Meaningful Surface Analysis

Meaningful Surface Analysis

Dr. Charles A. Doswell IIIDoswell Scientific Consulting – Norman, Oklahoma

Lubbock Severe Weather ConferenceLubbock, Texas – 17 February 2010

What does meaningful mean?

• Helps with the diagnosis of the current weather situation

• Critical to a scientific understanding• Forecast = diagnosis + trend• Formal: Q is a forecast quantity

€

Q t = to +δt( ) =Q t = to( )diagnosis

1 2 4 3 4 +∂Q

∂t toδt

trend1 2 3

+K

Surface analysis• The industry standard:

The standard analysis

• The “Norwegian School” Conceptual model of an extratropical cyclone (ETC) – low pressure

• Polar Front model was an essential element– Cold fronts, warm fronts, occluded fronts– Definition of a front: a 1st order discontinuity in

density – The line on a map - on the warm side of a

baroclinic zone (large thermal gradient)

Dropping isotherm analysis …

• [surface temperature] is often neither representative nor conservative. It is not representative because of many local or orographic influences, and it is not conservative on account of the preponderance of nonadiabatic irreversible processes in the air closest to the earth’s surface. Petterssen (1940) – Weather Analysis and Forecasting

Evolution of this concept

• Secondary characteristics– Cyclonic wind shift across a front– A “kink” in the isobars

• Airmass boundaries– Drylines – moisture discontinuities– Outflow boundaries– Land/sea breeze fronts– Others: gravity waves, density currents, etc.

• Weakly baroclinic troughs in the pressure field

Uncertainty in frontal analysis

Uccellini et al. (1992 AMS Bull.)

An ensemble of analyses …

• It may be inevitable … but good or bad?– Standard isobars + fronts– Isotherms and isodrosotherms– Pressure changes

• Consensus is right most of the time – why?– Diversity of plausible ideas is valuable

• Low probability, high-impact event potential

– Analysis and forecasting as a team activity

What tells you the most?

Surface analysis and severe weather

• A simplified version … Ingredients for severe convective weather:

– Deep moist convection … ingredients:• Moisture• Instability (conditionally unstable lapse rates)• Lift

– Supercell-based severe convective weather• Vertical wind shear

– Non-supercellular severe convective weather• ??

} CAPE

Manually-analyzed surface features

• Boundaries– Fronts (baroclinic, by definition)– Drylines (baroclinity is a function of time of day)– Windshifts (weakly baroclinic)– Other … sea breeze fronts, gravity waves, etc.

• Thermal features ( is preferred)• Moisture (r is preferred)• Pressure changes (effects on surface wind)

Diagnosis of features

• Diagnosis – interpretation of data in terms of physical processes

• Bad diagnosis usually bad short-term forecast

• Use of all available information• Any interpretation of the data is necessarily

provisional … the best you can do at the time• Recognition of bad data

Maddox’s Requirements for Making a Short-Term Forecast

1. An accurate diagnosis, including continuous monitoring, of the current situation

2. An extensive physical understanding of the phenomena occurring, including any anticipated developments

Triple points …

Ingredients: Moisture, Instability, Lift, Vertical wind shear

Objective analysis aids

• What’s the purpose of manual analysis?• What do you obtain from machine analysis?

– Many variables of interest aren’t easily produced for hand analysis (divergence, vorticity, etc.)

• Keep the notion of ingredients in mind, though

– Machine analysis is consistent (reproducible)– Fast and requires no personal effort at all

• Something of a 2-edged sword

Case example: 22 May 2008

e versus dewpoint

T/Td versus /r

Indices and parameters

Source: SPC

More indices/parameters

… and what happened …

What can sfc diagnosis do for you?

• Not perfect forecasts!• Keeps you connected to the data• Forces you to think like a meteorologist, not a

technician• Pattern recognition• Ingredients-based thinking

Principles of team forecasting

• Don’t bring your ego to the office!– No one is always right and no one is always wrong– Disagreement is fine, but keep it scientific, not personal– No one succeeds when any one of you fails to do a good

job

• Shift change is a critical time– Listen carefully and ask questions, if necessary– Ever inherited a mess from the previous shift?– Ever leave your relief with a mess?

Surface analysis as a briefing tool

• Surface analysis generally is idiosyncratic• Every few hours, and especially just prior to a

shift change, a “standard” surface analysis should be created– Record of your diagnosis and how it has evolved– A fast way to illustrate your concept of what is

going on– Everyone should agree on its format

Thank you!!

• [email protected]• http://www.flame.org/~cdoswell

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