Introduction

The planetary boundary layer (PBL) is that part of the atmosphere that interacts with Earth's surface on a time scale of about an hour. This rapid interaction is a direct result of turbulence, which is an essential feature of the PBL. The sources of turbulence are wind shear and convection. Defining characteristics of turbulence are its chaotic fluctuations and diffusiveness. That is, trace constituents released into a turbulent fluid are rapidly diffused, and the small-scale patterns of this diffusion cannot be predicted. Because of the randomness and the large range of scales of PBL turbulence, processes in the PBL are often described in terms of statistical averages of fluctuations. This means that most measurements of PBL structure need to be spatially or temporally averaged before they can be quantitatively interpreted.

If generation of turbulence by convection is occurring in the PBL, it is known as an unstable or convective boundary layer (CBL); if the hydrodynamic stratification of the PBL acts to suppress or dissipate turbulence, it is known as a stable boundary layer (SBL). When relative humidity reaches 100% within the PBL, clouds form that can have a dramatic effect on its subsequent evolution.

2 BOUNDARY LAYER EVOLUTION

Over land, the daily solar cycle determines the PBL evolution. In the morning, the sun starts to warm the ground, which has been cooling through the night by infrared radiation. Clear air is nearly transparent to the sun's short-wave (visible) radiation

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

and thus is warmed only slightly by direct solar radiation. Instead, the ground absorbs most of the solar radiation and then warms the air above it mostly through convection, which is the upward movement of buoyant parcels of air warmed by contact with the surface, in combination with compensating downward transfer of cooler, more dense air from above. This process generates turbulence that, in turn, increases the efficiency of transport of atmospheric constituents in the CBL. Efficient mixing means that the lapse rate, which is the rate of change of temperature with height, is nearly adiabatic throughout much of the CBL. This means that vertical displacements of an air parcel do not change the buoyancy of the parcel relative to its environment. Figure 1 shows the structure of the CBL using virtual potential temperature, which is constant with height in an adiabatic layer, and a scalar variable with a surface source and negligible concentration above the CBL.

The CBL continues to deepen typically at least until early afternoon to perhaps 1 to 3 km. Generally, the relative humidity in the upper part of the CBL tends to

Figure 1 Convective boundary layer. On the left, the sublayers that make up the boundary layer are shown, along with a schematic of flow patterns—upward-moving buoyant thermals forming near the top of the surface layer, extending through the mixed layer, and dissipating in the upper part of the boundary layer. Part of their kinetic energy is dissipated in the entrainment zone by entraining warmer, more buoyant air from above into the boundary layer. The thermals have a smaller total area, and consequently a larger velocity magnitude than the compensating downward-moving air in between the thermals. The flux profiles in the middle panel show the virtual potential temperature (0,.) flux (which has a universal shape), and the flux of a scalar # which (in this case) has a source at the surface and whose mean concentration decreases with height throughout and immediately above the boundary layer. The virtual potential temperature can normally be assumed to be a conserved variable in a well-mixed clear boundary layer. The mean virtual potential temperature and scalar concentration profiles, including their jumps across the top of the boundary layer, are shown on the right.

FLUX MEAN

Figure 1 Convective boundary layer. On the left, the sublayers that make up the boundary layer are shown, along with a schematic of flow patterns—upward-moving buoyant thermals forming near the top of the surface layer, extending through the mixed layer, and dissipating in the upper part of the boundary layer. Part of their kinetic energy is dissipated in the entrainment zone by entraining warmer, more buoyant air from above into the boundary layer. The thermals have a smaller total area, and consequently a larger velocity magnitude than the compensating downward-moving air in between the thermals. The flux profiles in the middle panel show the virtual potential temperature (0,.) flux (which has a universal shape), and the flux of a scalar # which (in this case) has a source at the surface and whose mean concentration decreases with height throughout and immediately above the boundary layer. The virtual potential temperature can normally be assumed to be a conserved variable in a well-mixed clear boundary layer. The mean virtual potential temperature and scalar concentration profiles, including their jumps across the top of the boundary layer, are shown on the right.

increase through the day from moistening due to surface évapotranspiration and turbulent mixing. If the humidity reaches saturation, clouds develop at the top of the CBL.

As the solar heating decreases late in the day, convection disappears and radiative cooling at the surface again dominates over solar warming. Eventually, the ground becomes cooler than the overlying air, which radiatively cools much more slowly than the ground. At this point, the SBL develops, which is considerably shallower (ranging from a few tens to a few hundred meters deep) and less turbulent because buoyancy is now suppressing the turbulence generated by shear. As a result, turbulent transport is less efficient. Above the surface layer, turbulence becomes intermittent and, because of the stable stratification, gravity waves may become important. The top of the SBL is not as well defined as the daytime CBL since the turbulence decreases much more slowly with height. Because of the low turbulence, discrete layers with their own characteristic properties may form and advect at different speeds. After sunrise, the ground begins to warm, and the cycle repeats.

Over the ocean, the large heat capacity and effective heat conductivity keep the ocean temperature nearly constant over the daily cycle; thus the daily cycle is often insignificant in the marine boundary layer (MBL). This means that the MBL typically has much less production of turbulence energy by buoyancy and is usually shallower (perhaps 0.5 to l .5 km) than the daytime CBL over land. On average, there is less cloudiness at the top of the MBL during the day than at night because absorption of solar short-wave radiation warms the MBL directly and thus reduces the production of turbulence by buoyancy and the degree of mixing.

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