Numerical study of the diurnal variation of the meteorological parameters in the planetary boundary layer Part I Diurnal variation of winds

The diurnal variation of various meteorological parameters in the Planetary Boundary Layer at different latitudes was studied adopting the basic framework of the simple one dimensional model of Estoque and modifying it in the light of the latest theories of atmospheric turbulence. Following are the results concerning the variation of wind: i) The phase angle of the diurnal wind speed wave shifts with height, the rate of shift varying with latitude. The latter is negative at latitudes north of 30°N., zero at about 30°N., and becomes positive south of 30°N. ii) Low level wind maximum occurs before midnight in midlatitudes, slightly after midnight at 30°N., at sunrise at 17.5°N., and later farther south. iii) The amplitude of the diurnal wind speed wave increases from north to south, reaches a maximum a little below 3O°N., and then decreases rapidly. The super-geostrophic winds are strongest between 40°N. and 20°N., suggesting that these latitudes are more favorable for the occurrence of low level jet than any others. The height of the low level wind maximum is below 500 m. north of 30°N., at about 550 m. between 30°N., and 12.5°N. and higher farther south. iv) The winds attain an absolute minimum value by sunrise north of 30°N., and only a relative minimum by about sunset south of 30°N. v) A semidiurnal oscillation of wind speed occurs in the layers below 400 m. north of 30°N., but is not noticed at latitudes south of say 30°N. vi) The Ekman layer appears to be shallower in latitudes south of 30°N. than in more northern latitudes