The Propagation Of Mesoscale The Propagation Of Mesoscale Convective ComplexesConvective Complexes

The Corfidi Vector The Corfidi Vector MethodMethod

Mike PelusoHoward Manges

20 April 2006

Corfi.. What?Corfi.. What?• Corfidi Vectors are a tool that operational

meteorologists can use that MAY help more accurately predict:– the motion of MCS’s

• More specifically the meso beta convective elements (MBE)

– heavy rain– flash flooding– damaging winds

What do these vectors represent?

The “Original” Corfidi MethodThe “Original” Corfidi Method

• Sum of two vectors:– Advective – The mean flow of the

850-300mb cloud-layer wind Newton and Katz(1958) and Chappell (1986)

– Propagation – Represents new cell formation opposite in direction to the low-level jet but equal in magnitude

Merritt and Fritsch, Corfidi et.al (1996)

Advective ComponentAdvective Component• Simplified example of

vertical wind profile throughmean cloud layer.

• Mean flow vector would be

270°/30kts

• Mathematically: (Vcl)

(V850+V700+V500+V300 )/4

850-hPa

700-hPa

500-hPa

300-hPa

Propagation ComponentPropagation Component

• Represents new cell formation opposite in direction to the low-leveljet but equal in magnitude

850-hPa wind barb• LLJ 240°/30kts

• Therefore the vector representative of propagation will be from 60°/30kts

Addendum to Addendum to Corfidi Vector MethodCorfidi Vector Method

• A major shortcoming was found in this method.

• MCS propagation is not as straight forward as 2 vectors being added or subtracted.

• Case studies indicate that the cold pool spatial relationship to the low level inflow (LLJ) has an additional effect on propagation patterns.

Thus….

• Assuming that new cell development (propagation) is ALWAYS opposite the LLJ is incorrect.

• The effect of this spatial relationship of the meso-high to the inflow (LLJ) plays a major role in new cell development– Cumulogenesis occurs along the leading

edge of the cold pool and hence the region of greatest LLC Corfidi, WAF (2003)

Cold Pool ComponentCold Pool Component

Cold Pool ComponentCold Pool Component

Corfidi et.al (2003)

Modes of MCS PropagationModes of MCS Propagation• Two Types

• Quasi-stationary –a.k.a back-building or upwind

propagating–Significant flood threat

• Progressive –a.k.a. forward or downwind

propagating–Reduced threat of flooding

• Schematically……

Movement of MCS-BackbuildingMovement of MCS-BackbuildingMean flow vector (V

adv )

LLJ

(Vpr

op.)

Region of Greatest LLC Favored MCS

motion700-hPa

500-hPa

850-hPa

300-hPa

•Region of greatest LLC dictates MCS motion.•Assuming sufficient instability and favorable thermodynamic environment Corfidi (2003)

• Called “training” storms• Provide significant risk for

flash flooding.• Occurs when propagation

exceeds advection– The result of mean cloud

layer winds being nearly parallel to outflow boundary.

– Strong normal component of the LLJ

Mean flow vector (Vadv )

LLJ

(Vpr

op.)

Region of Greatest

LLC

Favored MCS

motion

Corfidi Wea. Forecasting 12/03

Movement of MCS-BackbuildingMovement of MCS-Backbuilding

Movement of MCS- ProgressiveMovement of MCS- Progressive

Mean flow vector (Vadv)

LLJ (Vprop )

LLC

Reg

ion

700-hPa

500-hPa

850-hPa

300-hPa

Favored MCSmotion

Movement of MCS-ProgressiveMovement of MCS-Progressive• Storm motion is opposite the

LLJ along the downwind side of cold pool.

• Vector length is directly related to rate of cumulogenesis along gust front.

• Not usually associated with heavy rainfall or flooding.

Mean flow vector (Vadv)

LLJ (Vprop )

LLC

Reg

ion

MCSmotion

Forecast Magnitude of MBE Forecast Magnitude of MBE motionmotion

• A straight forward calculation can be utilized to determine the magnitude of motion for the MBE (Mesoscale Beta Element).

• |VMBE| = { |Vcl|2 + |-VLLJ|

2 -2( |Vcl| . |-VLLJ| ) cos Φ}1/2 • Φ = angle between mean cloud layer flow and LLJ

MBE Forecast DirectionMBE Forecast Direction

• φ = sin-1

• φ = angle between mean cloud layer flow and MBE motion

{(|-VLLJ| . sinΦ) / |VMBE|}

850-300mb thickness contours

MBE Motion

Mean flow vector (VCL)

(-VLLJ ) =(VPROP)

MCC φ

The Role of Dry AirThe Role of Dry Air• Dry air allows for:

– increased evaporative cooling – Sublimation and melting

• These create density differences and stronger DD’s

• Strong cold pools demonstrate the potential for favoring progressive MCS movement

• Weak cold pools favor Quasi-stationary or Backbuilding MCS’s that occur in moist or nearly saturated low-level environments.

SummarySummary• 3 factors influencing motion:

– Advective component – Propagation component– Cold pool (outflow boundary).

• The spatial relationship between them influences MCS motion

• 2 propagation modes:– Upwind or Backbuilding– Downwind or Progressive

• Distinguished by orientation of the gust front to the mean wind and the LLJ.

• 1 grade A+

ReferencesReferences

• Corfidi, Stephen F., 2003: Cold Pools and MCS Propagation: Forecasting the Motion of Downwind-Developing MCS’s. Wea. Forecasting., 18, 997-1017.

• Merritt, J.H. and Fritsch, J.M., 1994: Predicting the Movement of Mesoscale Convective Complexes. Wea. Forecasting., 11, 41-46.

• Corfidi, Stephen F., 1998: Forecasting MCS Mode and Motion. Prepared for the 19th Conference Severe Local Storms, Minneapolis, MN 1998.