Wind and Temperature Profiles in the Radix Layer: The Bottom Fifth of the Convective Boundary Layer

Abstract

In the middle of the convective atmospheric boundary layer is often a deep layer of vertically uniform wind speed (M UL), wind direction, and potential temperature (UL). A radix layer is identified as the whole region below this uniform layer, which includes the classic surface layer as a shallower subdomain. An empirical wind speed (M) equation with an apparently universal shape exponent (A) is shown to cause observations from the 1973 Minnesota field experiment to collapse into a single similarity profile, with a correlation coefficient of roughly 0.99. This relationship is M/M UL ϭ F(z/z R), where F is the profile function, z is height above ground, and z R is depth of the radix layer. The profile function is F ϭ (z/z R) A exp[A(1 Ϫ z/z R)] in the radix layer (z/z R Յ 1), and F ϭ 1 in the uniform layer (z R Ͻ z Ͻ 0.7z i). The radix-layer equations might be of value for calculation of wind power generation, wind loading on buildings and bridges, and air pollutant transport. The same similarity function F with a different radix-layer depth and shape exponent is shown to describe the potential temperature () profile: (Ϫ UL)/(0 Ϫ UL) ϭ 1 Ϫ F(z/z R), where 0 is the potential temperature of the air near the surface. These profile equations are applicable from 1 m above ground level to the midmixed layer and include the little-studied region above the surface layer but below the uniform layer. It is recommended that similarity profiles be formulated as mean wind or potential temperature versus height, rather than as shears or gradients versus height because shear expressions disguise errors that are revealed when the shear is integrated to get the speed profile. G. This layer is above the boundary layer. Next is the entrainment zone, a region of subadiabatic temperature profiles, overshooting thermals, intermittent turbulence, and wind shear (Deardorff et al. 1980). Further down is a region where wind speed and direction are nearly uniform with height, z. This is the uniform layer (UL), where the wind speed is M UL and the potential temperature is UL. The wind is subgeostrophic because thermals communicate surface drag information via nonlocal transport. Traditionally, the term ''mixed layer'' is reserved for the whole turbulent region between the surface and the average ML top, z i ; hence,

Diurnal Cycles of Surface Winds and Temperatures as Simulated by Five Boundary Layer Parameterizations

“…In this regard, the BLK scheme produces an entrainment layer (i.e., 1000-1400 m), a nearly uniform flow layer (100-1000 m), and a convective root layer below, with more momentum transferred downward to accelerate the surface flow, leading to the generation of strong V SFC during the daytime. This vertical wind profile in the PBL is very similar to that discussed by Santoso and Stull (1998).…”

Use of Synthetic Data to Test Flight Patterns for a Boundary Layer Field Experiment

“…During convective conditions with statically unstable air, one can identify subdomains of the convective mixed layer (ML) having different similarity scalings. Using wind speed as an example, winds are zero near the ground and smoothly increase until finally becoming tangent to the vertically uniform winds in the mid-ML (Santoso and Stull 1998). In this middle layer, called the uniform layer (UL), wind speed and direction are nearly uniform, but subgeostrophic, with height.…”

“…There is a region or gap between the top of the SL and the bottom of the UL where some researchers feel that SL similarity theories fail. To better explain this portion of the ML, Santoso and Stull (1998) analyzed data from the 1973 Minnesota field experiment (Izumi and Caughey 1979) and were able to define a radix layer (RxL) as the region between the surface and the bottom of the UL that obeys a similarity different than MO. The classic SL is a subdomain within the bottom of the RxL.…”

“…Based on these Minnesota data, the following empirical relations (Santoso 1993;Santoso and Stull 1998) were found to describe the vertical profiles of mean wind speed M and potential temperature within and above the radix layer:…”

“…Field experiment data for the RxL are very limited. As was shown by Santoso and Stull (1998), it is impossible to use SL tower data at low altitudes with rawinsonde data aloft, because there is usually an artificial gap between the two profiles at altitudes where accurate RxL data are needed most. The first system gives good time averages while the second system gives snapshots with large sampling error.…”