The Boundary Layer

“The lowest layer of the atmosphere is called the troposphere.  The troposphere can be divided into two parts:  a planetary boundary layer, PBL, extending upward from the surface to a height that ranges anywhere from 100 to 3000 m, and above it, the free atmosphere.  The boundary layer is directly influenced by the presence of the Earth’s surface, responding to such forcings as frictional drag, solar heating, and evapotranspiration.  Each of these forcings generates turbulence of various-sized eddies, which can be as deep as the boundary layer itself, lying on top of each other.   Consider the case of solar heating:  solar heating of the ground on a sunny day creates thermals of warmer air that rise over colder air causing vertical mixing and turbulence.  Therefore, in any weather prediction model, the PBL must be parameterized as a mechanism for turbulence (Stull, 1988).
A good surface forecast is often critically dependent on accurate estimates of surface fluxes, and in turn, on reasonably accurate soil moisture and temperature estimates.”


“This movie is a combined visualization of the PBL and wind dynamics over the LA basin for a one-month period. Vertical motion of the PBL is represented by the gray “blanket”. The height of the PBL is largely driven by convection associated with the changing surface temperature of the Earth (for example, rising during the day and sinking at night). The colored arrows represent the strength and direction of winds at different altitudes”


“Physical laws and equations of motions, which govern the planetary boundary layer dynamics and microphysics, are strongly non-linear and considerably influenced by properties of the Earth’s surface and evolution of the processes in the free atmosphere. To deal with this complicity, the whole array of turbulence modelling has been proposed. However, they are often not accurate enough to meet practical requests. Significant improvements are expected from application of a large eddy simulation technique to problems related to the PBL.

Perhaps the most important processes, which are critically dependent on the correct representation of the PBL in the atmospheric models (Atmospheric Model Intercomparison Project), are turbulent transport of moisture (evapotranspiration) and pollutants (air pollutants). Clouds in the boundary layer influence trade winds, the hydrological cycle, and energy exchange.”


2 thoughts on “The Boundary Layer

  1. Relative to the thunderstorm which will soon disipate, I once saw the upper portion, with supercooled droplets, separate from the lower portion, with droplets whose temperatures were above 0C, once the droplets suddenly froze and liberated the latent heat of crystallization and the surrounding colder atmosphere suddenly lifted the upper portion away from the bottom portion like a second stage rocket to the base of the tropopause where it mushroomed. At least that was my imagined assumption.

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