Momentum and mass transfers in the surface boundary layer

Abstract

Abstract Results are presented from six micrometeorological studies conducted over a grass turf at Davis, California, in 1966 and 1967. Highly reliable surface drag and evaporation data from very sensitive lysimeters of 6·1 m diameter afforded excellent opportunity to evaluate several parameters important to aerodynamic‐prediction equations. For the six studies the mean von Kármán constant, k, ranged from 0·40 to 0·44, strongly supporting continued acceptance of k at around 0·42. The Monin‐Obukhov (1954) universal ϕ M function was found to vary as ∣Ri∣ −1/3 under near‐free convection, indicating significantly greater diabatic profile effects than suggested in the form of the KEYPS profile as given by Sellers (1965). Empirical relationships providing excellent fit to experimental data for the range −3·5 < Ri < 0·3, were ϕ M = (1–16 Ri) −1/3 and ϕ M = (1 + 16Ri) 1/3 respectively for unstable and stable conditions. For ϕ w corresponding expressions were ϕ w = ·885 (1–22 Ri) −.40 and ϕ w = ·885 (1+34 Ri) .40 . The ratio KW/KM showed a systematic drop from 1·13 at neutral to a value around 0·75 under strongly stable conditions, which conflicts with recent reports of no change of KH/KM within a wide range of stable conditions (Laykhtman and Panomareva 1969; Webb 1970; Oke 1970). The UCD results are in general agreement with the literature in that KW/KM systematically increased with increasing instability. In the range 0 > Ri > −2·0, KW/KM increased from 1·13 to 1·6. No expressions relating ϕ M and ϕ W or KW/KM to z/ L are presented due to uncertainties in Davis z/ L data. However, use of Davis Ri data in several published z/ L (Ri) relationships, allowed a limited comparison with results of previous studies.