Mixing‐height differences between land use types: Dependence on wind speed

Abstract

[1] Mixing height and its spatial variation are important for a variety of issues and applications. In a previous study in the region around Nashville, Tennessee, we found a significant difference in the daytime mixed-layer inversion height z i between the area to the north and west of the city and the area to the south and east. These observations were from 3 days of very light daytime winds. The subregion to the north and west is deciduous forest, and that to the south and east is agricultural and mixed forest land. The differences in z i , which we are referring to as Á z i , were attributed to differences in surface heat flux characteristics arising from the land use variation. In the present study we investigate the effects of increasing wind speed on Á z i between the subregions, using a sample of 15 dry days from Southern Oxidants Study 1995. As convective boundary layer (CBL) wind speeds increased from ]1 to 6 m s À1 , Á z i decreased from 600 m to negligible. We found that an increase in shear-generated turbulence in the CBL was a probable factor in this decline. Two light-wind days that followed days with widespread rainfall were also noted. The Á z i on these days were small. Thus either strong winds or widespread surface moisture availability were able to nullify the effects of the land use/surface heat flux differences on Á z i between the two subregions.

Boreal lakes moderate seasonal and diurnal temperature variation and perturb atmospheric circulation: analyses in the Community Earth System Model 1 (CESM1)

“…The mean summer planetary boundary layer (PBL) height decreased significantly by up to 70 m near the location of maximum decrease in 850 hPa temperature. This decrease is consistent with observations and modelling studies that show a lower PBL depth over wet locations (Banta and White, 2003;Patton et al, 2005) and with decreased sensible heat flux (Shin and Ha, 2007). Hi (GLWD lake area)ÁLo (default CLM4 lake area) offline monthly average surface flux anomalies for Canada (land north of 488N and between 1758 and 3258E) under present climate: latent heat flux, sensible heat flux, rate of increase in subsurface enthalpy, upwelling longwave radiation and absorbed shortwave radiation.…”

The Influence of Idealized Heterogeneity on Wet and Dry Planetary Boundary Layers Coupled to the Land Surface

“…The local PBL depth is lower (higher) over the wet (dry) patches, which is consistent with recent observations that show large z i variability from heterogeneous forcing under low wind conditions (Banta and White 2003). Entrainment of low water vapor mixing ratio is observed over the region of high soil moisture and entrained dry air extends all the way to the ground (cf.…”

Vertical Velocities and Available Potential Energy Generated by Landscape Variability—Theory

“…Banta and White (2003) observed that the mixing height difference caused by small-scale landscape variability decreases from 600 m to negligible values as the ambient wind increases from 1 to 6 m s Ϫ1 . De Ridder and Gallée (1998) found that, when the soil moisture availability increases in a semiarid region like southern Israel, there is a reduction of the thermal diurnal amplitude, and an enhancement of the moist convection and precipitation.…”

“…To illustrate, the thermally forced secondary circulations are weakened or suppressed by the ambient flow (Segal and Arritt 1992), the ambient flow modulates the convective initiation (Ziegler and Rasmussen 1998), the surface roughness plays no role in the development of the convection (De Ridder and Gallée 1998), the surface roughness changes contribute to the vertical fluxes up to midtropospheric levels (Kustas et al 2005), the mixing height difference decreases as the ambient wind increases (Banta and White 2003). To establish the range of validity of the above results as a function of wavelength and ambient flow intensity, a very large number of numerical simulations would be necessary.…”