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Predicting Severe Hail in the WFO LWX County Warning Area Toward Increased Accuracy in Hail Size Forecasts Matthew R. Kramar WFO Baltimore/Washington Jeffrey Waters The Pennsylvania State University History • Studies correlating freezing level and height of 50 dBZ echo core in relation to hail size have been pursued for Northern Plains (Donavon and Jungbluth 2007 [DJ07]) and Southern Plains/West Texas (Porter et al. 2005) following the method described in DJ07 • 50 dBZ core used as proxy for updraft strength • DJ07 suggested numerically that use of freezing level instead of wet bulb zero height provided better relationship • Also showed relationship adequately modeled linearly for hail ≤ 2.00 inch diameter. History • Porter et al. found good correlation for Southern Plains/West Texas, different (shallower) regression line equation • Regional variations in topography, moisture depth, shear and EML influence contribute to differences in slope of regression equations, but technique should work Motivation • * ATHAIL 426 TO PM THE EDT... SIZE NATIONAL WEATHER OF criterion PENNIES AND Scheduled 1 January 2010 change to 1.00” for SERVICE DOPPLER WIND GUSTS TO 60 severe hail in Eastern Region RADAR INDICATED MPH CAN BE A THUNDERSTORM EXPECTED IN THE – Quantify relationship for LWX forecast areaSEVERE CAPABLE OF AREA. PRODUCING WARNED STAY – Help forecasters with transition after manyPENNY years of smaller SIZE HAIL... INDOORS AND AWAY DAMAGING WINDS WINDOWS change threshold 40000 ft 50000 ft AND FROM OF 60 MPH. 0.5 deg 0.9 deg IN EXCESS UNTIL THE STORM HAS PASSED. • Several offices in ER guilty of SVR-ly underestimating hail size in SVR products – Provide starting point for including hail size estimates in warnings and statements 60000 ft 1.3 deg Methodology • Compile reports – – – – Storm Data from Jan 2005 – early June 2009 used All hail reports documented (hail size, time, location) Elevation of report location estimated from topographic maps Stratified into bins following DJ07 • Small [0.75 -0.99 inch] • Medium [1.00 - ≤ 2.00 inch] • Large [≥ 2.00 inch] • Evaluate upper air sounding data and synoptic charts – Determine which sounding (IAD, WAL, RNK) most representative of thunderstorm environment and assign freezing level – Assumption: freezing level height (MSL) more or less uniform Methodology • Evaluate radar – Level II data obtained for KDOX, KLWX, KAKQ, KFCX – 50 dBZ core height recorded subject to: • Sustained for two volume scans • Within 4 volume scans (~15-16 min) of report time • VCP 11/12 and their derivatives • Notes on storm structure were made (i.e. WER, rotation, etc.) VCP21 VCP11 www.srh.noaa.gov/radar/radinfo/vcp21.html www.srh.noaa.gov/radar/radinfo/vcp12.html – Reasons to reject a report: • • • • • Time/location errors that could not be resolved with confidence VCP 21/221 with inadequate ability to evaluate core height Report not largest within 10 mi/15 min of hail report Radar data not available Subjective assessment that reported hail likely smaller than maximum expected hail size in storm Methodology • Final adjustments – 50 dBZ core height in ARL converted to AGL at site of report – Freezing level in AGL at station converted to AGL at report site H2 F1 F2 E1 = elevation of radar site or sounding station F1 = height of freezing level ASL E2 = elevation of hail report site F2 = height of freezing level above report site H2 = height of 50 dBZ core ARL __________________________ • F2 = F1 + E1sounding – E2 E1 E2 Methodology geometry • 50 dBZ AGL = H2 + E1radar– E2 Methodology • 50 dBZ adjusted core heights in AGL plotted against adjusted freezing level in AGL • Least squares linear regression applied to scatter plot • Linear quantile regression also applied to data Results ─ 1.00 inch only All heights in AGL 45000 40000 Adjusted Core Height (fT) 35000 30000 y = 2.6249x - 697.57 R2 = 0.9011 25000 Adjusted Core Height (ft) 20000 Q0.05 Line 15000 Q0.10 Line Q0.20 Line 10000 Linear (Adjusted Core Height (ft)) 5000 0 5000 7000 9000 11000 13000 Adjusted Freezing Level (ft) 15000 17000 Results ─ All 1.00-1.99 inch All heights in AGL 45000 40000 35000 Adjusted Core Height (fT) 30000 y = 2.4184x + 2451.5 2 R = 0.8845 25000 Adjusted Core Height (ft) 20000 Q0.05 Line 15000 Q0.10 Line Q0.20 Line 10000 Linear (Adjusted Core Height (ft)) 5000 0 5000 7000 9000 11000 13000 Adjusted Freezing Level (ft) 15000 17000 Results ─ 1.50-1.99 inch All heights in AGL 45000.0 40000.0 35000.0 Adjusted Core Height (fT) 30000.0 y = 2.214x + 6139.2 R2 = 0.9504 25000.0 1.50-1.99 inch heights 20000.0 Q0.05 Line 15000.0 Q0.10 Line Q0.20 Line 10000.0 Linear (1.50-1.99 inch heights) 5000.0 0.0 5000.0 7000.0 9000.0 11000.0 13000.0 Adjusted Freezing Level (ft) 15000.0 17000.0 Results (heights in AGL) 45000 DJ07 slope: ~ 3.3 40000 Porter et al. slope: ~3.0 35000 Adjusted Core Height (fT) 30000 y = 2.4184x + 2451.5 Slope here: ~2.42!?!?! 2.62 Southern Plains typically has EML, so Porter et al. slope makes sense 2 R = 0.8845 25000 Adjusted Core Height (ft) WHOAAAAAA NELLIE!!! 20000 Q0.05 Line 15000 Q0.10 Line Q0.20 Line 10000 Linear (Adjusted Core Height (ft)) 5000 0 5000 7000 9000 11000 13000 Adjusted Freezing Level (ft) 15000 17000 Mid-Atlantic characterized by typically moister column; slope SHOULD be STEEPER! Why isn’t it? Results (heights in AGL) 45000 Q0.10 for large end of bin and small end of bin converge. 40000 35000 Meteorological? Consequence of radar elevation slice density? Adjusted Core Height (fT) 30000 25000 20000 1.00 inch core heights 1.25 inch core heights 15000 1.50 inch core heights 1.75 inch core heights 10000 Q0.10 Line 1.00 Q0.10 Line 1.50 & 1.75 5000 0 5000 7000 9000 11000 13000 Adjusted Freezing Level (ft) 15000 17000 Results • Next step: decide which quantile regression equation to use as a threshold (DJ07 used Q = 0.10) • POD/FAR balanced against quantile: – lower quantile = higher FAR, higher POD – higher quantile = lower FAR, lower POD • Could use against a statistically independent dataset (say, 2003 severe weather season data) to evaluate FAR, POD, lead time. Results • Goal this winter is to develop D2D toggle between AllTilts reflectivity and expected core height • Updated hourly based on RUC or LAPS freezing level • End result will be a sampled value at every point in the radar domain: Expected Core Height for 1.00” hail • When 50 dBZ core height at or above ECH for two scans, hail ≥ 1.00 inch expected References • Changnon, S.A., 1970: Hailstreaks. J. Atmos. Sci., 27, 109-125. • Donavon, R.A. and K.A. Jungbluth, 2007: Evaluation of a technique for radar identification of large hail across the Upper Midwest and Central Plains of the United States. Wea. Forecasting, 22, 244-254. • Porter, D.L., M.R. Kramar and S.D. Landolt, 2005: Predicting severe hail for the Southern High Plains and West Texas. Preprints, 32nd Conference on Radar Meteorology, Albuquerque, NM. Amer. Meteor. Soc.