drizzle and cloud cellular structures in marine stratocumulus over the southeast pacific: model...
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Drizzle and Cloud Cellular Structures in Marine Stratocumulus over the Southeast Pacific: Model Simulations Hailong Wang 1,2,3, Graham Feingold 2, Rob wood 4, and Jan Kazil 2,3
1 Pacific Northwest National Laboratory2 NOAA Earth System Research Laboratory3 CIRES, University of Colorado 4 University of Washington
Introduction
Results I: Diurnal Variations
Results II: on the Formation of POCs
A13J-0423
Vast areas of subtropical oceans are covered by stratocumulus (Sc) clouds, regulating solar heating to the ocean; It is argued that a 4% increase in the amount of Sc could offset the warming caused by CO2 doubling.
Satellite imagery of Sc shows the recurrence of open- and closed-cell patterns that have distinct differences in cloud albedo; What controls the formation of Pockets of Open Cells (POCs) that are embedded in closed cells has been unclear.
Based on VOCALS-REx observations, cloud-resolving model simulations are used to examine the effectiveness of potential factors at promoting drizzle and POC formation.
Fig. 1: Vertical profiles of potential temperature θ and water vapor mixing ratio qv observed during C-130 research flight 6 (RF06; POC-drift mission) in overcast (black circles), POCs (hexagons) and heavily precipitating boundaries (plus signs) region; Constructed “drier” (solid lines) and “wetter” (dotted lines) profiles are used to initiate model simulations.
We use the Weather Research and Forecasting (WRF) model (version 3.1) with a two-moment microphysics and the monotonic advection scheme (Feingold et al. 1998; Wang et al. 2009; Wang and Feingold 2009a,b).
Six experiments (D30, D150, D500, W30, W150 and W500) are performed to examine diurnal variations of cloud properties under “drier” (D) and “wetter” (W) conditions (Fig. 1) for given initial cloud condensation nuclei (CCN) number concentration s of 30, 150 and 500 cm-3 in 60×60 km2 domain for 36 hours.
Fig. 2: diurnal variations of (a) LWP, (b) cloud fraction, (c) cloud-base rain rate and (d) drop (solid lines) and total particle (dotted lines) number concentrations; Drizzle is more sensitive to temperature and moisture perturbations (Δθ=-1 K; Δqv=+0.9 g kg-1) than to a five-fold decrease in CCN (150 vs. 30 cm-3); The Sc deck that breaks up due solely to solar heating recovers at night; Precipitation is open-cell cases deplete CCN to an extent that cloud formation is significantly suppressed.
Fig. 3: same as Fig. 2 but for D30 and other four experiments with different CCN source strengths (3.6 cm-3 h-1 for SA1 and SP1; 0.72 cm-3 h-1 for SA2 and SP2) and start time (“A” and “P” indicates 6 am and 6 pm respectively; a weak source starting from the late afternoon is sufficient to maintain clouds, suggesting that some local/remote CCN sources are necessary for POCs to endure for days.
One control (CTRL) and eight sensitivity experiments are performed to explore the effectiveness of perturbations (as indicated on top of each panel in Fig. 4) in CCN, moisture and/or temperature, surface sensible and/or latent heat fluxes in the domain center (outlined by dotted lines) at promoting drizzle and POC formation.
Control experiment Δqv: 0~0.9 g kg-1CCN: 30~150 cm-3
Δqv+ Δθ Δθ: -1~0 K2x(Δqv+ Δθ)
LFX: 150~300 W m-2 SFX: 15~30 W m-2 ΔLFX + ΔSFX
CCN:150 cm-3 “Drier sounding”
148 W m-2 3 W m-2
Fig. 4: Cloud albedo fields at t = 6 h; Drizzle and POC formation respond faster to the CCN reduction than to the moisture increase, but the latter generates stronger and more enduring drizzle; The temperature decrease in the center promotes cloud thickening outside of the perturbed area even if it’s combined with the moisture increase; An increase of surface sensible heat flux can thicken local clouds and promote drizzle, even more effectively when latent heat flux is together enhanced, although the sole increase of latent heat flux does not help much in 6 hours.
Fig. 6: Snapshots of 200-m vertical velocity w (left), 200-m qv (middle) and cloud albedo (right) at t = 3, 6 and 9 h from an experiment that has the same initial conditions as in CTRL but with “2x(Δqv+ Δθ)” perturbations in the middle 15-km wide stripe; contours of rain rate (0.2, 5, and 20 mm day-1) are superimposed on the qv fields.
A circulation originated from the perturbations transports lower-level moist air out of the perturbed area. Air converges along the gust fronts and clouds are thickened; however, massive convergence and drizzle is triggered in a remote area where lower-level moisture transport is blocked by counter flow. This represents a potential mechanism for POC formation in closed cells near a broad region of open cells where strong precipitation can initiate such a circulation (Wang and Feingold 2009b).
Summary
References
Acknowledgements: This work was mostly supported by NOAA’s Climate Goal program when HW was at CIRES/NOAA ESRL; HW thanks PNNL for support to having it finished up. The authors thank the team of scientists, engineers, and support staff for their efforts in making VOCALS-Rex such a success.
Fig. 5: Same as Fig.1 but with mean profiles (t = 3-9 h) in open cells (D30; dotted lines) and closed cells (D500; solid lines) superimposed; Thick gray lines represent initial “drier” sounding; Precipitation in the open-cell case moistens and cools the lower-level air to the observed states in open-cell region.
Feingold, G., et al., 1998. Atmos. Res., 47–48, 505–528.
Wang, H., and G. Feingold, 2009a. J. Atmos. Sci., 66, 3237-3256.
Wang, H., and G. Feingold, 2009b. J. Atmos. Sci., 66, 3257-3275
Wang, H., W. C. Skamarock and G. Feingold, 2009. Mon. Wea. Rev.,137, 2547-2558.
Both open- and closed-cell clouds exhibit distinct diurnal variations;
A source is necessary to balance the depletion of CCN by precipitation to maintain open cells;
Drizzle over a broad region is more sensitive to the initial meteorological perturbations than to the microphysical perturbation;
Marked differences in lower-level temperature and moisture between overcast and open-cell regions could result from precipitation in open cells;
A local CCN reduction and moisture increase can initiate drizzle and open cells embedded in closed cells;
A temperature decrease can induce a circulation that prevents local drizzle formation but promotes it in a remote area; This represents a potential mechanism for POC formation in the Southeast Pacific stratocumulus region whereby the circulation is triggered by strong precipitation in adjacent broad regions of open cells;