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Measurements of evaporation and heat transfer in the lower atmosphere by an automatic eddy‐correlation technique
Author(s) -
Dyer A. J.
Publication year - 1961
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49708737311
Subject(s) - evaporation , eddy covariance , sensible heat , environmental science , latent heat , atmospheric sciences , energy balance , wind speed , atmosphere (unit) , precipitation , meteorology , thermodynamics , physics , ecosystem , biology , ecology
Following the development of an automatic instrument for the determination of evaporation and sensible‐heat transfer by the eddy‐correlation technique, a large number of measurements of these eddy fluxes have been made over level pasture‐land. The observations, taken at a height of 4 m, are in excellent agreement with the energy balance. Individual measurements, of 5‐min duration, are distributed about a strict energy balance with a standard deviation of 31 per cent, due mainly to horizontal gradients of wind, temperature, and humidity below 4 m height. Over a period of several hours these gradients average out to negligible values. The short response time of the sensing equipment (∽ 0·3 sec) ensures that, for conditions of moderate and high instability, significant deficiencies in eddy‐flux measurements occur only with winds exceeding 10 m sec −1 . A quantity ϵ, the ‘evaporation fraction,’ i.e., the ratio of evaporation to evaporation plus sensible‐heat transfer, is found to increase markedly after rain, and to gradually decrease during the succeeding dry period. This increase can be quantitatively related to the amount of rain. The average value of ϵ during February‐March 1960 was 0·56, whilst a value of 0·54 can be computed from annual totals of precipitation and net day‐time radiation income.