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Boundary Layer Clouds

Goal: To understand the physical processes of PBL (shallow) clouds and represent their effects in climate models. In the following we describe our observational and modeling studies of the different types of boundary-layer clouds. These cloud studies will continue to be coordinated with the GCSS (GEWEX Cloud System Study) program, which seeks to develop physically based parameterizations of cloud-related processes for climate and global numerical weather prediction models. We will also work as the NCAR CSM Atmospheric Modeling Working Group (AMWG) to improve PBL clouds in climate models.

Marine stratocumulus regime

One of the most climatologically important PBL cloud types is marine stratocumulus. Small changes in its fractional cloud cover or microphysical properties can drastically alter the amount of solar radiation input to the ocean surface. Hence, an accurate representation of this cloud regime in a coupled climate model is required to simulate accurately the energy budget of the Earth's surface. Current climate models treat clouds, turbulence and radiation separately using independently developed parameterization schemes, but these physical processes can interact strongly on a temporal (or spatial) scale that is smaller than the time step (or grid resolution) commonly used in current climate models. Our goal is to develop parameterizations that represent the net effect of all these processes.

One of the key issues in incorporating marine stratocumulus into climate models is the rate of entrainment of warm dry air from above the PBL into the stratocumulus-topped boundary layer (STBL). This rate determines the thermodynamic structure of the STBL, and hence the cloud amount. Our numerical and observational studies of marine stratocumlus have been focused on this particular issue. Based on large eddy simulations, we have recently developed an entrainment-rate formula, which differs from those developed elsewhere and requires testing with field observations. We have just conducted the DYCOMS-II experiment*LINK*where we focused explicitly on entrainment processes and will test different entrainment-rate formulae currently used for STBL parameterizations.

Another key issue in incorporating marine stratocumulus into climate models is the effect of mesoscale variations. Mesoscale variations, such as mesoscale cellular convection or cloud streets, are often observed in the marine stratocumulus region. These variations are likely to modify the grid-averaged cloud amount within a GCM mesh, but their effect has never been included in any GCM. Within the next few years, increasing computer power will allow us to simulate explicitly the mesoscale variations, along with the dominant turbulent motions (i.e., large turbulent eddies). Such simulated flow fields can be used to examine the effect of mesoscale variations on the cloud properties of marine stratocumulus.

Transition from marine stratocumulus to trade cumulus regime

As air moves downstream towards the equator over the eastern part of large oceanic basins, marine stratocumulus breaks up and gives way to cumulus. During this transition, along the air trajectory the cloud cover is drastically reduced and hence solar radiation input to the ocean is drastically increased. This transition between the two cloud regimes is another focus of our PBL cloud research within MMM.

In the incipient stages of this transition, the stratocumulus layer typically becomes "decoupled" from the well-mixed layer near the surface; here stratocumulus becomes only weakly linked to the surface process, and cumulus often develops under the stratocumulus deck. Important processes for this decoupling and development of cumulus under stratocumulus include evaporation of drizzle, short wave radiative warming of the stratocumulus, and surface heat flux. We will continue to investigate the roles of these processes.

Another mechanism that may also play a role in the transition is cloud-top entrainment instability. When evaporation of cloud due to entrained dry inversion air is significant, the mixture may become colder than its cloudy environment (that is, negatively buoyant), a process known as buoyancy reversal. Whether this buoyancy reversal process can lead to the transition from stratocumulus to cumulus regime is still debatable. Our recent large eddy simulations showed that buoyancy reversal did not lead to a total breakup of stratocumulus cloud deck but that it plays a dominant role in determining the simulated cloud fraction and liquid water path. We will continue looking for other important factors that determine the cloud amount and eventually develop a cloud scheme of the marine stratocumulus regime and its transition to the cumulus regime.

Fair weather cumulus

Fair weather cumulus over subtropical oceans is known to play a major role in the hydrological cycle of the Hadley Circulation. Trade cumulus transports moisture from the PBL to the low- to mid-troposphere, pre-conditioning the atmosphere for deep convection further downstream. MMM scientists have a long history of observational and modeling studies of this cloud regime, which will serve as a basis for further study. Examples of field studies in this regime that will continue to be used for comparisons with modeling studies include: BOMEX, GATE, and ATEX. Key issues include how to represent the cloud amount, which affects the global radiation budget, and moisture transport by cumulus, which affects the global moisture distribution.

Fair weather cumulus over land is also important because it modifies the land surface through its effect on incoming radiation. We intend to include fair-weather cumulus in our coupled PBL-land process model, which is described in the following section. We plan to also use observational data from ARM for comparison with modeling results.

The role of fair weather cumulus (over both land and ocean) on transport of biogenic hydrocarbons and other trace gas species, and their chemical reactions is also being investigated. Work is now underway to incorporate these processes in this cloud regime into our large eddy simulation code that is coupled with a chemistry transport model (see below).

Observing the boundary layer

Improvements in remote sensing capabilities being conducted jointly with ATD, as well as with NOAA and NASA, will provide new ways to observe the three-dimensional structure of both the clear and cloudy PBL. Water vapor differential absorption lidar (DIAL) aircraft data from SGP and other programs will be used to study the fine-scale structure of scalars in the PBL, as well as mesoscale variability of humidity and PBL structure. Fine-scale measurements of both radial velocity and scalars, for example from the lidars in flat terrain (LIFT) experiment, will be used to document PBL structure in the entrainment layer and provide data for comparison with numerical simulations of entrainment to develop improved parameterizations.

A fundamental limitation in LES modeling is the fidelity of the parameterizations used to represent sub-grid scale motions. This problem is especially acute near fluid interfaces--i.e., near the surface and near the PBL top. To address this problem, we are planning to conduct a series of observational studies that will measure the sub-grid scale motions and allow comparison with parameterizations of these motions used in large-eddy numerical models. The objective is to develop parameterizations that more accurately incorporate the sub-grid scale field of motion into the resolved field. The first of these sub-grid scale experiments, SGS-2000, has been carried out in the Central Valley of California in September 2000 using a two-dimensional array containing 14 three-dimensional sonic anemometers. We are starting to develop plans for a similar experiment to investigate sub-grid scale parameterizations in the entrainment region at the top of the PBL.