Turbulent Efficiency in Vegetation Canopies

Nicole Hervol '06

In order to find the efficiency of transport of heat and water vapor between carious levels in the atmosphere, the scalar roughness lengths of heat and water vapor, zoh and zov, respectively, must first be estimated. These quantities are defined as the height at which the temperature and humidity profiles, respectively, reach their measured values at the surface. To determine these quantities, according to past theory, the surface temperature must first be measured. This is usually done cia the use of remote-sensing satellites; however, the measured temperature often is dependent upon the view angle of the radiometer, as the radiometer “sees” different fractions of vegetation and soil from different angles. Thus, the quantities zoh and zov are loosely defined, and as a result, kB-1, defined as ln(z/zoh,v), is heavily dependent upon these temperature measurements.

A new model based on the complementary evaporation approach was developed to determine kB-1 without using unreliable surface temperature and humidity measurements. The model was applied to datasets from Kansas grassland and a Canadian boreal forest. The new approach, when applied to grasslands, gave estimates that are consistent with classic wind tunnel and theoretical value of kB-1, provided that the data were carefully screened to ensure that the assumptions of the complementary approach were valid. Use of the method on forest canopies requires further investigation.