Excitation of rainfall over the Pacific Warm PoolAn alternative view?

R.E. Carbone1,2 and Yanping Li 2,3

1N ational Center for Atmospheric ResearchBoulder, Colorado, USA2University of Hawaii

3Colorado State University

Lower boundary gradients of temperature and humidity are a principle forcing of thermally direct solenoidal circulations, which routinely excite moist convection under

(otherwise) weakly-forced conditions. (sea breezes, mountains, land surface complexity)

Background

JJA ’96 -’07

radar rainfall

(Carbone and Tuttle 2008)

Tiwi Islands Proxy?

Fairly flat, wet, neo-oceanic

Light surface wind, from nearly any direction on a given day, but

most often westerly.

Steady breezeΔTv = ~1-1.5ºC

1st rain nearly always on leeward shore

Lower boundary gradients of temperature and humidity are a principle forcing of thermally direct solenoidal circulations, which routinely excite moist convection under

(otherwise) weakly-forced conditions. (sea breezes, mountains, land surface complexity)

Might SST gradients be similarly influential in favorable oceanic regions?.

Background

Working Hypothesis

A disproportionate fraction of western Pacific warm pool rainfall

events are triggered over mesoscale SST gradients.

Study Domain

2006-2009 Timeseries

Global High Resolution SST (international team; JPL source)• Essentially all relevant satellites• Daily averages, 5km grid, IR, MW, • low-pass filtered by MW sensor footprints

32

29

8km

Soloviev and Lukas Wavelet Analysis

2006-2009 Timeseries

CMORPH Rainfall (NCEP CPC morphing technique)

• Includes essentially all relevant IR and MW instruments/platforms

• 3h/0.25º version of the product used

• GEOSAT IR is employed for temporal-spatial interpolation only • Polar, TRMM, and other low orbit microwave used for QPE.• First rain defined as a minimum of 5 pixels, central maximum, >1 mm/

main use is to detect the onset time and location of rainfall events

SST: Daily Reality vs 4Yr. Climatology

complex, multi-scale, sharp gradients, 2+ ºC dynamic range

SST Forcing and Rainfall Onset

Assumed Conditions Prior to rainfall onset

• typically low surface wind speed (~thermal equilibrium)

• hydrostatic pressure response to warm/cool SST patches• divergence commensurate with cross-gradient flow

Definition of Rainfall “Onset”• continuous events traced backward to first rain location• there are ~10,000 onset events (4 yrs) • events are 3hrs to 4 days in duration

Idealized Gradient

SST

Hydrostatic Pressure Perturbation

Amplitude of SST Gradient

Amplitude of Flow Perturbation

Laplacian (Divergence)

TSST

gHp

b

b

SST

dxx

SSTu

2

2

x

SST

2

2

x

SSTDiv

2019 July 09

Laplacian (yellow-red infer convergence)

Gradient Amplitude

SST

Co-evolution of LSST and SSTGJuly - October 2009

Systematic evolution of SST Laplacian dipoles (top)

SST gradients in the form of filaments, arcs, and closed ring-like shapes (bottom)

Estimation of local convergence Gravity current Approximations:

PBL climo H ~ 700m

assume equivalent He= 350m

assume equivalent

For cylindrically symmetric model,

A breeze-type density current tends to propagate at ~0.5 the speed of a "free" density current

18.1300

3.03508.91~

ms

T

TkgHv

CT oe 3.0~

153

2

1071050

8.12~

22

11

s

r

vrv

rt

A

Acon r

r

15105.3~ seconvergenc

Breeze Approximation:

PBL H=700m, equiv. He= 350m

–LSSTm~ 0.6oC/100km2,

Transport time

LSST

T

gHTT

T

gH

ppwvu

b

byyxx

b

b

yyxxzttyx

''

''

tLSSTT

gHwvu m

b

bzyx

t

15

2510454000

10

6.0

300

3508.91~

scon

Background convergence:

NCEP w at 925 hPa is ~

Mean Convergence ~ 3 x 10-6 s-1

13102 sm

~ 54000s(15h)

Typical monthly distributions of rainfall onset in relation to LSST and SST

January September

Total event rainfall increases with both the size and darkness of the dots

4-yr background distribution of SST

ocea

n

4-yr distribution of rainfall-onset SST and background SST

The ratio of onset frequency and background SST is minimized for the colder SSTs. This ratio decreases to ~1 for SST > 30.4oC.

Event onset (day before ,104 events)

4-yr distribution of rainfall-onset SST and background SST

The ratio of onset frequency and background SST is minimized for the colder SSTs. This ratio decreases to ~1 for SST > 30.4oC. SST decreases after onset.

SST,-LSST distribution

before & after onset events oc

ean

SST,-LSST distribution

before & after onset events

SST,-LSST distribution

before & after onset events

SST,-LSST distribution

Before Onset After Onset

Div Conv Div Conv

75% of rainfall event excitations are associated with antecedent -LSST

4-yr distribution of background SSTG orientation

Large scale southward-directed

gradients are dominant in Boreal Winter.

Distribution trends toward isotropic in

Boreal summer

4-yr distribution of orientation at onset

The onset orientation distribution is more E/NE

directed than background.

Windward Lee

In sfc westerly flow, this suggests a preference for onset of rainfall on the windward “shore” of a warm patch.

+

-

probable excitation location

SST Gradient Orientation; Probable Rainfall Excitation Location

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Warmer SST facilitates mean upward motion by increasing the low level moisture and thereby increasing the instability of the flow to moist convection. The observed low level convergence is determined by the absolute value of local SST (Ramanathan and Collins, 1991; Needlin and Held, 1987 ).

Whether SST or SSTG matters?

The SST gradient is more important than local SST in determining w. The low level circulation driven by the SST gradient leads to convergence of air in the boundary layer over the warmer oceans. (Lindzen and Nigam, 1987).

Both of these statements have basis in fact and are correct in so far as they go, identifying the production of moist static energy at a regional scale.

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Conclusions:

Observations and theory support the notion of “blue water breezes”

The breezes appear to be strong enough to routinely excite deep moist convection in a conditionally unstable atmosphere with low inhibition.

- LSST-estimated local convergence is 10 times the average background

- 75% of rainfall excitations are coincident with convergent LSST

- SST at rainfall onset is only ~0.25ºC higher than background.

But…..

Does any of this matter?

Maybe

The largest events propagate 1- 4 days, 500 to 3000 km.

Most of the rain is remote from the point of excitation

These rainfall systems are likely the most prolific producers of SSTG

There will be a systematic phase speed w.r.t. transient tropical waves, to which the genesis of new rainfall events has a relationship.

We are currently examining the timeseries of SST, SSTG, LSST, and transient deep tropospheric forcing for evidence of systematic coupling/feedback or absence thereof.

Thank you