1 1 1

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space AdministrationUse of High-Resolution

WRF Simulations toForecast Lightning Threat

Photo, David BlankenshipGuntersville, Alabama

E. W. McCaul, Jr.1, K. La Casse2, S. J. Goodman3, and D. J. Cecil2

1: USRA Huntsville2: University of Alabama in Huntsville3: NASA Marshall Space Flight Center

Huntsville, Alabama, USA

SPoRT Meeting

14 February 2007

2 2 2

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

Premises and Objectives

WRF: Weather Research and Forecast ModelCRM: Cloud Resolving ModelAdditional Forecast Interests

CI - convective initiationTi - First lightning (35 dBZ at -15C, glaciation)Tp - Peak flash rate VIL (Mass)Tf - Final lightning

Given:1. Precipitating ice aloft is correlated with LTG rates2. Mesoscale CRMs are being used to forecast convection3. CRMs can represent many ice hydrometeors (crudely)

Goals:1. Create WRF forecasts of LTG threat, based on ice flux in layers near -15 C, and on simulated reflectivity.

3 3 3

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

0 oC

Flash Rate Coupled to Mass in the Mixed Phase RegionCecil et al., Mon. Wea. Rev. 2005 (from TRMM Observations)

F15 SREF 3-hr COMBINEDF15 SREF 3-hr COMBINEDPROBABILITY OF LIGHTNINGPROBABILITY OF LIGHTNING

- Pr (CPTP) >= 1 x Pr (PCPN) >= .01”

Uncalibrated probabilityof lightning

SPC Experimental Product

5 5 5

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Lightning Threat Forecasts:Methodology

1. Use high-resolution (2-km) WRF simulations to prognose convection2. Develop diagnostics from model output fields to serve as proxies for LTG: graupel fluxes; reflectivity profiles3. Create 0-6, 0-8 or 0-12 h forecasts of LTG threat based on

WRF output for the selected diagnostics4. Calibrate WRF forecasts with actual total LTG rates from

HSV LMA and reflectivity from TRMM PR radar5. Assess WRF capabilities for forecasting LTG threat

6 6 6

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Lightning Threat Forecasts:10 December 2004

Cold-season hailstorms, little LTG

7 7 7

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Configuration (typical)10 December 2004 Case Study• 2-km horizontal grid mesh• 50 vertical sigma levels• Dynamics and physics:

– Eulerian mass core– Dudhia SW radiation– RRTM LW radiation– YSU PBL scheme– Noah LSM– WSM 6-class microphysics scheme

(graupel; no hail)

• 8h forecast initialized at 12 UTC 10 December 2004 with AWIP212 NCEP EDAS analysis;

• Also used METAR, ACARS, and WSR-88D radial vel at 12 UTC;

• Eta 3-h forecasts used for LBC’s

8 8 8

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Sounding, 2004121019Z

Lat=34.8Lon=-85.9CAPE~500

9 9 9

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

LTG ground truth: flash density

10 December 2004, 19Z

10 10 10

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Reflectivity at 1.25 km:

10 December 2004

11 11 11

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

LTG threat derived from WRF graupel flux at -15C

• Compute field of upward graupel flux at -15C

• Rescale dynamic range of values against observed LTG flash rates on the same 1 km grid; for experiments looked at thus far, scale coefficient is ~0.2 (this will change)

• With histogram truncation, could match areal coverage to observations; not done here because of loss of some simulated storms which appear to be important

• After applying scaling factor, obtain field of LTG flash rates in units of fl/(5 min)/km2

12 12 12

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

LTG threat methodology advantages

• Method based on LTG physics

• Method supported by solid observational evidence

• Can be used to obtain quantitative estimates of flash rates

• Method is fast and simple; based on basic model output fields; no need for complex electrification modules

• Graupel flux method can be expanded to include a flux threshold, which will allow truncation of flux histogram so as to allow matching of areal coverages of predicted and observed LTG activity, as model accuracy improves

13 13 13

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

LTG threat based onWRF graupel flux at -15 C

14 14 14

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Lightning Threat Forecasts:30 March 2002

Supercell ahead of Squall Line

15 15 15

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Sounding, 2002033003Z

Lat=34.4Lon=-88.1CAPE~2800

16 16 16

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

LTG ground truth: LMA flash rate

30 March 2002, 04Z

17 17 17

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

WRF Reflectivity at 1.25 km:

30 March 2002, 04Z

18 18 18

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

LTG threat based onWRF graupel flux at -15C

19 19 19

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

Conclusions:

1. On 100 km scales, WRF forecasts of LTG are useful in terms of location, timing and amplitude2. On 10 km scales, LTG forecasts are only as good as the model3. Inclusion of WSR-88D velocity data helps; so does dBZ if storms already up at t=04. WRF microphysics still too simple; need more ice categories; need double-moment or better microphysics5. Finer model mesh may improve updraft representation, and hydrometeor amounts6. Biggest limitation is likely errors in initial mesoscale fields

20 20 20

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

Future Work:

1. Expand catalog of simulation cases to obtain better statistics2. Test newer versions of WRF, when available: - more hydrometeor species - double-moment microphysics3. Run on 1-km or finer grids, whenever feasible4. In future runs, examine fields of interval-cumulative wmax, and associated hydrometeor and reflectivity data, not just the instantaneous values; for save intervals of 15-30 min, events happening between saves may be important for LTG threat5. Refine dBZ-based LTG threat computation to account for size of individual storms, not just storm systems6. Assess WRF utility in prognosing LTG onset, cessation

21 21 21

SPoRT, Feb. 2007

Earth-Sun System DivisionNational Aeronautics and Space Administration

Acknowledgments:

This research was funded by the NASA Science Mission Directorate’s Earth Science Division in support of the Short-term Prediction and Research Transition (SPoRT) Project at Marshall Space Flight Center, Huntsville, AL.