Surface and Planetary Boundary Layer Processes

What are Surface and Planetary Boundary Layer Processes?

The Earth's surface has many profound effects on the atmosphere that impact our ability to understand and predict its behavior. The rain that falls on your house originally evaporated from the surface (the ocean, a lake, or even a tree), and much of the heat that we receive from the sun is first absorbed at the Earth's surface and is then transferred to the atmosphere.

The lowest portion of the atmosphere (from surface to about 1 to 2 km high) is where surface effects are most evident. This region is known as the atmospheric boundary layer of our planet, or the planetary boundary layer or simply "boundary layer". The different ways in which the surface interacts with the boundary layer or the boundary layer responds to the surface are called surface and boundary layer processes. This heading also includes interactions between the boundary layer and the rest of the atmosphere.

The Earth has many types of surfaces - mountains, hills, lakes and seas, plants, and manmade structures. Each of the surfaces differs in how it interacts with the atmosphere. For example, more water will typically evaporate from a lake than from a pasture of the same size, and dark barren soil will absorb much more solar radiation than a bright snow surface. Surface and boundary layer processes determine how much heat, water, gases, and particles are exchanged between the surface and the boundary layer and between the boundary layer and the free atmosphere. Also of interest are how the winds and temperatures within the PBL are affected and where and when clouds start to form.

The Earth's surface is the source and often a depository for pollutants and various constituents in the atmosphere that can affect both the quality of the air we breathe and the ability for energy to pass through the air to the surface. The processes by which gases and small particles derive from and are deposited at the surface are also of interest to this ESRL theme.

Illustration of boundary layer processes.

Why is this important?

The Earth's surface is where the primary exchanges of heat and water that are starting points for driving the Earth's climate and weather systems occur.

Many important weather and climate phenomena (e.g., El Nino or hurricanes) are principally driven by atmosphere-surface interaction. Surface and boundary layer processes must be identified and understood in order to be replicated or simulated in computer models used to forecast daily weather, the extent of air pollution events, and the impact of human activity on future climate. These processes are often poorly resolved or are not even represented in current models. Accurate forecasts of the path and intensity of the next hurricane, the onset of monsoon rains, or the high temperature in Denver tomorrow require a careful accounting for PBL processes. For air quality, it is important to know the conditions under which pollutants enter the atmosphere, and the direction and distance they are transported.

What do we know?

Much is known about the basic mechanisms (physics) of surface and boundary layer processes. These include turbulent and non-turbulent air motions, radiative energy transport, thermodynamics, water phase changes, plant physiology, material reflectance, chemistry, and principles of conservation of mass, energy, and momentum.

Through extensive field observations, the behavior of the boundary layer has been investigated under many conditions. Those observations have been analyzed to demonstrate the extent to which basic physical mechanisms can be observed and understood at the surface and within the boundary layer. From these concepts, computer models have been developed and have been shown to reasonably replicate the many aspects of the observations. For example, we know that there are trade-offs between thermally driven lifting of air parcels and turbulent mixing of those parcels caused by higher speed winds over rough surfaces. This affects the rate of vertical transport of heat and mass from the surface into and through the boundary layer. Various vegetated surfaces have been studied to determine their particular impacts on the surface and boundary layer processes, as have other land and water surface types and conditions. The reflectance of the surface has been cataloged to allow the ratio of incoming solar radiation converted at the surface to be determined. Land-use surveys are conducted to further understand the distribution of surface properties. Studies of air-sea interaction processes have clarified the role of wind, waves, ocean currents, and whitecaps in the exchanges between the ocean and atmosphere.

The complete physical and chemical description of surface and boundary layer processes including their actual spatial and temporal description are far too complex to be handled by today's computers. So, simplifications called parameterizations have been derived to capture the essential aspects of these processes. Such parameterizations are an essential tool for predicting weather and climate.

What don't we know?

The need for information on very small spatial and temporal scales leaves many remaining gaps in our knowledge and capabilities. If we are to predict regional or global atmospheric phenomena on scales of minutes to millennia this information and understanding are crucial.

At ESRL we are working to fill the most critical of these gaps. Following is a summary of topics that we view as the most pressing in need of further study in the area of surface and boundary layer processes.

Cloud formation and transitions within the boundary layer
Impact of the exchange of heat and gases over the ocean on ocean temperatures
Concentration and transport of pollutants in realistically variable boundary layer structures
Factors affecting the vertical transport of heat and matter during the boundary layer daily cycle
Partitioning of various causes of climate changes in the polar regions
Interaction between the boundary layer and atmosphere above (free troposphere)
Effects of waves and sea spray on hurricane formation
Better representation of relevant surface features and their temporal variations in models
Accounting for the complete surface energy balance over various land surfaces.

What is NOAA's role?

As the primary agency for understanding and predicting the future state of the atmosphere, NOAA has long addressed these issues and has contributed greatly to the current level of understanding of surface and boundary layer processes.

ESRL is developing improved remote sensing instrumentation to observe surface and boundary layer features. We conduct field campaigns to collect information in regions and under conditions not previously well observed and documented. We are carrying out long-term monitoring observations of surface energy exchange processes at global remote and regionally representative fixed surface sites. Several groups in ESRL are working with satellite-based programs to improve and extend their global and regional observations of surface properties. Ocean voyages are made to investigate aspects of the interactions and exchanges between the boundary layer and ocean in different climatic zones and geographical regions. Extensive work is being done to introduce and test methods of incorporating what has been observed and explained about the surface and PBL processes into research and operational atmospheric forecast models.

What will we need to know in the future?

Information available to describe the current state of the planet (e.g., land surface changes due to urbanization or forest cutting) must be sufficient for expanding modeling capabilities, and for validation of model predictions.

We must continue to have the capability to observe changes that have not been anticipated or predicted. Our observing capabilities need to expand and improve to meet the need for more highly resolved information on the boundary layer and surface.

What are the benefits for society?

By thoroughly understanding surface and boundary layer processes, scientists are able to include those effects into computer simulations of weather, climate, and air quality.

The interactions and feedbacks between processes are better represented and predictions will be more confidently accurate. Improved weather and air quality forecasts, due to better representation of surface and boundary layer processes, will allow asufficient warning for catastrophic and common events, further minimizing adverse effects on people, property, and the environment. Being better prepared for catastrophic events would inherently improve the quality of life for many individuals who might otherwise have been more adversely affected. In the longer term it is expected that the climate simulations will benefit from improved parameterizations of the surface and boundary layer processes such that those predictions can be more confidently used in policy and planning for the future.