Simulated Stratocumulus Research Articles

Cloud droplet size and liquid water path retrievals from zenith radiance measurements: examples from the Atmospheric Radiation Measurement Program and the Aerosol Robotic Network

Abstract. The ground-based Atmospheric Radiation Measurement Program (ARM) and NASA Aerosol Robotic Network (AERONET) routinely monitor clouds using zenith radiances at visible and near-infrared wavelengths. Using the transmittance calculated from such measurements, we have developed a new retrieval method for cloud effective droplet size and conducted extensive tests for non-precipitating liquid water clouds. The underlying principle is to combine a liquid-water-absorbing wavelength (i.e., 1640 nm) with a non-water-absorbing wavelength for acquiring information on cloud droplet size and optical depth. For simulated stratocumulus clouds with liquid water path less than 300 g m−2 and horizontal resolution of 201 m, the retrieval method underestimates the mean effective radius by 0.8 μm, with a root-mean-squared error of 1.7 μm and a relative deviation of 13%. For actual observations with a liquid water path less than 450 g m−2 at the ARM Oklahoma site during 2007–2008, our 1.5-min-averaged retrievals are generally larger by around 1 μm than those from combined ground-based cloud radar and microwave radiometer at a 5-min temporal resolution. We also compared our retrievals to those from combined shortwave flux and microwave observations for relatively homogeneous clouds, showing that the bias between these two retrieval sets is negligible, but the error of 2.6 μm and the relative deviation of 22% are larger than those found in our simulation case. Finally, the transmittance-based cloud effective droplet radii agree to better than 11% with satellite observations and have a negative bias of 1 μm. Overall, the retrieval method provides reasonable cloud effective radius estimates, which can enhance the cloud products of both ARM and AERONET.

Assessing regional scale predictions of aerosols, marine stratocumulus, and their interactions during VOCALS-REx using WRF-Chem

Abstract. This study assesses the ability of the recent chemistry version (v3.3) of the Weather Research and Forecasting (WRF-Chem) model to simulate boundary layer structure, aerosols, stratocumulus clouds, and energy fluxes over the Southeast Pacific Ocean. Measurements from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) and satellite retrievals (i.e., products from the MODerate resolution Imaging Spectroradiometer (MODIS), Clouds and Earth's Radiant Energy System (CERES), and GOES-10) are used for this assessment. The Morrison double-moment microphysics scheme is newly coupled with interactive aerosols in the model. The 31-day (15 October–16 November 2008) WRF-Chem simulation with aerosol-cloud interactions (AERO hereafter) is also compared to a simulation (MET hereafter) with fixed cloud droplet number concentrations in the microphysics scheme and simplified cloud and aerosol treatments in the radiation scheme. The well-simulated aerosol quantities (aerosol number, mass composition and optical properties), and the inclusion of full aerosol-cloud couplings lead to significant improvements in many features of the simulated stratocumulus clouds: cloud optical properties and microphysical properties such as cloud top effective radius, cloud water path, and cloud optical thickness. In addition to accounting for the aerosol direct and semi-direct effects, these improvements feed back to the simulation of boundary-layer characteristics and energy budgets. Particularly, inclusion of interactive aerosols in AERO strengthens the temperature and humidity gradients within the capping inversion layer and lowers the marine boundary layer (MBL) depth by 130 m from that of the MET simulation. These differences are associated with weaker entrainment and stronger mean subsidence at the top of the MBL in AERO. Mean top-of-atmosphere outgoing shortwave fluxes, surface latent heat, and surface downwelling longwave fluxes are in better agreement with observations in AERO, compared to the MET simulation. Nevertheless, biases in some of the simulated meteorological quantities (e.g., MBL temperature and humidity) and aerosol quantities (e.g., underestimations of accumulation mode aerosol number) might affect simulated stratocumulus and energy fluxes over the Southeastern Pacific, and require further investigation. The well-simulated timing and outflow patterns of polluted and clean episodes demonstrate the model's ability to capture daily/synoptic scale variations of aerosol and cloud properties, and suggest that the model is suitable for studying atmospheric processes associated with pollution outflow over the ocean. The overall performance of the regional model in simulating mesoscale clouds and boundary layer properties is encouraging and suggests that reproducing gradients of aerosol and cloud droplet concentrations and coupling cloud-aerosol-radiation processes are important when simulating marine stratocumulus over the Southeast Pacific.

Determination of cloud liquid water distribution using 3D cloud tomography

The cloud microwave tomography method for remotely retrieving 3D distributions of cloud Liquid Water Content (LWC) was originally proposed by Warner et al. in the 1980s but has lain dormant since then. This paper revisits and extends the cloud tomography method by rigorously examining the nature of the resultant mathematical problem and its close relationship to the physical configuration of microwave radiometers. The singular value decomposition (SVD) analysis reveals that the retrieval of cloud LWC fields from microwave emission is highly ill‐posed, and requires special techniques to solve it. The truncated SVD approach along with the L‐curve method for choosing the optimal truncating point is used to obtain a better retrieval of cloud LWC. A group of sensitivity studies show that the retrieval accuracy is determined by several factors, including the number of radiometers, the spatial resolution of output, the number of scanning angles, the radiometer characteristics (e.g., noise level, beam width), the physical arrangement of radiometers, and the uncertainty in the ancillary temperature and water vapor mixing ratio data. When more radiometers and/or more scanning angles are used, and/or the radiometer beam width is reduced, and/or when a coarser output resolution is acceptable, the retrieval problem becomes less ill‐posed, and a better retrieval can be obtained. Moreover, the observation system simulations demonstrate that the cloud tomography method is able to retrieve the cloud structures generated by cloud resolving models with a good accuracy. For a setup consisting of four microwave radiometers of typical noise level 0.3 K, the tomography method is capable of retrieving the LWC to within 5% of the maximum LWC in the simulated stratocumulus and broken cumulus cases, with a spatial resolution of a few hundred meters.

The effect of temperature and humidity fluctuations on the liquid water path of non‐precipitating closed‐cell stratocumulus clouds

AbstractThe spatial variability of the liquid water path (LWP) is analyzed from a large‐eddy simulation of the diurnal cycle of stratocumulus clouds as observed during FIRE I. In stratocumulus clouds, the temperature and the total specific humidity cannot fluctuate independently, but are tightly connected to fluctuations in the liquid water potential temperature. If the latter are relatively small, a strong positive correlation between the temperature and the total specific humidity can be expected. The effect of temperature fluctuations on the magnitude of liquid water fluctuations must be considered to properly compute the LWP distribution in stratocumulus clouds.The simulated stratocumulus cloud fields are used to compute the albedo inhomogeneity factor χ according to the effective thickness approach. During the day, the mean LWP decreases due to short‐wave radiative warming. Also, the probability density function for the LWP becomes positively skewed due to cumuli that have their base well below the mean stratocumulus cloud‐base height. For this situation, the inhomogeneity factor χ displays a minimum value of about 0.85. For solid (unbroken) cloud with an assumed Gaussian distribution for the optical depth, we find that the minimum inhomogeneity factor will be about 0.8. This minimum value for the inhomogeneity factor, and that from the LES results, are larger than reported from FIRE I observations. Copyright © 2008 Royal Meteorological Society

Orographic influences on the annual cycle of Namibian stratocumulus clouds

The impact of African orography on the persistence and seasonal cycle of Namibian stratocumulus is examined in the context of an atmospheric general circulation model. A control simulation with realistic orography and simulated stratocumulus is compared to a sensitivity experiment, in which African orography is removed. In the absence of orography, stratocumulus incidence off the Namibian and Angolan coast is significantly reduced during austral winter and spring. It is argued that through the combined effects of obstruction and deflection of atmospheric flow, orography acts to enhance static stability of the lower troposphere over the Atlantic off the Namibian coast, which provides an environment conducive to the development of stratocumulus in that region.

The Relationship between Drop In-Cloud Residence Time and Drizzle Production in Numerically Simulated Stratocumulus Clouds

This paper considers the production of drizzle in statocumulus clouds in relation to the boundary-layer turbulent kinetic energy and in-cloud residence times. It is shown that drizzle production in statocumulus of the order of 400 m in depth is intimately related to the vertical velocity structure of the cloud eddies. In a series of two dimensional numerical experiments with fixed cloud condensation nucleus concentrations, the effect on drizzle production of enhanced or diminished vertical velocities is simulated. Rather than do this by simulating clouds exhibiting more or less energy, we modify drop terminal velocities in a manner that conserves the fall velocity relative to the air motions and allows droplet growth to occur in a similar dynamical environment. The results suggest that more vigorous clouds produce more drizzle because they enable longer in-cloud dwell times and therefore prolonged collision-coalescence. In weaker clouds, droplets tend to fall out of the cloud before they have achieved significant size, resulting in smaller amounts of drizzle. In another series of experiments, we investigate the effects of the feedback of drizzle on the boundary-layer dynamics. Results show that when significant amounts of drizzle reach the surface, the subcloud layer is stabilized, circulations are weaker, and the boundary layer is not well mixed. When only small amounts of drizzle are produced, cooling tends to be confined to the region just below cloud base, resulting in destabilization, more vigorous circulations, and a better mixed boundary layer. The results strongly suggest that a characteristic time associated with collision-coalescence be incorporated into drizzle parameterizations.