• Entrainment Rates in Nocturnal Marine Stratocumulus

Related website: http://www.atmos.ucla.edu/~bstevens/publications.html

Stratocumulus clouds are a persistent feature over subtropical oceanic regions where the underlying ocean is much colder than the atmosphere. Stratocumulus clouds have long been recognized as having a significant impact on the radiative balance of the Earth, and thus on the Earth's climate. For this reason, there has been a considerable effort devoted to modeling their evolution and parameterizing the processes that control them. One of these processes is the rate of transport of relatively warm and dry free tropospheric air across the cloud top into the marine boundary layer; i.e., the entrainment rate. The entrainment rate determines whether the cloud thickens or thins with time. There is no concensus on a formulation that can accurately predict the entrainment. A primary objective of the second study of the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) experiment was to obtain data that could be used as a basis for comparison with predicted entrainment rates. The primary contributors to DYCOMS-II are Donald Lenschow and Chin-Hoh Moeng and Bjorn Stevens (University of California, Los Angeles), Ian Faloona (NCAR/ASP), Doug Lilly (University of Oklahoma), Bryon Blomquist (Drexel University), Gabor Vali (University of Wyoming), Allan Bandy (Drexel University), Teresa Campos (NCAR/ACD), Hermann Gerber (Gerber Scientific), Samuel Haimov (University of Wyoming), Bruce Morley (NCAR/ATD), and Donald Thornton (Drexel University). DYCOMS-II took place during July 2001 in the persistent stratocumulus about 500 km WSW of San Diego, CA. Using the NCAR C-130 aircraft, measurements of both mean and turbulent structure of the MBL, cloud microphysics, and long- and short-wave radiation were obtained. Initial analysis, as shown in Figure 34, indicates that observed entrainment rates are less than modeled rates when the cloud layer is observed to be unstable for mixing with free-tropospheric air.

Figure 34. Observed time series of cloud boundaries for DYCOMS-II Flight 1: cloud base, solid circles are lifting condensation levels (LCL) taken from level-leg averages in the MBL located in the central region of the study area while the northern and southern region LCLs are denoted by upward- and downward-pointing triangles, and the gray circles are LCLs outside the study area. The squares are from sounding data. The solid line is a least-squares fit to the central region data specified by the relation for z_b. For cloud top, the circles and triangles are obtained from downward-looking radar and lidar retrievals. The two dashed lines at cloud-top are fits by eye showing: 1) no change in cloud-top height, and 2) an increase of 7.5 m/hour.

Large-eddy simulation (LES) of a stratocumulus case from DYCOMS II

The case study described above, with cloud depth remaining steady or thickening slightly despite fulfilling the cloud-top entrainment instability criterion, provides an interesting test case to examine how LES simulates the so-called cloud-top entrainment instability process. Moeng, in collaboration with Bjorn Stevens (University of California, Los Angeles) and Gabor Vali (University of Wyoming), found that the simulated cloud layer also remains solid, and even thickens slightly in time, in agreement with observations. Other statistics, such as the entrainment rate, liquid water path, turbulent kinetic energy, velocity variances, and skewness are also in agreement with observations.

Two-dimensional modeling of boundary-layer convection

With the increasing use of two-dimensional (2D) modeling to simulate both deep and shallow convection in the atmosphere, it is important to understand how well its statistics can be made to agree with observations and LES simulations. Moeng, Peter Sullivan, and Richard Rotunno, in collaboration with Akio Arakawa and James McWilliams (both University of California, Los Angeles) and Jeff Weil (CIRES/Colorado University), are using the simplest atmospheric convective system, a convective planetary boundary layer (PBL) with and without shear, as an example, and LES as a database. They found that for the convective PBL without mean wind, a 2D model can be tuned to produce reasonable turbulence kinetic energy (TKE, a measure of the convective intensity) simply through the imposed sub-grid scale eddy viscosity. Once the TKE is properly captured, other statistical properties that are strongly tied to the TKE can be reasonably represented, including heat flux and its countergradient transport, convective mass flux, areal coverage of convective updrafts, and friction velocity.

Climate modification via modification of low-level marine clouds

John Latham developed a novel idea for the amelioration of global warming by the advertent and controlled enhancement of the albedo A and longevity L of low-level marine clouds. His provisional calculations and some limited computer modeling support the quantitative validity of the proposed technique, which involves increasing the droplet concentration in such clouds, with a corresponding increase in both A and L: and thus cooling. The idea involves the dissemination at the ocean surface of small seawater droplets in sufficient quantities to act as the dominant CCN on which cloud droplets form. Satellite control of the dissemination is envisaged. This work includes collaboration with Keith Bower, Tom Choularton & Alan Gadian (all UMIST, Manchester, UK), Alan Blyth and Mike Smith (University of Leeds, UK), Stephen Salter, (University of Edinburgh, UK) and Tom Wigley (NCAR/CGD). If this technique were to prove workable on the scales required, it could be of great societal importance.