Mesoscale & Microscale Meteorology Division of NCAR

Land surfaces are typically heterogeneous. This leads to significant horizontal variations in the contributors to the surface energy budget, and thus PBL structure. This, in turn, can result in errors in numerical climate and weather forecast models that do not incorporate these effects. In order to deal with this problem, variables that describe the surface and variations in surface properties need to be properly formulated to satisfactorily represent the effects of the surface on the atmosphere. High quality comprehensive datasets are critically needed for comparison with models and development of parameterization schemes. Techniques need to be developed for comparing observations of fluxes and other statistical properties of the boundary layer over horizontally heterogeneous land surfaces with model results. We address two complementary questions. First, how can the heterogeneity be accounted for in models with large-scale resolution, such as regional and global models? And second, how do we incorporate the effects of surface heterogeneity on the diurnal variation of PBL structure in regional and global models?

Surface heterogeneity includes spatial variations of soil types, soil moisture, vegetation, and topography. The regional variability of soil moisture is important in storm initiation and evolution, and flash floods, making surface-atmosphere interaction important to both the weather (USWRP) and climate (GCIP, ROCEW) communities. Observational data analysis will focus on surface, aircraft, and remotely sensed data collected from various field campaigns, such as the Boreal Ecosystem-Atmosphere Study (BOREAS), the Cooperative Atmosphere-Surface Exchange Study (CASES-97), and the Southern Great Plains (SGP-97) Experiment. Using remotely sensed soil moisture, the effect of spatial variations in soil moisture on the development of the atmospheric boundary layer will be investigated. In addition, the effect of the surface heterogeneity on stable boundary layers will be examined using the observational data collected during CASES-99. High spatial resolution models with land parameterization schemes will be used to examine model sensitivities and compare their performance with observations. Surface land parameterization schemes will be applied to LESs to study the influence of the subgrid surface heterogeneity on the development of the PBL.

In order to evaluate the performance of numerical models, area-averaged turbulent fluxes over heterogeneous surfaces will be estimated using observations from surface-based sites, aircraft, and satellites. Remotely sensed variables include soil moisture from airborne and satellite microwave sensors, long- and short-wave radiation (including radiative surface temperature), and biomass and land-surface types retrieved from satellite and aircraft imagery. In addition, sensitivity of this scale-up process will be investigated using the BOREAS and SGP-97 data sets. Improved formulations of the bulk formulae for estimate of subgrid turbulent fluxes will also be developed.

Surface heterogeneity also plays an important role in the exchange of carbon dioxide between the atmosphere and terrestrial biosphere. This is very important from a global climate perspective. We will investigate the spatial variation of horizontal and vertical transport of carbon dioxide and the role that they play in carbon dioxide budgets, especially in nocturnal stably stratified boundary layers, which are a particular problem for carbon dioxide budget estimates.

Both climate and weather forecast models have particular difficulty predicting the air temperature close to the surface, and surface energy fluxes during the morning and evening transitions, and at night. As a result, the statistically steady state and homogeneous turbulence assumptions for Monin-Obukhov (M-O) similarity are violated during these periods. Since M-O similarity theory is the current basis for parameterizing land-atmosphere interactions in numerical models, we will be working on new schemes that parameterize temperature and fluxes during the transition and nocturnal periods emphasizing the effects of vegetation, soil moisture, land use, and topography on the evolution of the PBL. One approach will be to examine the temporal and spatial variability of air temperature, wind, and water vapor, and their vertical transport during the diurnal cycle using data from two field programs conducted during the spring (CASES-97) and summer (SGP-97) over the Great Plains. We will use these data to test surface-process parameterization schemes, linked surface-PBL schemes, and mesoscale models. We will also focus on understanding the nocturnal stable PBL, which is especially difficult to parameterize because of intermittent turbulence, by analyzing the field data collected from CASES-99. We are exploring participation in IHOP to further studies of PBL water-vapor evolution.

In order to treat the ocean and the atmosphere as one system, we need to understand the turbulent processes on both sides of the interface. Since the surface fluxes are the link between these two media, accurate representation of the surface fluxes is our primary goal in ocean-atmosphere interaction studies. Recent field measurements suggest that ocean waves can dynamically alter the turbulent kinetic energy budget in the atmospheric surface layer. Depending on the relative magnitude of the wave phase speed and the local wind, surface waves can either be a source or sink of momentum. As a result, the traditional relationship between surface fluxes and mean atmospheric profiles is altered. The effects of surface gravity waves on turbulence in the atmospheric and oceanic PBLs, and in particular on M-O scaling, will be investigated using LES with a nested-grid, high-resolution surface layer and a moving surface fitted grid. Our goal is to gain understanding of wave effects and then develop a parameterization that links the ocean and atmospheric PBLs as a system that includes the wavy (interface) effects.

Over the coastal zone, variations in oceanic bottom topography lead to shoaling waves. Existing numerical models for surface stress in the shoaling zone fail because of their inability to properly account for wave age, shoaling, and internal boundary layer development. The fetch-dependent wave field in the shoaling zone cannot be adequately studied without information on the spatial variation of the wind and stress fields. Two goals to be pursued are: to study the relationship between the spatial varying mean wind, stress, turbulence structure, and surface wave fields by analyzing the field data collected from the Shoaling Wave Experiment (SHOWEX); and, to model effects of wave age, shoaling, and internal boundary layer development on the drag coefficient and momentum transfer between the waves and the atmosphere.

The Earth's surface is the source and sink of many trace atmospheric constituents. The PBL acts as a conduit between the surface and the overlying free atmosphere, and as reactor for many of these constituents, which have both natural and anthropogenic sources. These transport and transformation processes occur in both clear and cloudy PBLs, and the budgets of many of these constituents are determined by physical processes in the PBL such as turbulent diffusion, entrainment, PBL growth rate, and cloud cover and transport. These processes will be studied using data from several field programs such as the Aerosol Characterization Experiment (ACE-1) and the Pacific Exploratory Mission (PEM-Tropics).

The effect of boundary-layer processes on the mixing and chemistry of biogenic hydrocarbons and their reaction by-products, particularly over forest canopies, is also being investigated. Hydrocarbons emitted from vegetation are relevant for climate because of their role as sources of ozone and aerosols in the troposphere. By coupling a PBL LES with biogenic hydrocarbon chemistry, an understanding of the influence of boundary-layer mixing in chemical constituents can be attained. By combining a forest canopy LES with simple decay chemistry, the role of the forest canopy and homogeneous and heterogeneous sources of hydrocarbons may be assessed. The role of small cumulus upon the fate of ozone and its precursors via boundary layer venting, aqueous chemistry, or scattering of solar radiation will be determined by including cloud microphysics with the PBL LES and biogenic hydrocarbon chemistry model. This coupled LES cloud and chemistry model will then be used as a tool to understand interactions between clouds, chemistry, and aerosols in the marine boundary layer as well as the convective boundary layer over land.