The diurnal cycle and MCSs:

Is the Eastern Piedmont like the High Plains?

Matthew D. Parker, NC State University

(with David A. Ahijevych, NCAR)

CSTAR planning meeting

NWS-RAH, 3 August 2006

Study uses NOWRAD dBZ composites:dt=15 min; dx=dy=2 km

• 1996-2000, 2002-2005

• 365 days per year (not just warm season)

• 66-92°W; 32-42°N

Surface elevation (m)

The “Rockies-High Plains, diurnal-nocturnal, convection-MCS cycle” is well-established…

What about the eastern U.S.?

• WV/VA/NC: Appalachian spine rises roughly 1 km above Atlantic coastal plain

Surface elevation (m)

Surface elevation (m)

The “Rockies-High Plains, diurnal-nocturnal, convection-MCS cycle” is well-established…

What about the eastern U.S.?

• WV/VA/NC: Appalachian spine rises roughly 1 km above Atlantic coastal plain

• Convective echoes diurnally maximized over Appalachian chain; observed to propagate downslope toward the east

• In the summer months, boundary layer moisture is readily available over the coastal Plain

• As the sun sets, eastward-moving convection arrives at the comparatively warm coastal waters of the

Atlantic, with the Gulf Stream only slightly farther offshore

Principal concern:

The main afternoon maximum in the diurnal cycle overwhelms any other signals in the data!

Storm frequency

(a value of 0.02 means “dbz ≥ 40” 2% of the time at a given pixel)

Hovmoller diagram

Moving eastward in time

Moving westward in time

Principal concern:

The main afternoon maximum in the diurnal cycle overwhelms any other signals in the data!

One approach:

Remove the diurnal cycle from the data

EOF1 = Principal diurnal cycleEOF2 =

“Earliness” (+) or “lateness” (-) of local max

EOF1 = Principal diurnal cycle

EOF2 = “Earliness” (+) or “lateness” (-) of local max

Residual (once EOF1/diurnal cycle is removed)

Residual storm frequency (once EOF1/diurnal cycle is removed)

N. Carolina coastCumberland Plateau (TN)

Blue Ridge Mts & Gt. Smoky Mts (NC,VA,WV, MD)

Diurnal max is early and pronounced

Residual storm frequency (once EOF1/diurnal cycle is removed)

9 m/s

13 m/s

9 m/s

13 m/s

10 m/s

10 m/s

-8 m/s

Carbone et al. streaks: 9-13 m/s E. of Miss.

Preliminary statistics based on 2003-2005

A convective episode has echoes ≥40 dBZ, lasts ≥ 3h, and spans ≥ 100 km

Hot off the presses: caveat emptor!

15 episodes present

15 episodes present

Preliminary statistics based on 2003-2005

A convective episode has echoes ≥40 dBZ, lasts ≥ 3h, and spans ≥ 100 km

• 740 episodes (~247/year)

• 354 “long” (≥ 250 km span) episodes (~118/year)

• avg. span 281 km, avg. duration 6.3 h, avg. speed 11.1 m/s

• # starting W. of the Blue Ridge front: 478

• # of these ending E. of Blue Ridge front: 106 (22%; ~35/year)

• avg. ending point for episodes initiated over the Blue Ridge mountains: ~Raleigh

Hot off the presses: caveat emptor!

Preliminary statistics based on 2003-2005

A convective episode has echoes ≥40 dBZ, lasts ≥ 3h, and spans ≥ 100 km

• Peak months for Blue Ridge crossers (in order): July, May, August, June

• Peak initiation times for Blue Ridge crossers: 17-21 UTC (peak @ 18Z)

Hot off the presses: caveat emptor!

Preliminary statistics based on 2003-2005

A convective episode has echoes ≥40 dBZ, lasts ≥ 3h, and spans ≥ 100 km

• Fraction of episodes that survive past (E. of) the Blue Ridge front:

• starting west of Cumberland Plateau: 6%

• starting over Cumberland Plateau: 25%

• starting over the Blue Ridge mountains: 75%

Hot off the presses: caveat emptor!

May be an under-estimate because the episode detection scheme doesn’t handle dissipation and reformation particularly well.

Preliminary statistics based on 2003-2005

A convective episode has echoes ≥40 dBZ, lasts ≥ 3h, and spans ≥ 100 km

• # of Westward-moving episodes: 64 (9%; ~21/year)

• # of these initiated in the coastal zone: 36 (56%; ~12/year)

Hot off the presses: caveat emptor!

Storm frequency(a value of 0.02 means “dbz ≥ 40” 2% of the time at a given pixel)

“other”

MCS generated by Apps

Generation by Apps + synoptic features

“Remnants” cross Apps and phase with new convective dev.

(classic)

dBZ

MCS generated by Apps

dBZ

Generation by Apps + synoptic features

dBZ

“Remnants” cross Apps and phase with new dev

dBZ

Remnants of T.S. “Cindy”

“other”

Concluding thoughts (work in progress)• Eastward-moving convective “episodes” signal (Carbone et al. 2002) is difficult to detect in the eastern U.S. because the afternoon maximum overwhelms every other mode in terms of amplitude

• Preliminary results: by removing the principal diurnal cycle, the eastward motion of orographically generated convection is revealed

• A particularly interesting problem in this part of the country…

afternoon convection along the Atlantic sea breeze front may also persist, possibly leading to cell interactions

afternoon maximum in convection over the Appalachians appears to be phased with the arrival of eastward-moving remnant precipitation systems that were produced during the previous day over the central U.S.

timing of the convective systems' arrival over the Gulf Stream appears to be phased with the expected offshore maximum in nocturnal convection that is observed throughout the southeastern U.S. coastal waters

Supplemental images