Determination of tornado intensity from forest damage

Transkript

1 Determination of tornado intensity from forest damage Veronika Beck 1,2, Nikolai Dotzek 2, Robert Sausen 2 1 Physik-Department,, Garching, Germany 2 DLR-Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany 24th AMS Conference on Severe Local Storms Savannah, Georgia, October 2008

2 Overview Description of the Letzmann tornado model and his theoretical tree damage patterns Presentation of the model structure Case study: tornado damage analysis Summary

3 Motivation 1) Johannes Letzmann, 1923: Das Bewegungsfeld im Fuße einer fortschreitenden Wind- oder Wasserhose Dissertation Thesis, Acta Comm. Univ. Dorpat AIII, ) Holland et al., 2006: A Simple Model for Simulating Tornado Damage in Forests, Journal of applied Meteorology and Climatology, 45, Determination of the velocity components and the intensity of the tornado wind field from forest damage patterns Tornado Micheln, DE, 23 June 2004 Diedorf Tornado, DE, 1 October 2006

4 Letzmann's tornado model (1923) (I) Analytical tornado model of Johannes Letzmann of the year 1923 that is based on a Rankine vortex with linear velocity increase in the tornado core and hyperbolical velocity decay in the tornado mantle, but depends on the parameters G max, α, and v trans. G max α v trans ratio between circular and translation velocity component angle between wind direction and direction of pressure gradient translation velocity of the tornado velocity [m/s]

5 Letzmann's tornado model (1923) (II) G max = 0.75 α = -60 velocity [m/s] v trans = 45 ms G max = 1.0 α = -60 velocity [m/s] v trans = 40 ms G max = 1.5 α = -60 velocity [m/s] v trans = 32 ms -1 Dependence of the velocity field on the parameter G max shows a separation into two calm points at G max = 1.0 For tornadoes both in USA and Europe G max is expected to lie in the region of G max = (Letzmann, 1925)

6 Letzmann's tornado model (1923) (III) G max = 1.5 α = -60 v trans = 32 ms G max = 1.5 α = -120 v trans = 32 ms velocity [m/s] velocity [m/s] Dependence of the velocity field on the parameter α: change from inflow to outflow at α = -90 (constant value of α inside the velocity field). Definition of a false and a genuine core by variation of the angle α inside the velocity field of the tornado.

7 Letzmann's tornado model (1923) (IV) Genuine core outflow of the vortex center, "two-cell vortex", decreasing angle α from the vortex core velocity [m/s] velocity [m/s] False core inflow to the vortex center, "two-cell vortex", increasing angle α inside the vortex core, decreasing angle α outside the vortex core Genuine core G max = 6.0 v trans = 11ms -1 False core G max = 6.0 v trans = 11ms -1

8 Theoretical damage patterns - Swath types Classification of tree damage patterns according to intensity, G max and α into four different swath types for α < 90 (inflow) For α > 90 (outflow) the tree damage patterns all have divergent behavior ψ is the falling angle of the tree ψ = 0 indicates a converging or diverging line in the tree damage pattern ψ = 180 indicates a converging line in the tree damage pattern Letzmann Beck (2008) (1923)

9 Structure of the model Tree model wind velocity tree parameters dbh, height, age, species, form, stand Wind field model max. tangential velocity max. radial velocity translation velocity bending moment tree resistance α G max v max vortex-model R max critical wind velocity for stem breakage Comparison wind velocity If the velocity of the wind field exceeds the critical velocity for stem breakage of the tree model, the tree is considered to fall in direction of the instantaneous wind.

10 Damage analysis - F3 Milosovice tornado

11 Damage analysis - F3 Milosovice tornado Estimation of the translation speed of the thunderstorm cell producing the Milosovice tornado from radar observations to ms -1.

12 Damage analysis - F3 Milosovice tornado Estimation of the translation speed of the thunderstorm cell producing the Milosovice tornado from radar observations to ms -1.

13 Damage analysis - F3 Milosovice tornado N Photo courtesy of Martin Setvak Forest damage pattern of the Milosovice tornado (path width m). Main tornado (1) and three smaller vortices (2), (3) and (4).

14 Main tornado damage pattern analysis Fujita-Scale distribution along the path G max = 4.0, α = -150, v trans = 16.5 ms -1 R max = 80 m, v max = ms -1 model-derived peak intensity: F3 G max = 4.0, α = -140, v trans = 16.5 ms -1 R max = 80 m, v max = ms -1 model-derived peak intensity: F3 G max = 5.0, α = -140, v trans = 16.5 ms -1 R max = 80 m, v max = ms -1 model-derived peak intensity: F4

15 Reassessment of F3 Milosovice tornado - Verification of F3 rating of Milosovice tornado - Localization of trace and diverging/converging line of the vortices - v trans = 16.5 ms -1 from radar observations - Damage patterns simulated by a Rankine vortex - Main vortex: R max = 80 m v max = ms -1

16 Reconstruction wind fields F3 Milosovice tornado zone I zone II zone III zone IV Reconstructed tornado near-surface wind fields of the main tornado (I) and the smaller vortices showing a continuous wind field for the vortices (I), (II) and (IV) and part of a tornado life cycle for vortex (III). Vortex (II) and (IV) have anticyclonic sense of rotation while vortex (I) and (III) are cyclonic. The spatial extension of the main vortex and vortex (II) is similar while vortex (III) and vortex (IV) have smaller spatial extension.

17 Castellcir tornado, Spain, 16 October 2006 Analysis of the damage patterns of the total damage path shows a high F1 or low F2 tornado Tornado trace (Bech et al. 2007)

18 Conclusions - Tornado near-surface wind field model based on a Rankine vortex depending on the parameters G max, α and v trans was used for the simulation of forest damage patterns. - Translation velocity of the thunderstorm cell producing the tornado was determined by radar observations. - From comparison of real damage patterns with the simulated damage patterns conclusions of the values of G max, α, v max, vortex structure, sense of rotation and tornado intensity can be drawn. - Maximum wind speed of the tornado is determinable with uncertainties of 10 ms -1 (half-width of Fujita-scale classes). But compared to tornado intensity determination from damage, additionally information about the near-surface wind field of the tornado can be achieved. Online available at: