A Review of the Precipitation Distribution Associated with the TROWAL and its

Application to a New England QPF Event

Frank M. NoceraNOAA/NWS Taunton, MA

• Poorly modeled heavy rain event with widespread 0.50 to 0.75 qpf. Isolated amounts of over 1 inch and approaching two inches in Southeast MA.

• Would have captured more attention if it was all snow!

Highlights of the New England TROWAL event - Nov 6 2008

OBJECTIVES

• Become familiar with the structure and conceptual model of the TROWAL to improve model QPF for Enhanced Short Term Forecasting operations.

• Update on latest research of TROWAL.

• New AWIPS Trowal procedures/applications.

• Supplement AWOC and VISIT training material.

24 HOUR PRECIP ENDING 12Z NOV 7 2008

Combination of METAR and COOP data.

00Z GFS 24 HR QPF /12ZThu-12ZFri/ 06Z GFS 24 HR QPF /12ZThu-12ZFri/

MODEL QPF

12Z 11/6 ECMWF 24 hr QPF 06Z 11/6 NAM12 QPF

• 06z NAM indicating much higher QPF across Southern New England.• Notice the usually superior ECMWF doesn’t have the signature of an occlusion in its QPF field.

09Z SREF 24 HR QPF PROBS

SREF 24 HR POP > 0.25 SREF 24 HR POP > 0.50

STRUCTURE AND CONCEPTUAL MODEL OF THE

TROWAL

ATTRIBUTES OF A TROWAL • Defined as a TRof Of Warm Air aLoft – warm anomaly aloft.

Basically a westward extension of the Warm Conveyor Belt (WCB). The WCB splits into two branches – a cyclonic air stream and an anticyclonic air stream.

Area where warm moist air has wrapped cyclonically around the Low, which is denoted by the “Comma head”. Indicates a strongly wrapped system.

It’s an area with locally high moisture and a minimum in stability, so if there is a lifting mechanism present, it’s fairly easy to reach saturation and produce precip.

Trowals are typically associated with slow moving/Occluded Cyclones – hence, large QPF potential.

Trowals are not unique. Nearly every occluded cyclone has some form of a Trowal, as it’s a basic structure of an occluded cyclone.

STRUCTURE OF TROWAL

Source – VISIT teletraining

(Canyon of warm air)

Mid level fronts are not perpendicular to the TROWAL, but rather parallel.

LATEST RESEARCH ON TROWALS

FRONTAL STRUCTURE OF THE TROWAL (Han et al. MWR May 2007)

WHITE FRONTS = Mid levels. BLACK FRONTS = Surface.• These mid level fronts serve as a lifting mechanism for precipitation within the TROWAL.

RADAR & SATELLITE 18Z

Notice how mode of precip (convective vs. stratiform) matches the conceptual model from the previous slide - Stratiform precip over Southern NH & VT associated with the mid level warm front and convection over CT/RI & southeast MA associated with the mid level cold front.

SURFACE OBSERVATIONS & ANALYSIS

18Z SURFACE MAP

21Z SURFACE MAP

LET’S LOOK AT SOME TRADITIONAL PARAMETERS

THETA-E at 850MB, 700 MB, 550 MB and WATER VAPOR IMAGERY at 18Z

18z GFS 850-500 mb THETA-E & Q-VECTOR CONVERGENCE

18z GFS SHOWALTER

• GFS hints at all three parameters (deep layer moisture, Synoptic scale lift and weak stability) overlapping at 18z across Northern CT and RI extending east in MA.

4 Panel FGEN (pressure surfaces)

• Model capturing frontogenesis along both mid level fronts.

LET’S LOOK AT SOME NEW PARAMETERS

PRESSURE ON 320Ke SURFACE

Pressure ridge on Theta-E surface (canyon of warm air).

Steep frontal slope implies low stability.

Data void areas – Theta-E surface likely vertical or folding over.

18Z RADAR

PRESSURE ADVECTION ON 1.5 PVU SURFACE

PV Anomalies

PRESSURE ADVECTION ON 1.5 PVU SURFACE

Dynamic Trop down to 430 mb.

Strong pressure advection into Southeast New England.

18Z JET ANALYSIS ON THE DYNAMIC TROPOPAUSE – 1.5 PVU SURFACE

Eastern MA and RI near LFQ of upper level Jet. Also, very short half wavelength between trof and ridge, providing lots of curvature enhancing divergence along

with ageostrophic flow & circulations.

DIAGNOSING STABILITY

850-500 MB THETA-E LAPSE RATES AND RH at 18Z

Negative Theta-E lapse rates = Theta-E decreasing with height

850-500 MB THETA-E LAPSE RATES AND RH at 00Z

Weak stability in a near saturated environment

00Z RADAR

00Z RADAR18Z RADAR

BUT WHY DOES PRECIPITATION INCREASE IN COVERAGE AND INTENSITY?

LET’S EVALUATE –

• The interaction between the mid level cold front and the upper level jet/PV anomaly along with stability and deep layer moisture.

Steep mid level frontal slope

PV anomaly and pressure advection remain well south of New England

Upper level jet approaching. Instability axis offshore

Interaction between upper level jet/PV anomaly and mid level cold front yielded a deep coupled circulation /deep column of lift/ combined with weak stability in a near saturated environment.

Steep frontal slope slides northward

Strong pres advection

LFQ of jet providing lots of curvature and divergence

Weak stability and deep layer moisture collocated

00Z RADAR18Z RADAR

SUMMARY/CONCLUSION• Knowledge of the trowal structure and conceptual model

would not have yielded a perfect forecast, but would have provided an opportunity to improve model qpf and understand why the NAM solution was different.

• When model qpf is not verifying, apply the trowal conceptual model to satellite and radar to assist with enhanced short term forecasting.

• Incorporate new AWIPS Trowal procedures/applications into forecast operations.

• This event supports other cases by Han et al. that suggest the classical conceptual model of the trowal, at least in some cyclones needs to be revised to include the possibility that the warm, moist airstream aloft may sometimes be bounded on its south side by an upper-level front rather than a surface-based cold front.

ACKNOWLEDGMENTSSincere thanks to:

Dan St.Jean / SOO WFO Gray, ME

Phil Schumacher / SOO WFO Sioux Falls, SD

Dave Novak / SOO HPC

Joe Dellicarpini / SOO Taunton, MA

Dave Radell / ERH-SSD

Frank M. Nocera

NOAA/NWS Taunton, MA

RECOMMENDED READING

• Mesoscale Dynamics of the Trowal and Warm-Frontal Regions of Two Continental Winter Cyclones – MWR May 2007 MWR May 2007 (Han et al.).

• The Occlusion Process in a Midlatitude Cyclone over Land – MWR April 1993 MWR April 1993 (Schultz & Mass)..

QUESTIONS?

FRANK.NOCERA@NOAA.GOV508-823-1983