On Convective Severe Weather Forecasts. Empirical Relations ...foralps/Other Events/EMS/21 On...Work part-financed by the European Union Community Initiative INTERREG III B Alpine

1

Title and Authors

On Convective Severe Weather Forecasts.

Empirical Relations vs the Theoretical

Approach

by(1)Gallai I., (1,2)Giaiotti D. B., (3,4)Gladich I., (1,2)Stel F.

[email protected]

(1)ARPA FVG - OSMER - Visco (UD) - ITALY

(2)ESSL - Wessling - GERMANY

(3)University of Trieste - Trieste - ITALY

(4)ICTP - Trieste - ITALY

Work part-financed by the European Union Community Initiative INTERREG III B Alpine Space - project FORALPS

7th EMS Annual Meeting/ 8th ECAM - El Escorial, Spain, 01-05 October 2007

2

Outline of the presentation

• Common aspects of the atmospheric hazards related to convective severe weather events.

• Expected improvements of convective severe weather events forecasts.

• Brief review and discussion of the most used empirical forecasting rules.

• Important aspects connected with the empirical rules used for severe convection forecasts.

• A strategy for forecasts improvement.

• Interdisciplinary studies stimulated by the specific problem.

• Conclusions.



3

Common aspects of convective severe weather events

Convective Severe Weather Events



Large Hail Flash FloodsIntense

Donwndrafts Tornadoes

Intense Electrical

Activity

Intense Vertical Vortex

(Mesocyclones, Tornadoes)

4

Expected improvements of convective severe weather events forecasts.



Improvement of convection ignition

forecast

Improvement of boundary conditions

forecast

Improvement of instability forecastNumerical Models

LAMs

Empirical Rules

Theory of Severe

Convection

Atmospheric Convection

Severe Atmospheric Convection Tools for Improvement

LAMs Outputs

quality

Severe Convection

Forecast qualityNot sufficient,

but necessary condition

5

Theoretical basis of empirical rules for intense vertical vortex forecast

•How to generate intense vertical vortexes at the mesoscale?



The basic idea is to tilt the horizontal environmental vorticityand then stretch the vorticity along the vertical

Lagrangian

differential

approach

since Davies-Jones

Klemp & Rotunno

Weisman & Klemp

works in 1980’s

This consideration leads to the study (and as a consequence to forecast)

the environment where vortexes form: wind shear, thermal-moisture

boundaries and buoyancy.

6

The most used empirical rules for intense vertical vortex forecast



Rasmussen and Blanchard

Wea. Forecasting 1998

Brooks et al


7

The most used empirical rules for intense vertical vortex forecast (cont’d)



Bulk Richardson Number

Vorticity Generation Parameter

VGP = S (CAPE)1/2

Rasmussen and Blanchard


8

Discussion on the most used empirical forecasting rules.

The empirical rules in use have been calibrated on specific data sets



NSSL - Brooks H., courtesy

Weakening of the

empirical rule

Missing alarms are frequent

9

Discussion on the most used empirical forecasting rules (cont’d)

The functions used as forecasting rules have high sensitivity to the variables

uncertainty in those regions of the domain where we are mostly interested in.



10

Important aspects connected with deep severe convection forecasts.

The empirical rules come from integral (global) properties of the environment,

even if they have been inspired by differential considerations.



DisadvantagesRelevant small scale and time fluctuations may be smoothed out

AdvantagesEasy to apply

• from routine measurements

• from numerical model outputs

• for climate change downscaling

Expected due to

climate change

Actual climatology

11

A strategy for forecasts improvement (both weather and climate change).

• Increase spatial resolution. Boundary

Layer (LES Moeng C.H. 2007 EMS meeting)

• Physics of the model (microphysics)

(WRF community)



Numerical Models

LAMs

Empirical Rules

Theory of Severe

Convection

Areas of the Improvement

• Integral approach to the vertical

vorticity generation (Gladich, Giaiotti, Stel)

• Bifurcations of small scale fluctuations

(Gladich, Giaiotti, Stel)

• Extend the statistics to all areas

(ESWD, Pucillo, Gladich, Giaiotti, Stel)

• Interdisciplinary experiments

(Sommer A. P.)New forecast tools

+

+

=

12

Interdisciplinary studies stimulated by the specific problem

The understanding of reality sometimes pass through the the study of analogies among very different phenomena.



Evaporation of a water drop containing polystyrene nanospheres, in particular experimental conditions. The distribution of the spheres reveals the presence of intense vertical vortexes in the drop during the evaporation process. Experiment made by Sommer A. P. Ulm University; Germany (Picture taken form Cryst. Growth Des. March 2007, courtesy by Sommer A. P.)

13

Interdisciplinary studies stimulated by the specific problem (cont’t)

Bifurcations in the trajectory of the representative point on the

empirical rules diagrams



Bifurcation point

Vortex occurrence

14

Conclusions



• Nowadays, the operational forecasts of convective severe weather events are based on two relevant elements:

- High resolution numerical model forecasts- Empirical rules

• Improvements in the convective severe weather forecast should come from:

- Improvement of high resolution numerical model forecasts- Theory of intense vertical vortex formation at mesoscale- Adaptation of the empirical rules to new data sets

• New insights into the empirical rules could come from fields not strictly related to the atmosphere physics (interdisciplinarity)

•There are attractive perspectives in the application of the empirical rules to speculate projections on the future convectivesevere storms climatology from climate change scenarios

15

Title of slide

Regional Agency for Environmental Protection

of



Documents

On Convective Severe Weather Forecasts. Empirical Relations ...foralps/Other Events/EMS/21 On...Work part-financed by the European Union Community Initiative INTERREG III B Alpine