Gunnar Senum

Observations of marine stratocumulus microphysics and implications for processes controlling droplet spectra: Results from the Marine Stratus/Stratocumulus Experiment

During the Marine Stratus/Stratocumulus Experiment, cloud and aerosol microphysics were measured in the eastern Pacific off the coast of northern California on board Department of Energy Gulfstream-1 in July 2005. Three cases with uniform aerosol concentration and minimal drizzle concentration were examined to study cloud microphysical behavior. For these three cases, the average droplet number concentration increased with increasing altitude, while the average interstitial aerosol concentration decreased with altitude. The data show enhanced growth of large droplets and spectral broadening in cloud parcels with low liquid water mixing ratio. Three mixing models, including inhomogeneous mixing, entity type entrainment mixing, and circulation mixing proposed in this study, are examined with regard to their influences on cloud microphysics. The observed cloud microphysical behavior is most consistent with the circulation mixing, which describes the mixing between cloud parcels with different lifting condensation levels during their circulations driven by evaporative and radiative cooling. The enhanced growth and spectrum broadening resulting from the circulation mixing reduce cloud albedo at the same liquid water path and facilitate the formation of precipitation embryos.

Characteristics of vertical velocity in marine stratocumulus: comparison of large eddy simulations with observations

We simulated a marine stratus deck sampled during the Marine Stratus/Stratocumulus Experiment (MASE) with a three-dimensional large eddy simulation (LES) model at different model resolutions. Various characteristics of the vertical velocity from the model simulations were evaluated against those derived from the corresponding aircraft in situ observations, focusing on standard deviation, skewness, kurtosis, probability density function (PDF), power spectrum, and structure function. Our results show that although the LES model captures reasonably well the lower-order moments (e.g., horizontal averages and standard deviations), it fails to simulate many aspects of the higher-order moments, such as kurtosis, especially near cloud base and cloud top. Further investigations of the PDFs, power spectra, and structure functions reveal that compared to the observations, the model generally underestimates relatively strong variations on small scales. The results also suggest that increasing the model resolutions improves the agreements between the model results and the observations in virtually all of the properties that we examined. Furthermore, the results indicate that a vertical grid size <10 m is necessary for accurately simulating even the standard-deviation profile, posing new challenges to computer resources.

Error analysis techniques for perfluorocarbon tracer derived multizone ventilation rates

Abstract Several mathematical schemes for calculating ventilation flows and their associated errors using the multiple perfluorocarbon tracer (PFT) method developed at Brookhaven National Laboratory are presented and methods are suggested for their implementation on microcomputer systems. A first order error analysis method is described along with a matrix method which yields identical results but is computationally much faster. Both methods are compared to the Monte Carlo error analysis technique. The computation of an optimal condition number is suggested as a means of gauging the sensitivity of the flow solutions.

Cloud microphysical relationships and their implication on entrainment and mixing mechanism for the stratocumulus clouds measured during the VOCALS project

Cloud microphysical data obtained from G-1 aircraft flights over the southeastern Pacific during the Variability of the American Monsoon Systems Ocean-Cloud-Atmosphere-Land Study Regional Experiment field campaign were analyzed for evidence of entrainment mixing of dry air from above cloud top. Mixing diagram analysis was made for the horizontal flight data recorded at 1 Hz and 40 Hz. The dominant observed feature, a positive relationship between cloud droplet mean volume (V) and liquid water content (L), suggested occurrence of homogeneous mixing. On the other hand, estimation of the relevant scale parameters (i.e., transition length scale and transition scale number) consistently indicated inhomogeneous mixing. Importantly, the flight altitudes of the measurements were significantly below cloud top. We speculate that mixing of the entrained air near the cloud top may have indeed been inhomogeneous; but due to vertical circulation mixing, the correlation between V and L became positive at the measurement altitudes in midlevel of clouds, because during their descent, cloud droplets evaporate, faster in more diluted cloud parcels, leading to a positive correlation between V and L regardless of the mixing mechanism near the cloud top.