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Conference CallApril 11, 2012

• Modeling update

• Rough draft: slides for Tropical Workshop

• Climatology: What has been completed/what needs to be done

• Focuses over next month/open discussion

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Decay over Land

• Many participants in last call suggested looking at decay based on quadrant

• Using same data sets, broken down by quadrants:– Center of storm based on interpolated hourly best

track position– Points NE of storm: quadrant 1– Points NW of storm: quadrant 2– Points SW of storm: quadrant 3– Points SE of storm: quadrant 4

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Decay over Land

• Brief overview of results (full results to be shared on blog in interest of time)– Interesting variations based on quadrant– Slope of reductions over time varies rather

significantly for various quadrants

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Modeling Update

• Having trouble running WRF-LES simulations on local machine

• NCAR computational information systems laboratory

• Will be working to run simulations on this system, while our system is being upgraded

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Modeling Update

• Examine lowest scan from Morehead (available until time of landfall, then radar goes down for several hours)

• Examine eye signature of wind field

• Compare to WRF/HWind analyses to radial velocity returns for radar; see “true” basic structure of eye

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Rough Draft: Training Slides

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TC Sustained Wind and Gust Forecasting

• RCL file provided by NHC: contains 64/50/34 knot wind radii out to day 5

• TCMWindTool uses these values, along with background model, “pie slices,” and land reduction factor provided by forecaster to create raw wind forecast

• Lack of “scientific” thought results in large degrees of spread and error in sustained wind speed and gust forecasts

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Hanna (2008): Sample NDFD Forecast

(Issued 00z 5th September, Valid 06z 6th September)

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Study Objectives

• The overall goal of this study is to improve the wind speed and wind gust forecasts associated with tropical cyclones.

• The primary focuses for improvement are in the land reduction factors and gust factors used in the TCMWindTool and forecast process.

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NDFD Verification• “Latest” NDFD sustained wind speed forecast was

compared to both ASOS and HWind Surface Analyses for storms affecting the region 2005-2010

• General trend of overprediction of surface wind speeds

• WFOs clearly come out in wind speed error analysis, indicative of lack of consistency in land reduction and gust factors used

• CAD situations often lead to a poor wind forecast for a given region, as seen in Hanna (2008) and Ernesto (2006)

11Valid at DATE September 2008 TIME

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Preliminary Results• Preliminary results suggested increase in land

reduction factors necessary for almost all areas

•Raleigh WFO appears to have smallest difference between forecast and HWind analysis (where forecasters used strongest land reduction factors of 33%)

NDFD – HWIND Analysis at (a) 0730 27 August, (b) 1330 27 August, (c) 1930 27 August, and (d) 0130 27 August (UTC)

(a) (b)

(c) (d)

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Decay over Land

• Show how the wind speeds of “typical” landfalling tropical cyclones decay once over land

• RUC analyses chosen– Available from NCDC since 2002 – Stay tuned: looking at other analyses for

consistency in results

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Decay over Land—Strong StormsIsabel (2003) & Irene (2011)

Isabel (2003)

Irene (2011)

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Decay over Land—Weaker Storms

Kyle (2002) Gaston (2004)

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• ADD QUADRANT SLIDE HERE

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Decay over Land

• Strong storms show sharp reduction in wind over first 6-24 hours once over land; little change afterwards– Timing of sharpest decrease in winds

depends on angle with respect to coastline

• Weaker storms show little reduction once over land– Sometimes, outer radii actually have slightly

higher winds than closer to storm

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Wind Speed ClimatologyNumerous past studies have shown surface winds can be best described by a Weibull distribution. The two-parameterWeibull distribution is defined as:

where:η = scale parameter,β = shape parameter (or slope),

•Using the NC State Climate Office CRONOS database, all available wind observations 2000-2010 were obtained

•Weibull distributions were then fit to the data for each station•A spatial analysis was conducted on the data, examining the parameters that define the Weibull distribution

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•Areas of highest elevation and locations near the coastline have the largest shape and scale parameters, indicative of highest mean wind speed values as well as a large spread in the wind speed distributions in these areas.

•There is a distinct minimum in mean wind speeds as well as the spread in the wind speed distributions for much of Virginia and west-central North Carolina.

•Topographic influences lead to significantly different mean wind speeds and distributions for areas in the same local region.

INSERT LEGEND HERE FOR COLOR SCALES

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• ADD COMPARISON TO WHEN STORM AFFECTING GIVEN AREA HERE

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Gust Factor Analysis

• ASOS 1-minute winds acquired for Irene (2011)

• Gust factor calculated as ratio: (Gust Speed / Sustained Wind Speed)

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•The highest gust factors (>1.4) were observed in areas where the sustained wind speeds were much weaker. This occurred in areas further inland as well as after the storm passage in many areas. Weaker wind speeds were also consistent with higher variability in the gust factors.

•In areas with strongest sustained wind speeds, gust factor values were much lower, near 1.2, and variability in the gust factors was much lower.

•Many coastal sites and areas near the mountains did not observe a significant increase in average values and variability in gust factors after the storm passed.

•Gust factor analysis currently in the process of being conducted for other storms

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A Modeling Approach: Where We are Headed

• High resolution modeling studies are in the process of being conducted for select cases

• Studies will examine the influence of various thermodynamic and environmental conditions for a given area that alter sustained wind speeds and gusts for a given area

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Base Modeling Case Study: Irene (2011)

HWIND ANALYSIS, 8/27/1330 UTC Model Run 7, 8/27/1300 UTC

Doppler Radar, Valid at 1300 UTC Simulated Radar, Valid at 1330 UTC

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Summary

• A statistical and climatological-based analysis of land reduction and gust factors has been conducted and is being finalized

• High resolution WRF-LES simulations are beginning

• Goal is a scientifically-based guidance for land reduction and gust factors for landfalling TCs

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Output Grid: Land Reduction Factors Ouput Grid: Gust Factors

Grid File: Land Use/Terrain Data

Basic Storm InformationStrength of Storm

Size of StormAngle of storm approachStorm propagation speed

NHC Best Track Data

Grid File: Thermodynamic/Environmental ConditionsStatic Stability

Boundary layer conditionsET Transition?

Cold Air Damming?

Goal for Final Product

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Climatology Paper: Where We Stand

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Climatology Paper Outline

• Results– Weibull distributions of wind speeds and gusts

during times of landfalling TCs vs. climatology• Purpose: Show “natural” wind speed distribution

for given area and how this changes during time of TC influence

• Completed: Weibull distributions for climatology• Needs to be done: Weibull distributions for times

storm affecting the given area

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Climatology Paper Outline

• Results– Land decay using RUC analyses for recent storms

• Purpose: Show how storms decay as storm moves over land

• Completed: – Comparison of storm vs. weak storms affecting region

– Decay for four quadrants of storm

• Needs to be done– Compare to other analyses, including NARR and/or CFSR data

– Extend results for more storms, possibly using other data set

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Climatology Paper Outline

• Results– Frequency of missing observations from

CRONOS data• Purpose: Determine if CRONOS database ok to

use for NDFD verification• Completed:

– General “sense” for frequency of missing observations, based on wind gust analysis and comparison to NDFD data

• Needs to be Done: – Final statistics on frequency of missing data

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Climatology Paper Outline

• Results– NDFD verification for storms (using both CRONOS

and HWind Analyses)• Purpose: Need to use both for verification (storm outages

right near center of storm, HWind analyses only available near time of landfall)

• Completed:– Bias calculations for all available storms

– General sense of bias

• Needs to be Done:– Create final plots for paper

– Normalize relative to wind speed verification?

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Climatology Paper Outline

• Results– Spatial gust factor analysis for storms using

ASOS data• Purpose: Verify gust factors for recent storms

affecting the region• Completed:

– Analysis for Irene (2011)

• Needs to be Done: – Analysis for other storms– Compare gust factors for various storms, based on storm

“general characteristics”