Presentation on theme: "FLURRIES OR FEET? An Analysis of QPF Errors in an Eastern North Carolina Snowstorm December 2-4, 2000 Jason Caldwell South Carolina State Climate Office."— Presentation transcript:
1 FLURRIES OR FEET? An Analysis of QPF Errors in an Eastern North Carolina Snowstorm December 2-4, Jason Caldwell South Carolina State Climate Office
2 WINTER WEATHER FORECASTING CSTAR ProjectNational Weather Service Offices (NC, SC, VA)Focus on improving forecast skill in CAD, CFEvaluation of the MM5 Real-Time Forecast SystemNCSU, NC SCO, NCSC, Capitol BroadcastingFocus on sources of model error on the synoptic and mesoscale related to CAD, CFExamine model performance relative to Eta
4 MM5 vs. ETA Quantitative Precipitation Forecasts (24-30 hour forecasts valid Z on 3 December 2000)Insert MM5 24h Forecast here
5 TOTAL ACCUMULATED SNOWFALL 2-4 DECEMBER 2000 (graphics courtesy NWS Raleigh)
6 What we know:Model forecasts missed a precursor vorticity maximum at 500 mbModels predicted coastal front too close to coast (site for surface cyclone formation)Models under-predicted intensity of cold-air dammingModels produced heavy precipitation with a thermodynamic profile that was characterized by shallow cloudOperational sea-surface temperature analyses were too warm according to NCEP, could have been tied to coastal front errors
7 HYPOTHESIS ALead vorticity disturbance at 500 mb(under-forecast by models) pushed the coastal front seaward, strengthened CAD, dried atmosphere over central NC before main vorticity maximum arrived through evaporative processes and subsidenceEVALUATE MODEL ERRORS IN 500mb VORTICITYCOMPARE VERTICAL CHARACTERISTICS BEFORE & AFTER VORT PASSAGETEST FOR EVIDENCE OF SURFACE BASED RESPONSEHOW DOES THIS AFFECT QPF?
8 ETA 500mb Height/Vorticity 12h Forecast EDAS 500mb Height/Vorticity Analysisvalid 00Z 3 December 2000MM5 500mb Height/Vorticity 12h Forecastvalid 00Z 3 December 2000valid 00Z 3 December 2000
9 VERTICAL CROSS-SECTION DIAGRAM Yellow section indicates Central NC region; Black dot indicates location at max QPF gradient
10 EDAS Vertical Analysis (Omega,RH) EDAS Vertical Analysis (TAdv,Theta) valid 00Z 3 Dec 2000valid 00Z 3 Dec 2000EDAS Vertical Analysis (Omega,RH)EDAS Vertical Analysis (TAdv,Theta)Drying aloft due to SubsidenceCAA increasing w/timeIn the wake of lead vort max, AVA/NVA signals convergence at the 500mb levelSubsidence indicated below 500mb by drying over central NC and CAA increasing with timeReflection of accumulation of mass seen with increased depth of CAD, seaward extension of CADSubsidence also indicated offshore by 06Z in response to CAA in low levelsOMEGA strengthens slightly around 700mb attributed to Differential Temperature Advection ( mb) and drier air over moist air INCREASED BUOYANCY TERMTemperatures rapidly decreasing w/heightUVV as response to DiffThermAdvLow-level drying as CAD strengthensCAA increasing w/timevalid 06Z 3 Dec 2000valid 06Z 3 Dec 2000
11 EDAS FRONTOGENESIS AND SLP ANALYSES (PRE- & POST- 500mb VORT MAX) PRE-VORTPOST-VORTEDAS FRONTOGENESIS AND SLP ANALYSES (PRE- & POST- 500mb VORT MAX)valid 00Z 3 Dec 2000valid 06Z 3 Dec 20001000mb frontogenesis indicates CF pushed off-shore between 00Z and 06Z in wake of lead vort maxSLP between 1028 and 1032 over central NC at 00Z----- intensifies to 1030 to 1034mb by 12Zvalid 00Z 3 Dec 2000valid 12Z 3 Dec 2000
12 HOW DOES THIS AFFECT THE QPF? The strength and southern extent of the cold air damming was under-predicted by model forecasts and led to errors in QPF.HOW DOES THIS AFFECT THE QPF?The decreased magnitude of CAD shifts the location of the coastal front and affects low-level thermal and moisture profiles inland- thermal gradient (baroclinic zone) is positioned farther west due to restricted areal coverage of cold dome- moisture availability is higher due to less airmass intrusion from parent high- lower atmosphere is less stable and more conducive to vertical motion- coastal front more likely to propagate inland as WAA overtakes weaker CAD
13 HOW DOES THIS AFFECT THE QPF? Model forecasts predicted the placement of the coastal front west of the actual location off the coast of North Carolina and resulted in QPF errorsHOW DOES THIS AFFECT THE QPF?Low pressure forms closer to the coast which extends the western boundary of precipitation into the Triad region- enhanced upward vertical motion inland due to surface convergence at CF- increased moisture transport with stronger and more easterly winds at low-levels- prolonged duration of precipitation as a result of proximity- prolific isentropic lift as warm air advection associated with cyclogenesis moves into NC over CAD at mid-levels
14 HYPOTHESIS BModel microphysics were overactive in producing precipitation with dry air aloft and a shallow cloudEVALUATE PRECIPITATION MECHANISMS PRESENTDIAGNOSE VERTICAL TEMPERATURE/MOISTURE PROFILE OVER CENTRAL NCHOW DOES THIS AFFECT QPF?
15 INGREDIENTS FOR WINTER PRECIPITATION FORECASTS FORCING FOR ASCENTLow/mid- level convergenceUpper-level divergence/diffluenceDifferential Temperature AdvectionAVAILABLE MOISTURE*Skew-T profiles *Satellite ImageryINSTABILITYTemperatures decreasing w/heightThermal and Moisture AdvectionTEMPERATUREDetermines p-type and snow/water ratioEFFICIENCYBased on cloud Temperature and ice generation*Max occurs at –15C w/strong forcing for ascent
16 MUCH DRIER THAN FORECAST Cross-section of Omega/RH EDAS Analysisvalid 12Z 3 December 2000MUCH DRIER THAN FORECASTMUCH DRIER THAN FORECASTCross-section of Omega/RH ETA 24h Forecastvalid 12Z 3 December 2000
17 24-hour MM5 and ETA Forecast Soundings and EDAS RAOB Raleigh-Durham International, NC (valid 12Z 3 December 2000)Cloud top T ~ -10CShallow Cloud LayerModels too moist below 850 mbCloud depth over-predicted by modelsLess drying above 600 mb than in models
18 HOW DOES THIS AFFECT THE QPF? The ETA model microphysics scheme allowed ice crystal growth at –10 degrees C which allowed precipitation generation to occur in shallow, super-cooled water droplet clouds.HOW DOES THIS AFFECT THE QPF?Dendrite growth occurred in an non-conducive thermal environment generating spurious precipitationIncreased moisture availability in the models through an extended depth over-quantified precipitation totals, when in reality the atmosphere was dry above 700mb
19 SUMMARY AND CONCLUSIONS Classification of QPF Error Sources(1) Synoptic-scale features (i.e. 500 mb vorticity)(2) Circular feedbacks from multiple factors(3) Model parameterizationsModel Inter-comparison+ MM5 achieved better QPF forecasts than Eta yet MM5 also under-estimated the CAD and vort max+ MM5 also out-performed the Eta in coastal front location, strength of CAD, and track of the cyclone
20 ( & OTHER CONSIDERATIONS) DIRECTIONS FOR FUTURE RESEARCH( & OTHER CONSIDERATIONS)Sensitivity experiments using: - High-resolution SST - PBL and Microphysics optionsExamine low-level moisture influx in model forecasts - Relationship to latent heat releaseStatistically quantify the relative weight of each proposed hypothesis in generating model QPF errors
22 Vertical Motion & Moisture Cross-Section EDAS vs Vertical Motion & Moisture Cross-Section EDAS vs. ETA 12h Forecast valid 00Z 3 Dec 2000Low-level drier air as a result of subsidence in CAD region over central NCRH values increase over eastern NC around 700mb in region of ascent where OMEGA= -7mb/sModel indicates region of subsidence isolated west of RDU adjacent to the mountainsTo the east, upward vertical motion (up to -11mb/s) predominates in the entire mb layerRH too low over central NC at 500mb indicating drying and instability between mbRH too high around 700mb level continues mid-level instability12hr model forecasts indicate downward omega west of RDU over central NC/southern VA with UVV above 850mb east of the CAD region approaching –11mb/s in eastern NC in the mb levelRH forecasts low values over RDU at this time below 850 with higher RH over mts and aloft around 700mb
23 Vertical Motion & Moisture Cross-Section EDAS vs Vertical Motion & Moisture Cross-Section EDAS vs. ETA 18h Forecast valid 06Z 3 Dec 2000Positive values of OMEGA indicates subsidence at low-levels over central NC and offshoreUpward vertical motion (OMEGA=-9mb/s) strengthens near 700mb over eastern NCRH values increased in the mb layer across all of NCSubstantial drying below 850mb and aloft above 700mbModel indicates upward motion from SFC-500mbTwo OMEGA maxima (-12mb/s, -16mb/s) over central/eastern NC in the mb layerRH too high at low-levels in CAD region (possibly due to erroneous precipitation)Dry air at 500mb too far west over mountains; RH too high above 700mb east of mountains
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