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  • Author or Editor: Virendra Ghate x
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Bruce Albrecht, Ming Fang, and Virendra Ghate

Abstract

Observations made at the Atmospheric Radiation Measurement (ARM) Program’s Southern Great Plains (SGP) site during uniform nonprecipitating stratocumulus cloud conditions for a 14-h period are used to examine cloud-top entrainment processes and parameterizations. The observations from a vertically pointing Doppler cloud radar provide estimates of vertical velocity variance and energy dissipation rate (EDR) terms in the parameterized turbulent kinetic energy (TKE) budget of the entrainment zone. Hourly averages of the vertical velocity variance term in the TKE entrainment formulation correlated strongly (r = 0.72) with the dissipation rate term in the entrainment zone, with an increased correlation (r = 0.92) when accounting for the nighttime decoupling of the boundary layer. Independent estimates of entrainment rates were obtained from an inversion-height budget using the local time derivative and horizontal advection of cloud-top height together with large-scale vertical velocity at the boundary layer inversion from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis model. The mean entrainment rate from the inversion-height budget during the 14-h period was 0.74 ± 0.15 cm s−1 and was used to calculate bulk coefficients for entrainment parameterizations based on convective velocity scale w* and TKE budgets of the entrainment zone. The hourly values of entrainment rates calculated using these coefficients exhibited good agreement with those calculated from the inversion-height budget associated with substantial changes in surface buoyancy production and cloud-top radiative cooling. The results indicate a strong potential for making entrainment rate estimates directly from radar vertical velocity variance and the EDR measurements.

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Virendra P. Ghate, Mark A. Miller, Bruce A. Albrecht, and Christopher W. Fairall

Abstract

Stratocumulus-topped boundary layers (STBLs) observed in three different regions are described in the context of their thermodynamic and radiative properties. The primary dataset consists of 131 soundings from the southeastern Pacific (SEP), 90 soundings from the island of Graciosa (GRW) in the North Atlantic, and 83 soundings from the U.S. Southern Great Plains (SGP). A new technique that makes an attempt to preserve the depths of the sublayers within an STBL is proposed for averaging the profiles of thermodynamic and radiative variables. A one-dimensional radiative transfer model known as the Rapid Radiative Transfer Model was used to compute the radiative fluxes within the STBL. The SEP STBLs were characterized by a stronger and deeper inversion, together with thicker clouds, lower free-tropospheric moisture, and higher radiative flux divergence across the cloud layer, as compared to the GRW STBLs. Compared to the STBLs over the marine locations, the STBLs over SGP had higher wind shear and a negligible (−0.41 g kg−1) jump in mixing ratio across the inversion. Despite the differences in many of the STBL thermodynamic parameters, the differences in liquid water path at the three locations were statistically insignificant. The soundings were further classified as well mixed or decoupled based on the difference between the surface and cloud-base virtual potential temperature. The decoupled STBLs were deeper than the well-mixed STBLs at all three locations. Statistically insignificant differences in surface latent heat flux (LHF) between well-mixed and decoupled STBLs suggest that parameters other than LHF are responsible for producing decoupling.

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Marcus Klingebiel, Virendra P. Ghate, Ann Kristin Naumann, Florian Ditas, Mira L. Pöhlker, Christopher Pöhlker, Konrad Kandler, Heike Konow, and Bjorn Stevens

Abstract

Sea salt aerosol in the boundary layer below shallow cumulus clouds is remotely observed with a Ka-band cloud radar at the Barbados Cloud Observatory and is detected in 76% of the measurements over 1 year. Carried by convection, sea salt particles with a diameter larger than 500 nm show an upward motion of 0.2 m s−1 below shallow cumulus clouds for a 2-day case study. Caused by an increasing relative humidity with increasing altitude, the sea salt particles become larger as they move closer to the cloud base. By using combined measurements of a Ka-band cloud radar and a Raman lidar, the retrieved equivolumetric diameter of the hygroscopically grown sea salt particles is found to be between 6 and 11 μm with a total number concentration of 20 cm−3 near cloud base. Assuming a fixed shape parameter, a size distribution of sea salt particles under high-relative-humidity conditions below cloud base is estimated and agrees with measurements taken by a dry-deposition sampler and online aerosol observations. The methods outlined in this paper can be used in future studies to get a better understanding of the vertical and temporal sea salt distribution in the boundary layer and sea salt aerosol–cloud interaction processes.

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Robert Wood, Kuan-Ting O, Christopher S. Bretherton, Johannes Mohrmann, Bruce. A. Albrecht, Paquita Zuidema, Virendra Ghate, Chris Schwartz, Ed Eloranta, Susanne Glienke, Raymond A. Shaw, Jacob Fugal, and Patrick Minnis

Abstract

A common feature of the stratocumulus-to-cumulus transition (SCT) is the presence of layers in which the concentration of particles larger than 0.1 μm is below 10 cm−3. These ultraclean layers (UCLs) are explored using aircraft observations from 14 flights of the NSF–NCAR Gulfstream V (G-V) aircraft between California and Hawaii. UCLs are commonly located in the upper part of decoupled boundary layers, with coverage increasing from less than 5% within 500 km of the California coast to ~30%–60% west of 130°W. Most clouds in UCLs are thin, horizontally extensive layers containing drops with median volume radii ranging from 15 to 30 μm. Many UCL clouds are optically thin and do not fully attenuate the G-V lidar and yet are frequently detected with a 94-GHz radar with a sensitivity of around −30 dBZ. Satellite data indicate that UCL clouds have visible reflectances of ~0.1–0.2 and are often quasi laminar, giving them a veil-like appearance. These optically thin veil clouds exist for 1–3 h or more, are associated with mesoscale cumulus clusters, and likely grow by spreading under strong inversions. Active updrafts in cumulus (Cu) clouds have droplet concentrations of ~25–50 cm−3. Collision–coalescence in the Cu and later sedimentation in the thinner UCL clouds are likely the key processes that remove droplets in UCL clouds. UCLs are relatively quiescent, and a lack of mixing with dry air above and below the cloud may help to explain their longevity. The very low and highly variable droplet concentrations in UCL clouds, together with their low geometrical and optical thickness, make these clouds particularly challenging to represent in large-scale models.

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