COHERENT STRUCTURE OF DRIZZLE EVENTS

Chin-Hoh Moeng

National Center for Atmospheric Research

The RF02 Case

• Display time: ~ hour 4.5 – hour 6

• Fields plotted: – Drizzle rate at ~ 4 m– Surface theta flux– theta fluctuation at ~ 4 m

Drizzle near the surface Surface heat flux

Temp near sfc

SUMMARY of the standard case

• only a few drizzle events• these drizzle/cold pool/large sfc flux

events:– persistent– seem to move with the mean wind

Why larger surface flux in cold pool area?

u_1 uncorrelated

sfc flux ~dT

u* fixed' 'w

1

sfc flux ~dT/u_1

D

Case 2:RF02 with Ug=8m/s; Vg=0

• Compute u* --- MO theory

• f = 0; no mean V

• Display time: hr 3—hr 5.5 (every 400s)

Vertical cross-section

D w

th qt

z

Summary-1

• Drizzle events move with wind

- U_mean – U_gal ~ 3 m/s

- about 1 km every 400s

- last longer than 1.5 hours!

near-surface plane view again

larger sfc flx~ smaller phim~ smaller u_1

sfc flx ~ dT/u_1

MO theory

Summary-2

• Cold pool ~ larger sfc heat flux

- not an artifact of a fixed u*

- but is it a robust feature?

Case 3:

Ug=0.5m/s; Vg=0

Very weak wind

Hour 1

Hour 2

Hour 3

Hour 3.5

For the very weak wind case:

Cold pool ~ smaller surface flux !

Why smaller surface flux in cold pool area?

u_1 larger in cold pool

sfc flux ~ 1/u_1

' '0w

1

Case 2:strong wind Case 3: weak wind

cold pool~ large u_1??

?

•circulation flow?•density-current dynamics?

SUMMARY-1

• Drizzle events are rare/intermittent;

due to merger of cold pools?

• Cold pool may associate with either

larger or smaller surface heat flux

(Observations?)

SUMMARY-2

• If cold pool ~ larger sfc flx, drizzle events are larger and last longer (1-2 hours):

drizzle cold pool larger sfc flux

updrafts more drizzle

with rain: solid lines

with rain, ug=8 m/s

with rain, standard ug,vg