Observations of the Structure of Heavily Precipitating Marine Stratocumulus

vanZanten, Margreet C.; Stevens, Björn

doi:10.1175/jas3611.1

Cited by 74 publications

(65 citation statements)

References 24 publications

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“…Consistent with previous POC observations (Petters et al, 2006;Sharon et al, 2006;Van Zanten and Stevens, 2005;Wood et al, 2011a), all five of the POC cases examined here show that cloud droplet number concentrations (N d ) decrease by more than a factor of 8 from the OVC region (Table 4). Whereas the mean cloud-layer N d inside the POCs has a range between 4 and 13 cm −3 across cases, the N d in the surrounding OVC region range from 52 to 207 cm −3 , further showing that POCs can persist when surrounded by a wide range of microphysical environments.…”

Section: Cloud Microphysicssupporting

confidence: 91%

“…Inadequate sampling precludes attributing this difference to the diurnal cycle. The z i, base observed in the VOCALS POCs is substantially deeper than the 700-800 m inversion base heights of POCs observed previously over the northeast Pacific (see Table 2; Van Zanten and Stevens, 2005;Sharon et al, 2006). We conclude that POCs can be produced in boundary layers with a range of inversion heights.…”

Section: Inversion Height and Structuresupporting

confidence: 41%

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Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific

et al. 2014

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Abstract.Five pockets of open cells (POCs) are studied using aircraft flights from the VOCALS Regional Experiment (VOCALS-REx), conducted in October and November 2008 over the southeast Pacific Ocean. Satellite imagery from the geostationary satellite GOES-10 is used to distinguish POC areas, and measurements from the aircraft flights are used to compare aerosol, cloud, precipitation, and boundary layer conditions inside and outside of POCs. Conditions observed across individual POC cases are also compared.POCs are observed in boundary layers with a wide range of inversion heights (1250 to 1600 m) and surface wind speeds (5 to 11 m s −1 ) and show no remarkable difference from the observed surface and free-tropospheric conditions during the two months of the field campaign. In all cases, compared to the surrounding overcast region the POC boundary layer is more decoupled, supporting both thin stratiform and deeper cumulus clouds. Although cloud-base precipitation rates are higher in the POC than the overcast region in each case, a threshold precipitation rate that differentiates POC precipitation from overcast precipitation does not exist. Mean cloud-base precipitation rates in POCs can range from 1.7 to 5.8 mm d −1 across different POC cases. The occurrence of heavy drizzle (> 0 dBZ) lower in the boundary layer better differentiates POC precipitation from overcast precipitation, likely leading to the more active cold pool formation in POCs. Cloud droplet number concentration is at least a factor of 8 smaller in the POC clouds, and the ratio of drizzle water to cloud water in POC clouds is over an order of magnitude larger than that in overcast clouds, indicating an enhancement of collision-coalescence processes in POC clouds.Despite large variations in the accumulation-mode aerosol concentrations observed in the surrounding overcast region (65 to 324 cm −3 ), the accumulation-mode aerosol concentrations observed in the subcloud layer of all five POCs exhibit a much narrower range (24 to 40 cm −3 ), and cloud droplet concentrations within the cumulus updrafts originating in this layer reflect this limited variability. Above the POC subcloud layer exists an ultraclean layer with accumulationmode aerosol concentrations < 5 cm −3 , demonstrating that in-cloud collision-coalescence processes efficiently remove aerosols. The existence of the ultraclean layer also suggests that the major source of accumulation-mode aerosols, and hence of cloud condensation nuclei in POCs, is the ocean surface, while entrainment of free-tropospheric aerosols is weak. The measurements also suggest that at approximately 30 cm −3 a balance of surface source and coalescence scavenging sinks of accumulation-mode aerosols maintain the narrow range of observed subcloud aerosol concentrations.

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Section: Cloud Microphysicssupporting

confidence: 91%

Section: Inversion Height and Structuresupporting

confidence: 41%

Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific

et al. 2014

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“…Cloud-top temperature data is available for all of the times, while optical depths and reflectivities are available only for the daylight hours. We estimate the cloud liquid water path from the optical depth using the relationship L ϭ 2 ⁄3/r e (e.g., Stephens 1978), where r e is the effective radius, which we fix at the ISCCP default value of 10 m. This is consistent with values measured on flight days (vanZanten et al 2005). Cloudtop pressures from the ISCCP product represent a synthesis of atmospheric soundings derived from the Television Infrared Observation Satellite Operational Vertical Sounder and, because of biases in the latter, tend to be underestimated (Wang et al 1999).…”

Section: Satellite Measurementssupporting

confidence: 65%

“…The former is taken from vanZanten et al (2005) when available and using a sounding composited from all the flight data otherwise, while the latter is based on differences in liquid water content in the cloud-top versus cloud-base circles for the seven entrainment flights, and is not corrected for cloud fraction (which in any case was very near unity for all flights).…”

mentioning

confidence: 99%

On the Structure of the Lower Troposphere in the Summertime Stratocumulus Regime of the Northeast Pacific

Stevens

Beljaars

Bordoni

et al. 2007

Monthly Weather Review

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Data collected in situ as part of the second field study of the Dynamics and Chemistry of Marine Stratocumulus field program are used to evaluate the state of the atmosphere in the region of field operations near 30°N, 120°W during July 2001, as well as its representation by a variety of routinely available data. The routine data include both the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and NCEP-NCAR reanalyses, forecasts from their respective forecast systems (the Integrated and Global Forecast Systems), the 30-km archive from the International Satellite Cloud Climatology Project (ISCCP), the Quick Scatterometer surface winds, and remotely sensed fields derived from radiances measured by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), the Advanced Microwave Sounding Unit, and the Advanced Very High Resolution Radiometer. The analysis shows that outside of the boundary layer the state of the lower troposphere is reasonably represented by the reanalysis and forecast products, with the caveat of a slight warm bias at 850 hPa in the NCEP-NCAR products. Within the planetary boundary layer (PBL) the agreement is not as good: both the boundary layer depth and cloud amount are underpredicted, and the boundary layer temperature correlates poorly with the available data, which may be related to a poor representation of SSTs in this region of persistent cloud cover. ERA-40 also suffers from persistently weak zonal winds within the PBL. Among the satellite records the ISCCP data are found to be especially valuable, evincing skill in both predicting boundary layer depth (from cloud-top temperatures and TMI surface temperatures) and cloud liquid water paths (from cloud optical depths). An analysis of interannual variability (among Julys) based on ERA-40 and the 1983-2001 ISCCP record suggests that thermodynamic quantities show similar interannual and synoptic variability, principally concentrated just above the PBL, while dynamic quantities vary much more on synoptic time scales. Furthermore, the analysis suggests that the correlation between stratocumulus cloud amount and lower-tropospheric stability exhibits considerable spatial structure and is less pronounced than previously thought.

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“…Their origin is not fully understood, but significant differences in aerosol and cloud microphysical properties between POCs and the surrounding clouds are typically observed Petters et al, 2006;Sharon et al, 2006;Wood et al, 2008Wood et al, , 2011a. VanZanten and Stevens (2005), Sharon et al (2006), and Wood et al (2011a) found that POCs are characterized by enhanced drizzle, although drizzle itself seem insufficient for transition from closed-to open-cell circulations Wood et al (2011a).…”

Section: Published By Copernicus Publications On Behalf Of the Europementioning

confidence: 99%

Limited-are a modelling of stratocumulus over South-Eastern Pacific

et al. 2012

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Abstract. This paper presents application of the Weather Research and Forecasting (WRF) model to limited-area modeling of atmospheric processes over the subtropical southeastern Pacific, with the emphasis on the stratocumulustopped boundary layer. The simulations cover a domain from the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field project conducted in the subtropical south-eastern Pacific in October and November 2008. We focus on a day where the UK's BAe-146 research aircraft encountered Pockets of Open Cells (POCs) at the very western edge of its flight track, rather than on the entire campaign as investigated in previous limited-area modeling studies. Model results are compared to aircraft observations with the main conclusion that the simulated stratocumulus-topped boundary layer is significantly too shallow. This appears to be a combination of an already too shallow boundary layer in the dataset used to provide initial and lateral boundary conditions, and the inability of the WRF model to increase the boundary-layer height. Several sensitivity simulations, applying different subgrid-scale parameterizations available in the model, a larger computational domain and longer simulations, as well as a different dataset providing initial and lateral boundary conditions were all tried to improve the simulation. These changes appeared to have a rather small effect on the results.The model does simulate the formation of mesoscale cloud-free regions that one might consider similar to Pockets of Open Cells observed in nature. However, formation of these regions does not seem to be related to drizzle-induced transition from open-to closed-cell circulations as simulated by LES models. Instead, the cloud-free regions appear to result from mesoscale variations of the lower-tropspheric vertical velocity. Areas of negative vertical velocity with minima (a few cm s −1 ) near the boundary layer top seem to induce direct evaporation of the cloud layer. It remains to be seen in LES studies whether the mechanism seen in the model is realistic or if it is simply an artifact of interactions between resolved and parameterized processes.

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Observations of the Structure of Heavily Precipitating Marine Stratocumulus

Cited by 74 publications

References 24 publications

Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific

Aircraft observations of aerosol, cloud, precipitation, and boundary layer properties in pockets of open cells over the southeast Pacific

On the Structure of the Lower Troposphere in the Summertime Stratocumulus Regime of the Northeast Pacific

Limited-are a modelling of stratocumulus over South-Eastern Pacific

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