Mesoscale Banded Precipitation

EAX Winter Weather Workshop28 October 2008

Norwegian Cycle Model

● Relatively narrow ribbons of air flowing along sloping isentropic surfaces ● Represent air trajectories ● Produce clouds and precipitation

Three conveyor belts or airstreams:

Warm – transports warm air poleward - lifts warm/moist air over warm front

Cold – runs parallel to the warm front on the cold side

Dry – descending air stream on the southwest or backside of system

Conveyor Belt - Overview

WCB – locate ribbon of strongest winds starting in the warm sector near highest RH and extend parallel along θe lines until they reach RH max.

CCB begins where easterly flow becomes sufficiently deep…..note the slope

TROWAL (Warm Conveyor Belt) – Mature Cyclone

Portion of WCB turns cyclonicallyand is pulled into the circulation

Creates a trough of warm air aloft (TROWAL)

Narrow and focused – gives riseto banded precipitation

TROWAL is a sloping canyon ● Best seen on mid-level θe surfaces (800-600mb) ● Ascending warm air ● Feature of occluded systems ● Strong frontogenesis

Wrap-around part of cyclone Enhances precipitation associated with comma head

Don’t underestimate………

Warm Air Advection!!(shameless plug)

5 year cold season climatology

Axes of snowbands relative to cyclone

Heavy precipitation bands: ● Occluded wrap around portion of cyclone ● Far from warm and cold fronts

Midlevel Theta-e

How to find a TROWAL with a X-Section

Where’s the TROWAL?

November 29-30, 2006 Snow/Ice Storm

Clinton, MO - 18"Butler, MO - 18"Blairstown, MO - 18"Sedalia, MO - 16"Moberly, MO - 15"Boonville, MO - 15“MCI – dustingSouthern Johnson/Jackson – 10”

Horizontal Divergence• Divergence (Convergence) acts frontolytically (frontogenetically), always,

irrespective of isotherm orientation.

F<0 F>0

What is Deformation?

Process by which an air stream tends to stretch or alter the distribution of a tracer such as moisture.

Monitor for banded precipitation

Can result in frontogenesis…..this can force precipitation into narrow bands

One method is two opposing air streams at mid or upper levels are directed towards each other. As they approach they split and fan out in a different pattern.

Sloped Continuity of Frontogenesis (F)6hr ETA forecast valid

18z 15 OCT 01

• Presence of parallel axes of positive frontogenesis sloping upward toward colder air is a common aspect of heavy banded precipitation areas.

600mb

700mb

850mb

Sloped Continuity of Frontogenesis Forcing (cont.)

Several Important Implications:

1) Do NOT rely on any single level of frontogenesis to forecast an area of expected banded snow.

2) Several levels should be assessed for spatial continuity and orientation of F, to see if banding is likely to occur at a given time.

3) The sloped continuity tells us something about the structure of the wind field we can use to infer frontogenesis from single sounding (observed or model derived), VAD, or wind profiler data, and large-scale flow fields.

Role of Deep-Layer Shear Profile

Nature of environmental wind profile may be conducive to “seeder-feeder” mechanism and rapid precipitation generation / elongation of bands during initial development phase.

Cooling-Induced Frontogenesis

Cooling the air increasesthe temp gradient.

Resulting frontogenesisstrengthens vertical motions.

Melting and evaporationlead to convergence atthe surface.

Warm air up & overcold air creates mesoscale version of cold front resulting in banded precipitation.

Begins as rain

Changes to snow

Heaviest pcpn along frontal zone near the0°C surface wet bulbisotherm.

Locate the Deformation axis…… Frontogenesis

Answer

Precipitation Aligns with Axis of Dilation

► When applied within the context of ingredients based forecasting, frontogenesis (F) is useful for assessing potential for banded winter precipitation.

► Doesn’t require a strong cyclone, only a strong baroclinic zone, often developed through horizontal deformation and associated largely with unidirectional vertical shear in the low to mid levels.

► “Sloped continuity” in time and space of F leads most directly to strong banded precipitation.

► Best practice: interrogate 50mb layers of F starting with 800-850mb. AWIPS 4-panel procedure has been created under user ID “winter”.

► Look for unidirectional deep layer shear under 700mb as an indication that F may be significant……evaluate VWP, wind profilers, progged soundings. When frontogenesis is acting, precipitation will be aligned with the deep layer shear vector.

► Look for a TROWAL (warm conveyor belt) and locate its presence in the nw quadrant of the upper system lying within the deformation zone.

Summary

November 29-30, 2006 Snow/Ice Storm

Clinton, MO - 18"Butler, MO - 18"Blairstown, MO - 18"Sedalia, MO - 16"Moberly, MO - 15"Boonville, MO - 15“MCI – dustingSouthern Johnson/Jackson – 10”