THE EFFECTS OF MESOSCALE SURFACE HETEROGENEITY ON THE FAIR-WEATHER CONVECTIVE ATMOSPHERIC BOUNDARY LAYER

Open Access
Author:
Kang, Song-Lak
Graduate Program:
Meteorology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 07, 2007
Committee Members:
  • Kenneth James Davis, Committee Chair/Co-Chair
  • John Corry Wyngaard, Committee Member
  • Nelson Lawrence Seaman, Committee Member
  • David R Stauffer, Committee Member
  • Yvette Pamela Richardson, Committee Member
  • Andrew Mark Carleton, Committee Member
Keywords:
  • daytime atmospheric boundary layer
  • mesoscale surface heterogeneity
  • mixed-layer similarity
  • absence of mesoscale vertical velocity
  • mesoscale vertical flux
  • advection of moisture
Abstract:
New insight into the structure of the fair-weather daytime ABL (about 1130-1430 LST) over heterogeneous surfaces at a 10-km-order scale is obtained from observational and numerical studies of data collected by aircraft and at surface flux sites during the International H2O Project (IHOP_2002) and using a recently-developed large eddy simulation (LES). From theses studies, the key findings to the ABL structure over the mesoscale surface heterogeneity are the following. First, the ABL over mesoscale surface heterogeneity has a considerably different structure, depending on the heterogeneity intensity. In the ABL with a low amplitude of surface heat flux variation, the microscale (turbulent) variances and fluxes, after filtering out the mesoscale contribution, can fit to the values predicted by the mixed-layer similarity which is built based on the assumption of a homogenous ABL. However, the ABL with a high variation amplitude can contain temporally oscillating mesoscale horizontal flows and thus does not satisfy the quasi-steady state condition. Second, even in the ABL over mesoscale surface heterogeneity, the microscale (turbulent) vertical flux is found to be much more significant than the mesoscale vertical flux, the product of mesoscale fluctuations of vertical velocity and a scalar (potential temperature or water vapor mixing ratio), due to the absence of mesoscale fluctuation of vertical velocity. However, as the amplitude of surface heat flux variation increases, the interscale component of the vertical flux, the product of miscroscale (turbulent) vertical velocity and mesoscale fluctuation of potential temperature or water vapor mixing ratio, becomes significant and can be comparable to the microscale (turbulent) vertical flux at certain times only in the non-quasi-steady state ABL. Third, the advection of moisture by the surface-heterogeneity-induced mesoscale horizontal flows can be significant from a perspective of the environment for moist convection. Even for the ABL with a low amplitude (here 100 Wm-2 or lower) of surface heat flux variation, the amount of moisture transported from the cooler region (the region over the surface having heat flux below the domain average) to the warmer region (the region over the surface having heat flux above the domain average) is as much as 130 % of the amount of moisture supplied from the surface in the warmer region.