A Case Study of Decoupling in Stratocumulus Xue Zheng MPO, RSMAS 03/26/2008.

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Presentation transcript:

A Case Study of Decoupling in Stratocumulus Xue Zheng MPO, RSMAS 03/26/2008

Outline Motivation Observation Numerical simulation Summary and future work

Synoptic History January 25 th, 2008 A cold front passed this region before. Dominated by high pressure SST~ o C www-angler.larc.nasa.gov

Flight Plan Two soundings Six levels at constant pressure surface: above inversion level, cloud top, cloud layer, cloud base, mixed layer, near the surface Each level has 10-minute observation

Soundings Data Temperature is cooler at most part of BL after about 1 hour Inversion level rises about 100m Two mixed layers on the second sounding Decoupling process between 600m and 800m UTC 16:55-17:05 UTC 18:17-18:27 Mean values Decoupling layer

Liquid Water Content and Relative Humidity Two separated layers with liquid water content A relative dry region between those indicates the discontinuous feature is building up Probably due to the specifically location of this sounding

Surface Wind and Wind Shear 30m 1550m

Buoyancy Flux and Turbulence Momentum Flux Negative buoyancy flux within the decoupling layer Near-surface buoyancy flux Vertical velocity variance decreases within cloud layer Strong turbulence near the surface Six layers

Large Eddy Simulation System of Atmospheric Modeling (SAM6.6.5a, Khairoutdinov) –Initial Soundings –Vertical profiles of large scale U, V, W (NCEP) –SST, latent heat flux, sensible heat flux, and surface drag (Fairall, et al 1996) –128X128X96 (∆x=100m) –3-Hour simulation with 0.5s time step Numerical Experiment –The influence of surface fluxes –The influence of large scale tendencies

Preliminary Results Low level warms up (~0.2K/2hours) Inversion level rises Simulated cloud layer extends to 1500m level From simulated cloud base TKE increases and reaches maximum within the could layer (K) Cloud liquid water (g/kg) model Obs TKE (m 2 /s 2 )

Summary Observation: –A stratocumulus case with strong wind and turbulence near the surface –Decoupling at sub-cloud layer Numerical simulation ignoring large scale tendencies: –Decoupling at sub-cloud layer is not simulated –Lower level is warming –Thicker cloud layer is simulated –TKE increases within cloud layer instead of decreasing