Meteorology - Planetary Boundary Layer (PBL)

The Planetary Boundary Layer (PBL) is typically referred to as the lowest few kilometers of the troposphere, and is an integral component of the Earth’s dynamic lower atmosphere. The actual depth or thickness of the PBL changes rather significantly across daily, monthly and seasonal temporal scales. Understanding the dynamics of the PBL aids in the validation of flux and other parameterizations of the PBL in numerical weather prediction models, and increases our abilities to model the coupling between the atmosphere and the Earth's surface. The PBL has traditionally been calculated by inferences taken from upper air thermodynamic soundings, with new state-of-the-science advances and methods for acquiring this information now commercially available - most notably, Lidar atmospheric remote sensing technology and systems.

Sensing the Depth of the PBL

The PBL is a critically important component of the aggregate atmosphere-Earth system, as the boundary is where the coupling between the atmosphere and the Earth's surface occurs, and the dynamic depth of the layer modulates the transfer of wind energy, heat and moisture between them. The PBL also contains most of the aerosols and water vapor in the atmosphere and thus has a primary influence on radiative fluxes. Research involving atmospheric physics, climate and environmental modeling, therefore, requires a comprehensive understanding of the role of the PBL. 

The PBL exhibits a typical diurnal cycle under fair skies and predominantly fair weather. The Earth's surface warms during the local morning due to the absorption of solar radiation, and a convective boundary layer (CBL) forms, with an entrainment zone (EZ) located above. The PBL responds to these surface dynamics in a timescale of an hour or less. Physical atmospheric quantities including velocity, temperature and moisture typically display rapid fluctuations or turbulence during this phase of insolation, and vertical mixing of the layer is strong. Surface heating recedes as insolation ceases during the evening and overnight hours, and a stable boundary layer (SBL) typically forms near the surface. An intermittently turbulent layer, called the residual layer (RL), is usually evident directly above this vertically stable boundary layer.  An atmospheric inversion is often located above the residual layer, acting as yet another layer or buffer zone between the residual layer and the free troposphere.

Particulate Matter (PM)

Particulate matter (PM) is a complex mixture of extremely small particles and liquid water droplets. Also referred to as particle pollution, PM is typically comprised of a number of components including acids such as nitrates and sulfates, organic chemicals, metals, and soil or dust particles, and is resident within the Planetary Boundary Layer (PBL).

The size of these airborne particles is directly related to their potential for causing health problems. The U.S. Environmental Protection Agency (EPA) is primarily concerned with particles that are 10 micrometers in diameter or smaller, as those particles are generally small enough to be inhaled into the lungs. Once inhaled, these particles may also affect the heart and respiratory systems and pose potentially serious health effects. 

The EPA assigns particle pollution in two distinct categories:

  • "Inhalable coarse particles," including those found near major roadways and dust-generating industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter (PM10).
  • “Fine particles," including those found in biomass fuel or industrial smoke and haze, are 2.5 micrometers in diameter and smaller (PM2.5). These particles may be generated directly from sources including forest fires, or they may form when gases emitted from power plants, industries and automobiles react chemically in the atmosphere to produce organically generated pollutants.
Figure 1: Temporal evolution of relative attenuated backscatter measured by a Windcube 3D Scanning Doppler lidar pointing vertically. The lower layer (1km height) is composed of biomass-burning aerosols, while the aerosols above are dust from the Sahara (4-5km height).

Figure 1: Temporal evolution of relative attenuated backscatter measured by a Windcube 3D Scanning Doppler lidar pointing vertically. The lower layer (1km height) is composed of biomass-burning aerosols, while the aerosols above are dust from the Sahara (4-5km height).