The Unique Structure of Atmospheric Mixed Layers over the Northern South China Sea

1. The Unique Structure of Atmospheric Mixed Layers over the Northern South China Sea Paul E. Ciesielski and Richard H. Johnson Colorado State University Presented at Chinese Cultural University on (11 November 2010)

2. Goals for Talk 1. to demonstrate that after the onset of the summer monsoon, the characteristics of atmospheric mixed layers over the Northern South China Sea are quite different from such observations made in other tropical and subtropical regions 2. to consider the ramifications of these unique mixed layer structures

3. Vertical structure of oceanic, tropical atmosphere • 3 distinct layers in atmospheric boundary layer (ABL) • Surface layer (10-100m), strong gradients of winds, T, and moisture • Mixed Layer (ML)- (~500 m deep over oceans, can be much deeper in daytime over land); small vertical gradients • Transition layer (entrainment zone) - ~100 m deep, separates ML from free atmosphere ABL Surface fluxes

4. Characteristics of Atmospheric Boundary Layers • presence of frequent turbulence • turbulence generated by buoyancy (unstable stratification - surface warmer than air) and/or mechanical mixing (due to wind shear) • one of the main mechanisms for generating positive buoyancy is radiative heating at the earth’s surface • this energy surplus is transmitted upward to the atmosphere by conduction and convection • convective transfer usually takes the form of convective plumes

5. Development of mixed layers over land (Stull 1991) sunset sunrise • After sunrise, the mixed layer grows rapidly during mid-morning, then slows before noon as mixing reaches LCL and clouds develop. • Cooling at the earth’s surface results in a noctural stable boundary layer and less-turbulent residual layer containing former mixed layer air.

6. 8 pm 8 am  increases with height indicating a stable stratification Example of mixed layer over land in a monsoon environment Pingdong (May 2008) Signature of a ML on a SkewT diagram - T curve lies along dry adiabat ( is a constant) and Td curve lies along a constant mixing ratio line.

7. From ASTER dropsonde mission in TiMREX on 31 May 2008 • ABL is the medium through which the land/ocean surface and the free atmosphere are coupled. • Accurate prediction of when and where convection will occur depends on how well models handle energy exchanged at the surface and its coupling through the mixed layer to the free atmosphere; this is accomplished by land-surface models and boundary layer parameterizations.

8. In this study a subjective technique was used to identify ML tops (zi). Examples of mixed layer (ML) identification from TOGA COARE (Johnson et al. 2001) θ q • Here zi is the level at which  exhibits an abrupt increase and q a sharp decrease. • Both of these structures must be present in a sounding to be assigned a zi.

9. TOGA COARE was conducted over the West Pacific warm pool region from 11/92 – 02/93. Mean mixed layer statistics for 4 month Intensive Observing Period • from ~700 sondes examined in GATE conducted in the tropical East Atlantic, mixed layers were present in 79%with mean zi of424 m(Fitzjarrald and Garstang 1979) • ~75% sondes have mixed layers with a mean zi ~ 500 m

10. SCSMEX (South China Sea Monsoon Experiment) • Conducted in May and June 1998 • Two sounding networks established over the SCS to determine and contrast properties of convection in two distinct ocean regions. • Large SST gradient over Northern SCS. Integral part networks were two R/Vs, which had hourly flux measurements and 6-h soundings.

11. Summary of Mixed Layer Analysis in SCSMEX • At Ship 1, the frequency and mean ML depth are similar to that observed in other tropical regions. • At Ship 3, the frequency and mean ML depth are considerably fewer and lower than observed in previous studies. (southern) (northern) • These histograms, which show the distribution of mixed layer tops in the vertical at the two ships, clearly show the deeper mixed layers at Ship 1.

12. Time series of ML tops and various other fields F = S + .61cpTsE/L, F - is sfc buoyancy flux, S - sensible heat flux and E - latent heat flux CBH = 125*(T-Td), CBH is an estimate of the Cloud Base Height (m)

13. Time series at Ship 1 (southern ship) • Characteristics similar to other tropical locations being present 83% of time • Cruise 1: suppressed convection and light winds, under these conditions ML variability driven by radiative diurnal effects. ML depths were ~60 m deeper in afternoon and evening soundings • Cruise 2: enhanced convective activity (P=9.1 mm/day) led to greater variability in ML depths. • SSTs continue to rise while Ts slowly decreased likely from convective downdrafts transporting cool, dry air towards surface. • Resulted in larger buoyancy fluxes, stronger mixing and deeper ML.

14. Time series at Ship 3 (northern ship) • Two convectively active periods over northern SCS, the first was associated with monsoon onset. • MLs decreased dramatically following mid-May rainy peak, reflecting the presence of recovering precipitation downdraft wakes in regions of convective rainfall. • Frequency of ML during cruise 1 was 75%, during cruise 2 only 20%. Monsoon onset • Cruise 2: characterized by northward advection of warm, moist air with Ts > SST and RH > 85% which results in small or negative F, weak vertical mixing and a lack of MLs.

15. Boundary layer characteristics at Ship 3 mean without ML (stable) mean with ML 74 79 zi • Each profile with a mixed layer was scaled by zi to preserve its boundary layer structure. • The main distinction between these composites is that those without mixed layers have (1) stronger vertical shear, (2) increases monotonically with height indicating a high degree of stability at low-levels, (3)q decreases monotonically with height.

16. To review: • GATE (East Atlantic) , TOGA COARE (West Pacific warm pool), the southern ship in SCSMEX (SCS) had mixed layers about 75% of the time with average depth ~500 m. Such mixed layer characteristics were also observed prior to monsoon onset at the northern ship in SCSMEX. • Following monsoon onset at the northern ship, winds shifted to southwesteries allowing warm, moist air to flow over cooler water. This in turn resulted in weak or negative buoyancy fluxes and stable conditions. In the presence of this low-level stable air, few mixed layers were observed (20% of time) and they were shallow (340m).

17. To our knowledge, such anomalous mixed layer characteristics, namely, the lack of mixed layer structures, or conversely, the high frequency of stable boundary layers have not been previously observed in a tropical, oceanic environment. • A few key questions: • Are these features unique to SCSMEX, or do such conditions exist in other monsoon regimes and seasons? • What are some implications of these stable boundary layers?

18. TiMREX (Terrain influenced Monsoon Rainfall Experiment) conducted from 15 May – 25 June 2008 had upstream ship soundings and 15 dropsonde mission over SCS. dropsonde flight pattern TiMREX period was characterized by low-level south-southwesterly flow advecting warm air over cooler waters.

19. TiMREX mixed layer analysis • Results from TiMREX mixed layer analysis are similar to post-onset analysis from SCSMEX • The relative lack of mixed layer structures over the Northern SCS as observed in SCSMEX and TiMREX may, in fact, be a common characteristics in the post monsoon onset environment of this region.

20. Unfortunately the ships in TiMREX did not have any flux measurements, but there is a satellite product GSSTF (Goddard Satellite-based Surface Turbulent Fluxes) which provides estimates of latent and sensible heat fluxes over the ocean. • Some details of this product: • Uses SSM/I, TMI and AMSR-E retrievals of SSTs, winds and Wb (where Wb is the PW in the lowest 500 m). This information is combined with NCEP reanalyses surface air temperature and used in the bulk aerodynamic algorithms to compute latent and sensible heat fluxes. • Version 2b of this product provides these fluxes at 1° horizontal resolution over the global oceans on a daily basis for the period from July 1987 to Dec. 2008. (personal communication Chung-Lin Shie)

21. Time series of various fields at SW ship of TiMREX • around May 20, surface winds switch from a northerly to southerly direction indicating the onset of the summer monsoon. • warm, moist air advects over cooler waters (sfc air temperature > SST) • in this environment, surface fluxes drop dramatically • bouyancy fluxes are weak or negative IN SHORT, CONDITIONS SIMILAR TO SCSMEX

22. A few key questions: • Are these anomalous mixed layer characteristics (i.e., high frequency of stable boundary layers at S3) unique to SCSMEX? • Do they occur in other monsoon regimes and seasons? Yes, they also occurred TiMREX. • What some implications of these stable boundary layers? (OKAY, so what?)

23. Time series of wind speed and surface fluxes during SCSMEX at Ship 3 (observed vs JMA) onset • large difference between the JMA reanalysis fluxes and those observed at Ship 3 in the post-onset period suggest: • models had difficulty reproducing the correct BL structures under weak stability and mixing conditions • models are fluxing too much heat and moisture into the atmosphere JMA (7.3) Obs (1.8) JMA (104) Obs (37)

24. NESA • During post-onset convectively active period model rainfalls are nearly double the TRMM and GPCP estimates • excessive model fluxes  too much model rain

25. Rainfall maps (satellite-estimated on left vs JMA model on right) Satellite Model Onset period Post-onset In post onset period (3 - 9 June), JMA analysis has nearly double the rainfall over southern Taiwan as was estimated by TRMM.

26. SUMMARY • SCSMEX and TiMREX mixed layer analyses suggest that following the monsoon onset northward advection of warm, moist low-level air over the cooler waters of the northern SCS results in stable boundary layers with weak vertical mixing of heat and moisture from the surface. • Under these conditions of weak mixing, mixed layers are present ~25% of the time tending to be quite shallow (~350 m). • During SCSMEX, the models appeared to have a difficult time dealing with these stable boundary layers (and weak mixing) as evidenced by their excessive fluxes and rainfall. • Models may need to handle these stable boundary layers differently to accurately predict convection and lower-atmospheric variability. • for additional details see: Ciesielski and Johnson 2009, SOLA

27. Supplementary material

28. Skew-Ts from SCSMEX with (left) and without mixed layers (right) no ML ML • Low-level T lies along dry-adiabat (constant ) • Td lies along constant mixing ratio line • Low-level lapse rate is stable • ( increases with height) • Td decreases with height.

29. scaled mean ML profilesat the ships Created by scaling each sounding, which had an observed mixed layer, by its mixed-layer depth (has effect of preserving the boundary layer structure). • small vertical shear • , and q nearly well mixed • slightly unstable near the surface • shading in lowest ~50 m denotes some uncertainty in sfc obs. • mean profile for S3 is ~1C cooler likely due to cooler SSTs at this site. Ship 1 Ship 3

30. Dropsonde mission on 4 June 2008 (11 LT takeoff) SST 5 Only sondes 7 and 8 from this flight of 12 dropsondes showed a mixed layer structure; 7 and 8 occurred over warmer SSTs. 7 8

31. Example of dropsondes without (#5) and with (#7) a mixed layer structure 5 7