Directorate

• Contents
• EXECUTIVE SUMMARY
• 1. SCOPE OF THE PROGRAM
• 2. IMPROVING OUR UNDERSTANDING OF EARTH-SYSTEM PROCESSES
• 2.1 Scale Interactions Associated with Precipitating Convective Systems
• 2.2 Understanding the Dynamics and Predictability of Weather Systems on Time Scales of 0-48 h
• 2.3 Fundamental Research on Precipitation Processes, with an Emphasis on their Improved Representation in Numerical Models
• 2.4 Understanding the Dynamics, Physics and Chemistry of Multi-Scale Atmospheric Chemical Constituent and Particulate Transport, Dispersion and Transformations
• 2.5 Fundamental Research in Boundary Layers and their Interfacial Interactions
• 3. ADVANCING THE SCIENCE OF ATMOSPHERIC PREDICTION
• 3.1 Advancing Short-Range Weather Prediction
• 3.2Advancing Data Assimilation Research
• 3.3 Improving Forecasting of Pollutants and Hazardous Materials
• 4. ADVANCED COMMUNITY RESEARCH TOOLS
• 4.1 WRF Community Model
• 4.2 Community Data Assimilation Techniques
• 4.3 Nested Community Climate Model
• 5. SUMMARY
• Acronym List
• Print Version
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Mesoscale & Microscale Meteorology Division Science Plan:

2.5 Fundamental Research in Boundary Layers and their Interfacial Interactions

(contents)

Goal: To improve our understanding of oceanic and atmospheric boundary layers, and the exchanges that occur across the surface/atmosphere and the boundary layer/free atmosphere interfaces.

We now have a good understanding of the clear convective planetary boundary layer (PBL) over homogeneous terrain. But most horizontal heterogeneities introduce substantial additional complexities, for example:

• Waves modify the oceanic PBL structure in ways that depend on their amplitude, speed and direction;
• Clouds within or emanating from the PBL can markedly alter its structure;
• Surface heterogeneities, such as spatial variations of soil types, soil moisture, vegetation, and topography, lead to significant horizontal variations in the contributors to the surface energy budget, and thus PBL structure; and,
• Stably-stratified PBLs remain a largely unresolved problem because of their intermittent turbulence and sensitivity to gravity wave effects. This leaves us with a broad range of PBLs whose structures are not well quantified, which we plan to investigate under this research program.

One of the most climatologically important PBL cloud types is marine stratocumulus, which has long been a priority area for study in MMM. Small changes in its fractional cloud cover or microphysical properties can markedly alter the amount of solar radiation input to the ocean surface, which has both weather and climate implications. Two important processes that control stratocumulus evolution are entrainment and drizzle and evidence is accumulating that drizzle is associated with a characteristic mesoscale pattern of open cells. We are studying the role of these processes both via observations obtained from the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) and by large-eddy simulations (LES). Our goal is to develop better understanding of the fundamental processes and use this to improve parameterizations for larger scale models.

Over the major ocean basins, the characteristic of clouds in the trade wind regime changes towards the equator. Marine stratocumulus, which is commonly found near continental regions, breaks up and gives way to trade wind cumuli further downstream. Improved understanding and prediction of this variation is critical to an accurate assessment of the albedo for the global radiation budget, moisture transport, and rainfall amount and distribution. The recent field program for RICO studied the downstream regime and we will be using the resulting data set for comparison with modeling predictions. RICO was a large community effort and the research will include interactions with EOL and investigators from several universities.

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 the top of the PBL. We have begun to address this problem observationally using two-dimensional arrays of sonic anemometers. The first deployment, the Horizontal Array Turbulence Study (HATS) was over a flat and uniform land surface. This provided, for the first time, the basis for validating LES closure approximations. HATS was followed by a deployment over the ocean, the Ocean Horizontal Array Turbulence Study (OHATS), which enables us to examine the influences of ocean waves. It has been known for some time, both from modeling and observations, that ocean waves modify PBL structure in ways that are not predicted by current theories such as Monin-Obukhov similarity. We now have the basis for developing models which can predict PBL structure in this regime. The next step will be to carry out a similar experiment in a plant canopy, which is another regime for which no current theories adequately predict the turbulence structure.

A particularly problematic regime for air-sea interaction is high winds. Here breaking waves, spray droplets and air bubbles have significant impacts on the transfer process.

We propose to continue our study of this transfer process in both the atmospheric and oceanic boundary layers by a combination of LES modeling and analysis of observations, with the goal of developing better parameterizations for coupled larger-scale numerical models.

Surface heterogeneity leads to significant horizontal variations in the contributors to the PBL structure, which can result in errors in numerical models that do not properly incorporate these effects. Variables that describe the surface and variations in surface properties need to be properly formulated to satisfactorily represent their effects on the atmosphere. We have been involved in several field programs that have quantified these effects, and have begun to study their impact with LES. We will continue these efforts through the analysis of data, launching new observational studies, and simulating ever more complicated heterogeneous PBLs. A specific goal is to account for their effects in large-scale regional and global models, which we consider will lead to improvements in both climate and weather forecast models.

 

Next section: ADVANCING THE SCIENCE OF ATMOSPHERIC PREDICTION

 

 

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