Evaporation Ducts


Learning Objectives


The Atmospheric Boundary Layer (ABL) and the Atmospheric Surface Layer

The Atmospheric Boundary Layer (ABL) extends from the surface to the base of the inversion layer, at a location designated the inversion base height, or Zi. Air in the ABL feels the surface and is usually turbulent.

The lowest 10% of the ABL is called the Surface Layer. This has special properties that will be examined when we study evaporation ducts. This layer extends only 10's of meters above the surface.

Vertical fluxes (heat, moisture, momentum) are assumed to be constant in the surface layer. These fluxes can be determined from one measurement of temperature, humidity, and wind speed within the surface layer and another measurement at the surface itself. This is accomplished using equations based on Monin-Obukhov Similarity Theory (MO Theory). Unlike the ABL theories, MO Theory is well developed and quite accurate.

Similar to other regions, variations in the value of M (remember M?) are most strongly affected by variations in vapor pressure, e. A moist surface will produce a negative vertical gradient in e and also M near the surface. When the vertical gradient in M is negative, a trapping layer is present. This trapping layer produces what we call an evaporation duct. Although this duct always extends to the surface, it is called an evaporation duct not a surface duct to distinguish it from the surface ducts described earlier which are usually deeper and caused by different processes than evaporation ducts. The evaporation duct is especially important over the ocean or lakes for EM frequencies in the 1 - 100 GHz ranges. Lower frequencies are usually not much affected by evaporation ducts because the wavelengths are too large to be trapped in the shallow region of the evaporation duct.

In the plot to the right, you see the lowest 20 meters of the same profiles shown above of humidity and M. (In this case we're looking at relative humidity instead of e, but they both have the same logarithmic shape near the surface). The steep humidity gradient corresponds to where M has the greatest magnitude negative slope.

Higher up, but still in the surface layer, the humidity gradient becomes less and therefore M has a less negative slope. At some elevation, the top of the duct, the humidity gradient becomes weak enough that the decreasing pressure effects on M dominates over the humidity gradient effects and the gradient of M becomes positive. This location where the gradient of M switches from negative to positive indicates the top of the evaporation duct and is represented by the symbol Z*.

The vertical distance from the surface at the switchover location is called the evaporation duct height.

Another important parameter is the strength of the evaporation duct, ΔM. The strength is defined as the value of M at the surface minus the value of M at the top of the duct. Stronger ducts can trap more radiation.