-dimensional effects, and subsequent evidence of Coriolis effects 20 Feb 2018 H. Wernli 13

14 Illustration of diversity Frontal palette (cold, warm, occluded,.. & orientation) Nature of their sub-structures (frontal waves, rain bands,...) Event-to-event variations (can differ dramatically) 20 Feb 2018 H. Wernli 14

Illustration of intermediacy: example of easterly waves Mesoscale systems embedded within larger-scale, contain distinctive sub-structures Waves tend to form and propagate on the large-scale

15 Illustration of intermediacy: example of easterly waves Mesoscale systems embedded within larger-scale, contain distinctive sub-structures Waves tend to form and propagate on the large-scale inter-tropical convergence zone System s wavelength ~2000 km Each wave has a meso-b scale domain of convective activity Embedded within the overall domain are meso-g scale regions of enhanced convection 20 Feb 2018 H. Wernli 15

16 Why are there mesoscale systems? The range of conceivable causal mechanisms includes: (i) (ii) (iii) (iv) (v) External mesoscale forcing (e.g., orographic effects) Scale contraction due to larger-scale internal forcing (e.g., frontogenesis) Spawning of a new system by the influence of external forcing upon a pre-existing mesoscale system (e.g., front incident upon orography à lee cyclone) Intrinsic mesoscale instability (e.g., frontal waves & frontal cloud bands) Synergetic interaction of sub-mesoscale systems (e.g., self-organization of convective clouds à tropical cyclone) 20 Feb 2018 H. Wernli 16

17 External mesoscale forcing: Orographic effects 20 Feb 2018 H. Wernli 17

18 Scale contraction due to larger-scale internal forcing: Frontogenesis frontal zone components of deformation field 20 Feb 2018 H. Wernli 18

19 Spawning of new system External forcing (Alps) + pre-existing system (front) produce - lee cyclone - heavy precipitation 20 Feb 2018 H. Wernli 19

20 Intrinsic mesoscale instability: Break-up of a stratospheric PV streamer 20 Feb 2018 H. Wernli 20

21 Synergetic interaction of sub-mesoscale systems Self-organization of convective clouds within easterly wave à hurricane/typhoon? 20 Feb 2018 H. Wernli 21

22 How are mesoscale systems observed? Requisite : mesoscale space-time resolved observations Available : ROUTINE networks - conventional synoptic surface & free atmosphere - some mesoscale surface observational networks - zoo of radar & satellite measurements SPECIAL - suites of specialized instrumentation for deployment in targeted field programmes 20 Feb 2018 H. Wernli 22

23 Routine radio-sonde network over Europe (black and red circles) 20 Feb 2018 H. Wernli 23

24 Swiss automatic network of surface measurements 20 Feb 2018 H. Wernli 24

25 Alpine rain-gauge network 20 Feb 2018 H. Wernli 25

The CHN radar A vertically pointing micro rain radar: https://data.

26 The CHN radar A vertically pointing micro rain radar: Example: 20 Feb 2018 H. Wernli 26

27 Field Programmes Design: Subject to logistic, instrumental and funding challenges Desiderata : - establish well-defined scientific objective(s) - specify precise observational requirements - deploy appropriate tools / facilities. Proposed prototype set-up for MAP Experiment 20 Feb 2018 H. Wernli 27

expensive Swiss natural catastrophe Economic

28 Are mesoscale systems important? August Elbe Economic losses 16 billion Insured losses 3.4 billion August 2005 Alps THE most expensive Swiss natural catastrophe Economic losses CHF 2.1 billion Insured losses source: AP CHF 1.5 billion 20 Feb 2018 H. Wernli 28

29 Are mesoscale systems important? Contribution of hazard events to major natural disasters Number of events: % 25 % 6% 40 % 29 % Geological events Earthquake/tsun ami, volcanic eruption Weather related events Storm Floods Extreme temperatures 7% Deaths: 1.75 Million 7% 2% 36% 55% Economic losses: 1,700 bn. US$* 25% 25 % 6% 38% 31 % 5% 5% Insured losses: 340 bn. US$* 11 % 79% 79% *2005 values 2006 Geo Risks Research, Munich Re 20 Feb 2018 H. Wernli 29

30 20 Feb 2018 H. Wernli 30

31 Topic overview STE Evaporation Fronts (Extreme) precipitation Convection Radar Moisture transport Fronts 20 Feb 2018 H. Wernli 31

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