Synoptic Environments Associated with the Training of Convective Cells

Aylward and Dyer 2010

The main focus is investigating the dynamics resulting in synoptically forced training convective rainfall

Synoptic conditions necessary for the generation of an SCEPT event

Define a SCEPT event to which precipitation rate and duration period satisfy a given criteria

Gather selected atmospheric variables which have a statistically significant correlation to SCEPT events

Allow forecasters to better locate regions of training convection based on findings

Introduction

Synoptically forced Convective Extreme Precipitation Training

Events linked with frontal boundaries (typically warm fronts)◦ Isentropic lift

Deep convection develops in a baroclinic environment where differential cyclonic vorticity advection is present◦ Typically ahead of a short-wave trough or jet streak

500 hPa trough nearly stationary during training events LLJ helps initiate convection and heavy rainfall

SCEPT

Near-neutral to weakly positive differential vorticity advection

Location of 850 hPa maximum moisture convergence Precipitable water maximum (moisture tongue) good

indicator of heavy rainfall High precipitable water content (>1.42 in) required Midlatitude and upper-level lows positioned to the W

or NW Strong surface moisture convergence and UVV at

700 hPa

SCEPT

Multisensor Precipitation Estimates (MPE) using a Z-R relationship to diagnose precipitation rate for each event

SCEPT event location points were gathered from the National Centers for Environmental Prediction – National Center for Atmospheric Research (NCEP – NCAR) while atmospheric variables examined for each event were gathered from the North American Regional Reanalysis (NARR)

Data Acquisition

Methodology

Rainfall rates must be exceeding 17 mm/h for at least 3 consecutive hours to account as an event

Event location determined by where event occurred ◦ If multiple events occurred, location determined by data

cluster NARR data used to analyze synoptic conditions of

each event◦ diagnostic fields included geopotential heights (300, 500, and

850 hPa), 500 hPa differential vorticity, 300 hPa jet streaks, UVV at 700 hPa, 850 hPa temperature fields, and precipitable water

Methodology

For each height field, the trough axis either deepening, weakening or neutral and tilted positively, negatively, or neutral

A shortwave (long-wave) trough <8000km (>8000km) in amplitude and wavelength

Closed lows were recorded at the pressure level(s)

Axis of greatest precipitable water was noted as well

36 SCEPT events during from 2004 - 2006

Methodology

1) Closed Upper Level Low (CULL)2) Upper Level Trough (ULT)3) 850 hPa Trough-Low (850TL)

3 Cases

Contained a 500 hPa closed low Positively tilted & strengthening trough at

850 and 500 hPa Lows at 300 and 500 hPa nearly stacked 850 hPa jet generally over SCEPT event,

moving toward the NW Precipitable water averaged 36.2mm

CULL

Moisture convergence occurred along the flow of the 850 hPa jet from SW-NE

The SCEPT event was located along the warm front of the associated MLC

Accompanied by WAA Generally occurred from 7Z -16Z during

the cool season months

CULL

CULL

A) 300 hPa, B) 500 hPa, C) 850 hPa, D) Surface observations

500 hPa trough without a closed low Maximum UVV at 700hPa occurred along

the flow 850 hPa jet advecting moisture from the

SSW Precipitable water average 37.1mm

ULT

Accompanied by WAA 300 hPa trough located 915 km west of

event Generally occurred in the warm sector of

an MLC Strong, positively tilted troughs existed at

300 and 500 hPa

ULT

ULT

A) 300 hPa, B) 500 hPa, C) 850 hPa, D) Surface observations

Weak UL flow with an 850 hPa shortwave trough or low present

Trough/low averaged 565km to west of SCEPT event

Moisture convergence was greatest along flow of 850 hPa jet

Advection occurred from the SSW

850TL

Little to no UL forcing existed and no relationship could be found with the 500 hPa and 300 hPa flows

Precipitable water averaged 42.2mm Primarily a warm season phenomena

850TL

850TL

A) 300 hPa, B) 500 hPa, C) 850 hPa, D) Surface observations

Results

Each case has its own characteristics◦ However, Strong UVV at 700 hPa, WAA and High Moisture

content common to all Of the 36 events, 47% had a closed 500 hPa low

and 25% had a longwave 500 hPa trough ◦ Overall, 72% of the SCEPT events were synoptically forced with

a long-wave trough

SCEPT event locations were concentrated across the western region of study◦ Gulf moisture and Atlantic moisture transport influenced

events

Results

500 hPa trough was slightly positive or neutrally tilted for CULL and ULT events

SCEPT events developed right of the 300 hPa jet streak

Surface moisture convergence maximum within 230 km of event location, training parallel to 850 hPa flow

700 hPa UVV maximums within 270 km of SCEPT event with training at the 700 hPa on a WSW to ENE orientation

Conclusion

Why SCEPT events occurred during the early morning hours associated with the least thermal instability

Possibility of a weakening LLJ during morning leading to system speed reduction

Longer research period needed examining more variables

Finer look surface moisture convergence and UVV to reduce error

Discussion

Questions