Flux-gradient relationship of water vapor in the surface layer obtained from CASES-99 experiment

The flux-gradient relationship of water vapor φ_q was obtained from the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99) field experiment, which was conducted on a flat and grassy field in the southeastern part of Kansas, USA, during October 1999. The CASES-99 data include turbulence measurements of wind, temperature, and vapor density along with their time-averaged values at various levels above the ground. Quality control of the data on the turbulent wind velocity, sonic temperature, and vapor density was performed prior to flux calculation. The turbulent sensible heat and latent heat fluxes were calculated using a 30-min window, and they were subsequently corrected and checked for the fulfillment of the steady state and for turbulence intensity. Weak fluxes and through-tower wind data were also excluded from the analysis. Vertical gradients of the mean values were obtained by differentiating functions fitted to the measured mean profiles. It was found that φ_q is comparable to φ_T for weakly stable stratification and less than φ_T for strongly stable stratification. On the contrary, φ_q was found to be larger than φ_T by approximately 20% for the neutral and unstable stratifications. The best fitting functions for water vapor are found to φ_q = 1.21(1 - 13.1z/Z)^-1/2 and φ_q = 1.21(1 + 60Az/L)^1/3 for the unstable and stable stratifications, respectively.