Excitation of rainfall over the Pacific Warm Pool An alternative view?

1. Excitation of rainfall over the Pacific Warm PoolAn alternative view? R.E. Carbone1,2 and Yanping Li 2,3 1N ational Center for Atmospheric Research Boulder, Colorado, USA 2University of Hawaii 3Colorado State University

2. Background 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) JJA ’96 -’07 radar rainfall (Carbone and Tuttle 2008)

3. Tiwi Islands Proxy? Fairly flat, wet, neo-oceanic Light surface wind, from nearly any direction on a given day, but most often westerly. Steady breeze ΔTv= ~1-1.5ºC 1st rain nearly always on leeward shore

4. Background 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?. Working Hypothesis Adisproportionate fraction of western Pacific warm pool rainfall events are triggered over mesoscale SST gradients.

5. Study Domain

6. 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 Soloviev and Lukas Wavelet Analysis 8km

7. 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

8. SST: Daily Reality vs 4Yr. Climatology complex, multi-scale, sharp gradients, 2+ ºC dynamic range

9. 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

10. Idealized Gradient SST Hydrostatic Pressure Perturbation Amplitude of SST Gradient Amplitude of Flow Perturbation Laplacian (Divergence)

11. 2019 July 09 Laplacian (yellow-red infer convergence) Gradient Amplitude SST

12. 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)

13. Estimation of local convergence Breeze Approximation: PBL H=700m, equiv. He= 350m –LSSTm~ 0.6oC/100km2, Transport time 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 ~ 54000s(15h) Background convergence: NCEP wat 925 hPa is ~ MeanConvergence ~ 3 x 10-6s-1

14. 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

15. 4-yr background distribution of SST ocean

16. 4-yr distribution of rainfall-onset SST and background SST Event onset (day before,104 events) The ratio of onset frequency and background SST is minimized for the colder SSTs. This ratio decreases to ~1 for SST > 30.4oC.

17. 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.

18. SST,-LSST distribution before & after onset events ocean

19. SST,-LSST distribution before & after onset events

20. SST,-LSST distribution before & after onset events

21. SST,-LSST distribution Before Onset After Onset Div Conv Div Conv 75% of rainfall event excitations are associated with antecedent -LSST

22. 4-yr distribution of background SSTG orientation Large scale southward-directed gradients are dominant in Boreal Winter. Distribution trends toward isotropic in Boreal summer

23. 4-yr distribution of orientation at onset SST Gradient Orientation; Probable Rainfall Excitation Location probable excitation location + - Windward Lee The onset orientation distribution is more E/NE directed than background. In sfc westerly flow, this suggests a preference for onset of rainfall on the windward “shore” of a warm patch.

24. Whether SST or SSTG matters? • 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 ). 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. 25

25. 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….. 26

26. 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.

27. Thank you