Abstract
The height of the atmospheric boundary layer (ABL) is an important variable in both observational studies and model simulations. The most commonly used measurement for obtaining ABL height is a rawinsonde profile. Mesoscale or regional scale models use a bulk Richardson number based on profiles of the forecast variables. Here we evaluate the limitations of several frequently-used approaches for defining ABL height from a single profile, and identify the optimal threshold value for each method if profiles are the only available measurements. Aircraft measurements from five field projects are used, representing a variety of ABL conditions including stable, convective, and cloud-topped boundary layers over different underlying surfaces. ABL heights detected from these methods were validated against the ‘true’ value determined from aircraft soundings, where ABL height is defined as the top of the layer with significant turbulence. A detection rate was defined to denote how often the ABL height was correctly diagnosed with a particular method. The results suggest that the temperature gradient method provides the most reasonable estimates, although the detection rate and suitable detection criteria vary for different types of ABL. The Richardson number method, on the other hand, is in most cases inadequate or inferior to the other methods that were tried. The optimal range of the detection criteria is given for all ABL types examined in this study.
Highlights
The atmospheric boundary layer (ABL) is defined as the lowest layer of the troposphere that is affected by the presence of the underlying surface and responds to surface forcing on a time scale of 1h or less (Stull 1988)
We provide a range of optimal h detetection criteria for several commonly-used gradient-based ABL height-detection schemes based on relatively large datasets of temperature, wind, and cloud-water profiles and validate the criteria with the ‘true’ ABL height from the turbulence measurements
Rib is directly related to the generation/consumption of turbulence in the ABL, detecting h using Rib (Fig. 5c) does not give satisfactory results compared to the temperature gradient (TGRD) and wind-shear method (WDS) methods
Summary
The atmospheric boundary layer (ABL) is defined as the lowest layer of the troposphere that is affected by the presence of the underlying surface and responds to surface forcing on a time scale of 1h or less (Stull 1988) The depth of this layer, h, is a key variable in many applications such as air-pollution prediction and weather forecasting (Beyrich 1997). The ABL height is an important scaling length for normalizing ABL variables, such as fluxes and vertical gradients of wind, potential temperature, and moisture for model and observational analyses. We provide a range of optimal h detetection criteria for several commonly-used gradient-based ABL height-detection schemes based on relatively large datasets of temperature, wind, and cloud-water profiles and validate the criteria with the ‘true’ ABL height from the turbulence measurements.
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