The Influence of Convective Parameterization on Model Forecasts of an East Coast

Cyclone

Photo courtesy of NWS Raleigh

Kelly M. Mahoney and Gary M. Lackmann

AMS WAF/NWP Conference

05 August 2005

Motivation• How can forecasters know when they can best

add value to NWP prediction?

• NWP forecasts can be highly sensitive to model physics [e.g., convective parameterization (CP) schemes]

• How can forecasters identify model forecast features characterized by less certainty, such as those driven by a CP scheme?

Motivation: Case Study• 17 February 2004: challenging coastal

front/cyclone, winter weather event

• Large upper trough approached East Coast, CAD event underway

• Where/when would cyclone form along coastal front?

• How far inland would wintry precipitation spread?AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 305 PM EST MON FEB 16 2004

…WITH MODELS VERY DIVERGENT ACCUMULATIONS AND POSITION OF MAXIMUM AMOUNTS VERY VERY UNCERTAIN AND WILL NOT ISSUE ANY WATCHES OR ADVISORIES. AT BEST BELIEVE ADVISORIES MIGHT BE NEEDED BUT WITH SO MUCH UNCERTAINTY PREFER TO LET LATER SHIFT NAIL DOWN THE EXACT AREA OF CONCERN. A SPECIAL WEATHER STATEMENT WILL BE ISSUED.

Eta QPF

12 UTC 17 February 2004

Manual surface analysis

500 hPa height, vorticity

EDAS Analysis

Cold-air damming

Coastal front

00 UTC 18 February 2004

500 hPa height, vorticity

Manual surface analysis

EDAS Analysis

Cyclone has formed along coastal front

Operational Eta model forecast

12Z 16 Feb Run, Valid at 06Z 18 Feb: 6-hourly precip (inches)

Forecast temperatures < 0°C; frozen precipitation a concern

0.3+

0.2

0. 1

Operational Eta 30-h forecast

MSLP and 6-hourly convective precipitation (tenths of inches)18Z 16 Feb operational run –

Valid at 00Z 18 Feb

SLP minima co-located with CP precip maxima

Workstation Eta model sensitivity experiment

Coastal cyclones in Eta forecast co-located with convective precipitation maxima…

Coincidence, or should forecasters treat these features as being less reliable?

Test: Re-run case using workstation Eta:1.) Betts-Miller-Janjic (BMJ) run (as operational)2.) Kain-Fritsch (KF) run3.) BMJ but with shallow convection disabled

Results – Total PrecipitationBMJ: SLP, 3h QPF KF: SLP, 3h QPF

BMJ – KF SLP diff

BMJ – KF precip diff

Coastal Front Representation

BMJ

• less-defined

• farther offshore

• winds mostly defined by two surface lows

KF

• better-defined

• larger temperature gradient

• more convergence

• closer to coast

• brings precip farther inland

BMJ 21Z 17 Feb

KF 21Z 17 Feb

Temp (shaded), 10-m wind streamlines

BMJ – No Shallow

27-hour forecasts from workstation Eta: Sea-level pressure, 3-h convective precipitation (mm)

BMJ KF15 UTC 17 Feb - BMJ 15 UTC 17 Feb - KF

Character of precip, SLP

more similar

BMJ: Shallow mixing scheme

Why were there such significant differences?

Shallow mixing eroded cap more rapidly; produced convection earlier in forecast.

Inclusion of gridscale vertical motion produced convection in areas of large scale ascent.

KF: Gridscale vertical motion

F06

F12

F18

Black: Control BMJRed: No Shallow Mixing

Impact of CP scheme design:

(29.7, -80.9)

(29.7, -80.9)

(29.7, -80.9)

Potential Vorticity (PV) ToolsPV tendency equation:

Fkp

gv

kp

gp

gtd

Pda

ˆˆ

Vertical diabatic PV

redistribution

Horizontal diabatic

redistribution

Vertical diabatic term proportional to:

vertical heating gradient

background vorticity

Friction

Enhanced cyclonic convective response in cyclonic flow

BMJ 39-h forecast

Workstation Eta forecast of 900–700 hPa potential vorticity (PVU), shaded as in legend at left, SLP (hPa, dashed contours), 3-h convective precipitation forecast (mm, solid contours) valid 03 UTC 18 Feb.

Convective precip maxima in coastal-front zone:

Coastal front Larger background vorticity

More prone to diabatic “spinup” of lower PV maxima, cyclones

(relative to convection in areas with lower background vorticity)

BMJ Run 3-h convective precip (solid)

900-700 PV (shaded)

SLP (dashed)

L

L

Potential Vorticity (PV) Tools

So, what really happened?

Why is this plow scraping slush around on a bare parking lot??

Observed radar: 00Z 17 Feb – 12 Z 18 Feb (3h intervals)

Event Synopsis• Favorable upper-level jet streak dynamics produced a

quick burst of snow early across central NC…tapering off to mist for the remainder of the day

• Southern and eastern NC received a mix of sleet and freezing rain early, changing over to all rain by afternoon

SLP ComparisonRed: KF

Blue: BMJGreen: Edas

Double-barreled coastal low (BMJ) verified quite well early in event

Lower central pressure, concentrated center (KF) more accurate by 12 UTC 18th

00Z 18 Feb

12Z 18 Feb

Conclusions

• Sensitivity tests confirm CP scheme strongly tied to:– location, character of coastal front, – coastal cyclogenesis: very different from BMJ to KF

• Differences consistent with hypothesis that CP-driven features should be regarded with less certainty

• Diabatic PV spin-up enhanced in regions of larger background vorticity, such as coastal front zone

• Plot convective precipitation, lower-tropospheric PV, SLP to pinpoint such features

Acknowledgements

• Forecasters, NWS – Raleigh • NCEP – Workstation Eta • NCEP – Matt Pyle, for Eta IC data• NCEP – Brad Ferrier, for discussions

on shallow mixing in BMJ scheme• Drs. Mike Brennan, Al Riordan, and

Sethu Raman

The End

Operational Eta 30-h forecast

MSLP and 6-hourly convective precipitation (tenths of inches)18Z 16 Feb operational run –

Valid at 00Z 18 FebConvective precipitation maxima in cyclonic coastal-front zone (larger background vorticity) therefore more prone to diabatic “spinup” of lower PV maxima, cyclones

(relative to convection in areas with lower background vorticity)

Operational Eta model forecast

12Z 16 Feb Run, Valid at 06Z 18 Feb: MSLP(mb)

Why were there such significant differences?

BMJ-KF Lifted Index (f03) KF omega (cm/s) and 3-hr convective precipitation (f27)

BMJ: Shallow mixing scheme KF: Gridscale vertical motion

Other differences?03Z 17 FebruaryBMJ 2-m

dewpoint(°C),mslpKF 2-m dewpoint(°C),mslp

10m WS diff (kts) (BMJ – KF) 2m Td diff (C) (BMJ – KF)2m LHF diff (BMJ – KF)10m WS diff(kts)(BMJ – KF)

BMJ LHF (W/m2),mslp KF LHF (W/m2),mslp

•KF directly interacts with the gridscale environment via downdrafts and dry air entrainment.

•This likely contributes to the production of a greater LHFs due to cooler and drier surface temperatures in locations where the KF scheme has activated.

Comparison to Observations

• Purpose of study was to examine sensitivity

• Assessing accuracy important as well (to relate output to forecast considerations)

• EDAS analysis used

Precipitation comparisonBMJ 3h precip (mm),mslp 12Z 17 Feb KF 3h precip (mm),mslp 12Z 17 Feb

Radar mosaic 09Z 17 Feb Radar mosaic 12Z 17 Feb

Precipitation comparisonBMJ 3h precip (mm),mslp 00Z 18 Feb KF 3h precip (mm),mslp 00Z 18 Feb

Radar mosaic 21Z 17 Feb Radar mosaic 00Z 18 Feb

Future Work

• Perform a more in-depth observational analysis

• Complete further modeling studies to assess the contribution of other processes to coastal cyclogenesis and frontogenesis.

• Acquire more accurate analyses to better compare model output.

• Share results with NWS forecasters

EDAS sea level pressure analysis with ship and buoy observations at 18Z 17 February

SST 17 Feb

Outline

• Motivation• Event synopsis• Forecast considerations• Modeling study and results• Summary• Plans for future work

Results – CP Scheme PrecipitationBMJ KF

BMJ – KF SLP diff

BMJ – KF convectiveprecip diff

Convective precip at 15Z (“X” marks skew-T location, (35;-75))

Skew-T at 09Z at (35;-75)

Skew-T at 15Z at (35;-75)

Warming due to LH release…

Motivation: Case Study• 17 February 2004: challenging coastal

front/cyclone, winter weather event

• Large upper trough approached East Coast, CAD event underway

• Where/when would cyclone form along coastal front?

• How far inland would wintry precipitation spread?AREA FORECAST DISCUSSION...UPDATEDNATIONAL WEATHER SERVICE RALEIGH NC905 PM EST MON FEB 16 2004

MODEL REPRESENTATION OF THE COASTAL LOW DEVELOPMENT WILL LIKELY BE HIGHLY SENSITIVE TO CONVECTIVE FEEDBACK. THIS IS EVIDENCED IN THE ETA...WHERE THREE SEPARATE LOW CENTERS ARE SEEN OFFSHORE BY 21Z TUESDAY...EACH CO-LOCATED WITH A MAXIMUM IN CONVECTIVE PRECIPITATION.

Eta QPF