Mesoscale Atmospheric Systems - Introduction Spring Semester 2022 Heini Wernli
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Mesoscale Atmospheric Systems
Introduction
Spring Semester 2022
Heini Wernli
Outline of lecture course see also www.iac.ethz.ch/edu/courses/master/modules/mesoscale-atmospheric-systems.html 22.02.2022 Introduction; Precipitation measurements 01.03.2022 Extreme precipitation events 08.03.2022 Water vapour transport 15.03.2022 Radar I (U. Germann) 22.03.2022 Radar II (U. Germann) 29.03.2022 Surface fronts 05.04.2022 Upper-level fronts 12.04.2022 Frontal instability 19.04.2012 Easter vacation - no lecture 26.04.2022 Stratosphere-troposphere exchange 03.05.2022 Ocean evaporation 10.05.2022 Stable water isotopes 17.05.2022 Moist convection (S. Schemm) 24.05.2022 Gravity waves (S. Schemm) 31.05.2022 … still open … 22 Feb 2022 H. Wernli 2
Prerequisites Large-scale atmospheric dynamics (Q-vectors, PV concept, baroclinic instability) Atmospheric physics (thermodynamics) These topics are essential for this lecture course! This is an MSc course that covers a very broad spectrum of topics à please actively fill your gaps if they exist 22 Feb 2022 H. Wernli 3
The examination Oral exam in “Prüfungssession” 30 Min. with Heini Wernli and Sebastian Schemm Essential: • basic physical understanding • interpretation of diagrams (and equations) • precise scientific language • not so much about “learning things by heart” 22 Feb 2022 H. Wernli 4
Introduction
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?
22 Feb 2022 H. Wernli 5
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. 22 Feb 2022 H. Wernli 6
A Preliminary Definition of Mesoscale Systems 22 Feb 2022 H. Wernli 7
Internal Thermal Transition Zones
à “fronts” occur on different scales
Sub-synoptic tropopause-fold
Meso-a scale elongated cold front
Meso-b scale sea-breeze front
Meso-g scale gust front
22 Feb 2022 H. Wernli 8
Destructive wind systems
à also occur on different scales
Sub-synoptic
windstorm “Lothar”
Meso-b scale hurricane annular
wind maximum
Meso-g scale thunderstorm gust front
22 Feb 2022 H. Wernli 9
Mesoscale systems are complex 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 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 22 Feb 2022 H. Wernli 10
Mesoscale systems are embedded within larger-scale flow;
they contain distinctive sub-structures
Easterly 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
22 Feb 2022 H. Wernli 11Why are there mesoscale systems?
The range of conceivable causal mechanisms includes:
(i) External mesoscale forcing (e.g., orographic effects)
(ii) Scale contraction due to larger-scale internal forcing (e.g.,
frontogenesis à see lectures about surface fronts)
(iii) Generation 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)
(iv) Intrinsic mesoscale instability (e.g., frontal waves & frontal
cloud bands)
(v) Synergetic interaction of sub-mesoscale systems (e.g.,
self-organization of convective clouds à tropical cyclone)
22 Feb 2022 H. Wernli 12External mesoscale forcing: Orographic effects 22 Feb 2022 H. Wernli 13
Generation of new system
External forcing (Alps)
+ pre-existing system (front)
produce lee cyclone …
… and heavy precipitation
22 Feb 2022 H. Wernli 14Intrinsic mesoscale instability: Break-up of a stratospheric PV streamer Examples show WV satellite images 22 Feb 2022 H. Wernli 15
How are mesoscale systems observed?
Requisite : mesoscale space-time resolved observations
Available : ROUTINE
- conventional synoptic surface & free atmosphere networks
- some mesoscale surface observational networks
- zoo of radar & satellite measurements
SPECIAL
- suites of specialized instrumentation for deployment
in targeted field programmes
22 Feb 2022 H. Wernli 16Routine radiosonde network over Europe (black and red circles) 22 Feb 2022 H. Wernli 17
Swiss automatic network of surface measurements http://www.meteoswiss.admin.ch/home/measurement-and-forecasting-systems/land-based-stations/automatisches-messnetz.html 22 Feb 2022 H. Wernli 18
Alpine rain-gauge network
Frei and Schär 1998 (IJC)
22 Feb 2022 H. Wernli 19Field 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 setup
for MAP Experiment in 1999
22 Feb 2022 H. Wernli 20Are
Aremesoscale
mesoscalesystems
systemsimportant?
important?
August 2002 - Elbe Economic losses
€ 16 billion
Insured losses
€ 3.4 billion
August 2005 – Alps
THE most expensive Economic losses
Swiss natural CHF 2.1 billion
catastrophe
Insured losses
source: AP
CHF 1.5 billion
28
22 Feb 2022 H. Wernli 21Are mesoscale systems important?
Contribution of hazard events to major natural disasters 1950-2005
Number of events: 276 Deaths: 7%
1.75 Million
Geological events
Earthquake/tsun
6% 7% 2%
25% ami, volcanic
eruption
25 29 Weather related events
% % Storm 36%
Floods
55%
Extreme
40 temperatures
%
Economic losses: 1,700 bn. US$*
Insured losses: 340 bn. US$*
6% 5%
5% 11
31 %
25
25% %
%
38% 79%79%
*2005 values
© 2006 Geo Risks Research, Munich Re
22 Feb 2022 H. Wernli 2222 Feb 2022 H. Wernli 23
Topics
Topic overview
STE Fronts
Convection
Evaporation (Extreme)
precipitation
Fronts
Radar
Moisture transport
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22 Feb 2022 H. Wernli 24You can also read
