Determining Boundary-Layer Height from Aircraft Measurements

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.

Vertical profile observations of water vapor deuterium excess in the lower troposphere

“…We calculate the influence of these processes using R notation but present the results by converting to delta notation (δ = R sample /R standard − 1) using VSMOW as the standard. The Rayleigh distillation model describes the effects of equilibrium fractionation on the isotopic composition of a dehydrating air parcel, for example during ascent (Dansgaard, 1964). Condensate that is formed as an ascending air parcel expands and cools is enriched in the heavier isotope relative to the vapor, and in the open form of the Rayleigh model, is assumed to be immediately removed from the system (Eq.…”

“…Rehydration processes, like cloud and rain droplet evaporation, have been proposed as mechanisms that could produce negative d-excess anomalies (Bolot et al, 2013; Sodemann et al, 2017). HDO molecules preferentially evaporate relative to H 18 2 O molecules (Dansgaard, 1964). The result is a relatively positive vapor d excess, while the d-excess signature of the residual droplet becomes progressively more negative as it evaporates (Aemisegger et al, 2015).…”

“…The δ notation indicates the sample's heavy-tolight isotope ratio reported relative to an international standard (δ = R sample /R standard − 1), where δ is multiplied by 1000 to report in per mil (‰). Both equilibrium fractionation, which separates heavy and light isotopologues based on their unique vapor-pressure differences, and kinetic fractionation, which is controlled by differences in diffusion rates of the isotopologues, modulate the isotopic signature of atmospheric H 2 O v (Dansgaard, 1964). Isotopic fractionation processes act to enrich and/or deplete both HDO and H 18 2 O relative to H 16 2 O in atmospheric waters, resulting in co-varying δD and δ 18 O signatures.…”

“…This model can be used to calculate the degree to which condensate is removed from an air parcel as it cools, such as when it ascends from the surface to higher altitudes. Perturbations in δD and δ 18 O signatures often co-vary by a factor of approximately 8 because the ratio of the δD and δ 18 O equilibrium fractionation factors is approximately 8 : 1 at typical surface temperatures (Dansgaard, 1964). The second-order isotope parameter deuterium excess (d excess = δD-8 × δ 18 O) can be used to help identify the non-equilibrium kinetic fractionation processes.…”

Convective Boundary Layer Heights Over Mountainous Terrain—A Review of Concepts

“…This criterion is used as the working definition in many of the examples provided in this paper. It should be recognized however, that other definitions exist that are using either air pollution (Maughan et al, 1982;Baxter, 1991;Myrick et al, 1994;Seibert et al, 2000) or turbulence parameters (Holzworth, 1972;Norton and Hoidale, 1976;Deardorff et al, 1980;Cai and Steyn, 1993;Vogelezang and Holtslag, 1996;Beyrich, 1997;Adler and Kalthoff, 2014;Dai et al, 2014) to indicate the CBL top. For example, Seibert et al (2000) define the CBL top as "the height of the layer adjacent to the ground over which pollutants or any constituents emitted within this layer or entrained into it become vertically dispersed by convection or mechanical turbulence within a time scale of about an hour."…”