FLURRIES OR FEET?

An Analysis of QPF Errors in an Eastern North Carolina Snowstorm

December 2-4, 2000

Jason CaldwellSouth Carolina State Climate Office

WINTER WEATHER FORECASTING

CSTAR ProjectCSTAR Project

National Weather Service Offices (NC, SC, VA)

Focus on improving forecast skill in CAD, CF

Evaluation of the MM5 Real-Time Forecast Evaluation of the MM5 Real-Time Forecast SystemSystem

NCSU, NC SCO, NCSC, Capitol Broadcasting

Focus on sources of model error on the synoptic and mesoscale related to CAD, CF

Examine model performance relative to Eta

EDAS Surface Analysis

EDAS 500mb Analysis

EDAS 850mb Analysis

EDAS 250mb Analysis

SYNOPTIC SET-UP

VALID 12Z 2 DECEMBER 2000

Insert MM5 24h Forecast here

MM5 vs. ETA Quantitative Precipitation Forecasts(24-30 hour forecasts valid 12-18 Z on 3 December 2000)

TOTAL ACCUMULATED SNOWFALL2-4 DECEMBER 2000

(graphics courtesy NWS Raleigh)

What we know:What we know:

• Model forecasts missed a precursor vorticity maximum at 500 mb

• Models predicted coastal front too close to coast (site for surface cyclone formation)

• Models under-predicted intensity of cold-air damming

• Models produced heavy precipitation with a thermodynamic profile that was characterized by shallow cloud

• Operational sea-surface temperature analyses were too warm according to NCEP, could have been tied to coastal front errors

Lead 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 subsidence

HYPOTHESIS A

•EVALUATE MODEL ERRORS IN 500mb VORTICITY

•COMPARE VERTICAL CHARACTERISTICS BEFORE & AFTER VORT PASSAGE

•TEST FOR EVIDENCE OF SURFACE BASED RESPONSE

•HOW DOES THIS AFFECT QPF?

ETA 500mb Height/Vorticity 12h Forecast

MM5 500mb Height/Vorticity 12h Forecast

valid 00Z 3 December 2000

valid 00Z 3 December 2000

EDAS 500mb Height/Vorticity Analysis

valid 00Z 3 December 2000

VERTICAL CROSS-SECTION DIAGRAM

Yellow section indicates Central NC region; Black dot indicates location at max QPF gradient

Drying aloft due to Subsidence

Low-level drying as CAD strengthens

CAA increasing w/time

CAA increasing w/time

UVV as response to DiffThermAdv

Temperatures rapidly decreasing

w/height

EDAS Vertical Analysis (Omega,RH)

EDAS Vertical Analysis (Omega,RH)

EDAS Vertical Analysis (TAdv,Theta)

EDAS Vertical Analysis (TAdv,Theta)

valid 00Z 3 Dec 2000 valid 00Z 3 Dec 2000

valid 06Z 3 Dec 2000 valid 06Z 3 Dec 2000

valid 00Z 3 Dec 2000 valid 12Z 3 Dec 2000

valid 00Z 3 Dec 2000 valid 06Z 3 Dec 2000

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PRE-VORT POST-VORT

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

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

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 errors

Model microphysics were overactive in producing precipitation with dry air aloft and a shallow cloud

HYPOTHESIS B

•EVALUATE PRECIPITATION MECHANISMS PRESENT

•DIAGNOSE VERTICAL TEMPERATURE/MOISTURE PROFILE OVER CENTRAL NC

•HOW DOES THIS AFFECT QPF?

INGREDIENTS FOR WINTER PRECIPITATION FORECASTS

AVAILABLE MOISTURE

*Skew-T profiles*Satellite Imagery

INSTABILITY

Temperatures decreasing w/height

Thermal and Moisture Advection

FORCING FOR ASCENT

Low/mid- level convergence

Upper-level divergence/diffluence

Differential Temperature Advection

EFFICIENCYBased on cloud Temperature and ice generation

*Max occurs at –15C w/strong forcing for ascent

TEMPERATUREDetermines p-type and snow/water ratio

MUCH DRIER THAN FORECAST

MUCH DRIER THAN FORECAST

Cross-section of Omega/RH EDAS Analysis

valid 12Z 3 December 2000

Cross-section of Omega/RH ETA 24h Forecast

valid 12Z 3 December 2000

24-hour MM5 and ETA Forecast Soundings and EDAS RAOB Raleigh-Durham International, NC

(valid 12Z 3 December 2000)

Models too moist below 850 mb

Cloud depth over-predicted by models

Less drying above 600 mb than in models

Cloud top T ~ -10C

Shallow Cloud Layer

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 precipitation

Increased moisture availability in the models through an extended depth over-quantified precipitation totals, when in reality the

atmosphere was dry above 700mb

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 parameterizations

Model 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

DIRECTIONS FOR FUTURE RESEARCH

( & OTHER CONSIDERATIONS)

Sensitivity experiments using:- High-resolution SST- PBL and Microphysics options

Examine low-level moisture influx in model forecasts- Relationship to latent heat release

Statistically quantify the relative weight of each proposed hypothesis in generating model QPF errors

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 NC

RH values increase over eastern NC around 700mb in region of ascent where OMEGA= -7mb/s

Model indicates region of subsidence isolated west of RDU adjacent to the mountains

To the east, upward vertical motion (up to -11mb/s) predominates in the entire 850-500mb layer

RH too low over central NC at 500mb indicating drying and instability between 700-500mb

RH too high around 700mb level continues mid-level instability

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 offshore

Upward vertical motion (OMEGA=-9mb/s) strengthens near 700mb over eastern NC

RH values increased in the 850-700mb layer across all of NC

Substantial drying below 850mb and aloft above 700mb

Model indicates upward motion from SFC-500mb

Two OMEGA maxima (-12mb/s, -16mb/s) over central/eastern NC in the 700-500mb layer

RH 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