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DOE OSTI2025

Is There a Scalar Atmospheric Surface Layer Within a Convective Boundary Layer? Implications for Flux Measurements

Top‐down entrainment shapes the vertical gradients of sensible heat, latent heat, and CO 2 fluxes, influencing the interpretation of eddy covariance (EC) measurements in the unstable atmospheric surface layer (ASL). Using large eddy simulations for convective boundary layer flows, we demonstrate that decreased temperature gradients across the entrainment zone increase entrainment fluxes by enhancing the entrainment velocity, amplifying the asymmetry between top‐down and bottom‐up flux contributions. These changes alter scalar flux profiles, causing flux divergence or convergence and leading to the breakdown of the constant flux layer assumption (CFLA) in the ASL. As a result, EC‐measured fluxes either underestimate or overestimate “true” surface fluxes during divergence or convergence phases, contributing to energy balance non‐closure. The varying degrees of the CFLA breakdown are a fundamental cause for the non‐closure issue. These findings highlight the underappreciated role of entrainment in interpreting EC fluxes, addressing non‐closure, and understanding site‐to‐site variability in flux measurements.

eddy covariance fluxes of scalars↗

DOE OSTI2022

Effects of Horizontal Resolution, Domain Size, Boundary Conditions, and Surface Heterogeneity on Coarse LES of a Convective Boundary Layer

Atmospheric properties in a convective boundary layer vary over a wide range of spatial scales and are commonly studied using large-eddy simulations (LES) in various configurations. We examine how the boundary layer depth and distribution of variability across scales are affected by LES grid spacing, domain size, inhomogeneity of surface properties, and external forcing. Two different setups of the Weather Research and Forecasting (WRF) model are analyzed. A semi-idealized configuration uses a periodic domain, flat surface, prescribed homogeneous surface heat fluxes, and horizontally uniform profiles of large-scale advective tendencies. A nested LES setup employs a larger domain and realistic initial and boundary conditions, including an interactive land surface model with representative topography and vegetation and soil types. Subdomains of identical size are analyzed for all simulations. Characteristic structure sizes are quantified using the variability scales L 50 and L 95 , defined such that features smaller than that contain 50% and 95% of the total variance, respectively. Progressive increase in L 50 from vertical velocity to temperature and moisture structures is systematically reproduced in all simulation configurations. This dependence of L 50 on the considered variable complicates the development of scale-aware parameterizations for models with grid spacing in the “terra incognita”. In simulations using a larger domain with heterogeneous surface properties, the development of internal mesoscale patterns significantly affects variance distributions inside analyzed subdomains. Sizes of boundary layer structures also strongly depend on the LES grid spacing and, in case of heterogeneous surface and topography, on location of the subdomain inside a larger computational domain.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS2000

Large Eddy Simulation of Wake Vortices in the Convective Boundary Layer

The behavior of wake vortices in a convective boundary layer is investigated using a validated large eddy simulation model. Our results show that the vortices are largely deformed due to strong turbulent eddy motion while a sinusoidal Crow instability develops. Vortex rising is found to be caused by the updrafts (thermals) during daytime convective conditions and increases with increasing nondimensional turbulence intensity eta. In the downdraft region of the convective boundary layer, vortex sinking is found to be accelerated proportional to increasing eta, with faster speed than that in an ideal line vortex pair in an inviscid fluid. Wake vortices are also shown to be laterally transported over a significant distance due to large turbulent eddy motion. On the other hand, the decay rate of the, vortices in the convective boundary layer that increases with increasing eta, is larger in the updraft region than in the downdraft region because of stronger turbulence in the updraft region.

Lin, Yuh-Lang↗

NASA NTRS1989

Climatic equilibrium of the atmospheric convective boundary layer over a tropical ocean

A radiative-convective boundary layer model was developed by coupling a thermodynamic model of a partially mixed convective boundary layer (CBL) with a radiation model, and energy balance constraints were used to study coupled boundary layer (CBL) equilibrium over three timescales (about 1 day, about 10 days, and more than 100 days). It is shown that the variation in cloud top decreases with greater coupling to the atmosphere and the ocean. The slope of the latent heat flux with increasing SST decreases with more tropospheric coupling, and reverses sign with a coupled ocean.

Ridgway, W.↗

DOE OSTI2026

The Spatial and Temporal Variability of the Clear Convective Boundary Layer at the ARM SGP Supersite

The convective boundary layer (CBL), also known as the mixing layer, constitutes the critical lower segment of the atmosphere that significantly influences daily human activities. The growing demand for precise weather forecasts is driven by the requirements of agriculture, transportation, and routine societal functions. Here, to enhance understanding of the CBL, this study investigates the spatiotemporal variability in the CBL and its controlling factors using four-year Doppler lidar, surface flux, and profiling measurements at five ARM Southern Great Plains sites within a 100 km radius. This investigation utilizes data collected exclusively under clear-sky conditions or scattered low-cloud conditions. Results reveal significant spatial differences in CBL evolutions. Daily mixing layer heights (MLHs) vary up to 1 km (30% of the mean) in late afternoon. There is a clear east–west contrast: western sites (C1, E32, E37) exhibit higher summer MLH (1.9–2.1 km) and vertical velocity variances (1.0–1.2 m 2 s −2 ) than eastern sites (1.6–1.8 km), reversing in winter. Temporally, the MLH peaks at 70% of the sunrise–sunset interval, the lagging heat flux (HF) peaks at 50%; and the seasonal MLH maxima lag the HF by approximately one month, influenced by nighttime PBL (planetary boundary layer) properties. The HF and lower tropospheric stability are the main factors of influence for the CBL, but site-specific dependencies highlight the critical roles of local factors, underscoring the need for including them in CBL modeling.

ARM SGP site↗

NASA NTRS1993

Models of thermal/chemical boundary layer convection: Potential application to Venus

The upper boundary layer of Venus is comprised of at least two distinct chemical components, mantle and crust. Fluid dynamical models of convection within Venus' mantle were primarily of the thermal boundary layer type. Models assessing the ability of convective mantle flows to deform the crust were undertaken, but models exploring the effects of a variable thickness crust on mantle convection were largely lacking. A Venusian crust of variable thickness could couple back into, and alter, the mantle flow patterns that helped create it, leading to deformation mechanisms not predicted by purely thermal boundary layer convection models. This possibility is explored through a finite element model of thermal/chemical boundary layer convection. Model results suggest that a crust of variable thickness can serve as a mantle flow driver by perturbing lateral temperature gradients in the upper mantle. Resulting mantle flow is driven by the combination of free convective and nonuniform crustal distribution. This combination can lead to a flow instability manifest in the occurrence of episodic mantle lithosphere subduction initiated at the periphery of a crustal plateau. The ability of a light, near surface, chemical layer to potentially alter mantle flow patterns suggest that mantle convection and the creation and/or deformation of such a chemical layer may be highly nonseparable problems on time scales of 10(exp 8) years.

Lenardic, A.↗

NASA NTRS1989

Mean inversion strength of the convective boundary layer over the ocean

The present consideration of an equilibrium energy budget for the oceanic convective boundary layer indicates that the mean inversion strength in balance with a radiatively driven subsidence is of the order of 9 K, in keeping with the widely-observed value most recently reported by Kuo and Schubert (1988). Calculations are presented for the convective boundary layer's potential temperature budget.

Betts, Alan K.↗

DOE OSTI2026

Thermodynamics-guided machine learning model for predicting convective boundary layer height and its multi-site applicability

Accurate estimation of convective boundary layer height (CBLH) is vital for weather, climate, and air quality modeling. Machine learning (ML) shows promise in CBLH prediction, but input parameter selection often lacks physical grounding, limiting generalizability. This study introduces a novel ML framework for CBLH prediction, integrating thermodynamic constraints and the diurnal CBLH cycle as an implicit physical guide. Boundary layer growth is modeled as driven by surface heat fluxes and atmospheric heat absorption represented with the low tropospheric stability, using the diurnal cycle as input and output. TPOT and AutoKeras are employed to select optimal models, validated against Doppler lidar-derived CBLH data, achieving an R 2 of 0.84 across untrained years. Comparisons of eddy covariance (ECOR) and energy balance Bowen ratio (EBBR) flux measurements show the same prediction capability. Models trained on the ARM SGP C1 site with ECOR data and tested at E37 and E39 yield R 2 values of 0.79 and 0.81, respectively, demonstrating their adaptability. The ML model trained with all sites' data slightly enhances the performance compared with ML models trained over single-site data. The interquartile range for predicted CBLH is consistently narrower than that for DL-derived CBLH, reflecting lower variability in predicted CBLH compared to DL-derived CBLH, which is influenced by additional factors, which are not well represented with the model inputs. The model's generalizability across multiple sites at the ARM SGP site demonstrates its potential for transfer to greater distances, offering a scalable approach for enhancing boundary layer parameterization in atmospheric models.

Chu, Yufei [Stony Brook Univ., NY (United States)]↗

DOE OSTI2022

Aerosols consistently suppress the convective boundary layer development

Aerosols with different vertical distribution and various optical properties induce diverse heating rates and thereby affecting convective boundary layer (CBL) development. Our results showed consistent CBL-suppression of aerosols during daytime with numerical experiments, in which aerosols were specified at different heights with synthesized single scattering albedo from 64 studies and asymmetry factor from 20 studies globally. Absorbing aerosols concentrated below but close to the CBL top had the strongest suppression effect on CBL development relative to that concentrated near surface or above CBL. Aerosol cooling effect by attenuating incident solar radiation and surface heat flux exceeded its warming effect by reheating the atmosphere layer with absorbed shortwave radiation, and eventually declined net heating rate, which inhibited CBL development, lowered mixed-layer potential temperature and stabilized atmospheric stratification. Stove effect of absorbing aerosols (CBL enhancement) under a zero background aerosol extinction coefficient is negligible for dominant dome effect (CBL suppression) which consistently suppresses CBL development regardless of aerosol vertical height and background aerosol extinction coefficient. Our study also highlighted the importance of specifying background aerosol extinction coefficient in numerical experiments for accurate assessment of aerosol radiative forcing and CBL-aerosol interactions.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Improving the estimate of higher-order moments from lidar observations near the top of the convective boundary layer

Abstract. Ground-based lidar data have proven extremely useful for profiling the convective boundary layer (CBL). Many groups have derived higher-order moments (e.g., variance, skewness, fluxes) from high-temporal-resolution lidar data using an autocovariance approach. However, these analyses are highly uncertain near the CBL top when the depth of the CBL (zi) is changing during the analysis period. This is because the autocovariance approach is usually applied to constant height levels and the character of the eddies is changing on either side of the changing CBL top. Here, a new approach is presented wherein the autocovariance analysis is performed on a normalized height grid, with a temporally smoothed zi. Output from a large eddy simulation model demonstrates that deriving higher-order moments from time series on a normalized height grid has better agreement with the slab-averaged quantities than the moments derived from the original height grid.

Rosenberger, Tessa E. (ORCID:0000000333205873)↗

NASA NTRS1992

Convective boundary layer budgets of moisture and sensible heat over an unstressed prairie

An evaluation of convective boundary layer budgets of sensible heat and moisture were examined for two days over the unstressed vegetation of the tallgrass Konza National Prairie. In addition to the budget evaluation the study had these goals: to estimate the area-average surface fluxes and compare them to independent, ground-based measurements, to estimate the near surface evaporative fraction, and to compare different evaluations of the ratio of surface to inversion fluxes, i.e., the entrainment parameter. The budget analyses indicate that vertical and horizontal advection were significant terms in the budget and cannot be ignored.

Grossman, Robert L.↗

DOE OSTI2021

Editors’ Choice—Natural Convection Boundary Layer Thickness at Elevated Chloride Concentrations and Temperatures and the Effects on a Galvanic Couple

The natural convection boundary layer ( δ n c ) and its influence on cathodic current in a galvanic couple under varying electrolytes as a function of concentration (1 − 5.3 M NaCl) and temperature (25 °C−45 °C) were understood. Polarization scans were obtained under quiescent conditions and at defined boundary layer thicknesses using a rotating disk electrode on platinum and stainless steel 304L (SS304L); these were combined to determine δ n c . With increasing chloride concentration and temperature, δ n c decreased. Increased mass transport (Sherwood number) results in a decrease in δ n c , providing a means to predict this important boundary. Using Finite Element Modeling, the cathodic current was calculated for an aluminum alloy/SS304L galvanic couple as a function of water layer ( WL ) thickness and cathode length. Electrolyte domains were delineated, describing (i) dominance of ohmic resistance over mass transport under thin WL , (ii) the transition from thin film to bulk conditions at δ n c , and (iii) dominance of mass transport under thick WL . With increasing chloride concentration, cathodic current decreased due to decreases in mass transport. With increasing temperature, increased cathodic current was related to increases in mass transport and solution conductivity. This study has implications for sample sizing and corrosion prediction under changing environments.

25 ENERGY STORAGE↗

NASA NTRS1990

Radiative control on tropical convective boundary layer equilibrium

How the near-balance between the radiative cooling and the surface fluxes in the tropics controls boundary layer, the height of cloud-base and the top of the convective boundary layer over the tropical oceans is discussed. The impact of boundary layer cloudiness and upper level moisture on the equilibrium solutions is explored.

Betts, Alan K.↗

NASA NTRS1950

Analysis of turbulent free-convection boundary layer on flat plate

A calculation was made for the flow and heat transfer in the turbulent free-convection boundary layer on a vertical flat plate. Formulas for the heat-transfer coefficient, boundary layer thickness, and the maximum velocity in the boundary layer were obtained.

Eckert, E R G↗

NASA NTRS1995

The upper convective boundary layer

Progress made in understanding of the physics of the upper boundary layer of the sun, its influence on frequencies of five-minute oscillations, and its role in the excitation of the oscillations, are reviewed. The approaches taken for the seismological diagnosis of the properties of the layer are discussed. Information concerning the properties of the layer are obtained from Michelson Doppler imagery high-resolution data. The structure of the convective boundary layer is discussed considering high-resolution observations, three dimensional numerical simulations, the mean stratification of the convective layers, and the effects of momentum transfer by convection. The effects of convection on the oscillation frequencies, mode excitation, and mode damping, are discussed.

Kosovichev, Alexander G.↗

DOE OSTI2025

How Representative Are Uncrewed Aircraft System Measurements of the Convective Boundary Layer?

Abstract Uncrewed aircraft systems (UAS) demonstrate significant potential for filling data gaps in the atmospheric boundary layer. However, the extent to which UAS observations—typically vertical profiles taken over 15 min—are representative of the boundary layer as a whole remains poorly characterized. Using large eddy simulations (LES) of the daytime convective boundary layer (CBL), we quantify random errors in UAS measurements that occur due to insufficient statistical convergence of the time average to the true ensemble mean. Random errors in first‐order moments increase as the CBL becomes increasingly unstable, and are largest near the surface for most quantities. Errors are on the order of 2–6 m for wind speed, 15–60 for wind direction, 0.2–3 K for potential temperature, and 0.1–1 g for specific humidity, with errors in turbulent fluxes on the order of 50%–100%. Sampling strategies that mitigate random errors are discussed in light of our results.

Greene, Brian R. [Now at Verisk Extreme Event Solu↗

NASA NTRS2011

Investigating the Impact of Surface Heterogeneity on the Convective Boundary Layer Over Urban Areas Through Coupled Large-Eddy Simulation and Remote Sensing

Large-eddy Simulation (LES) was used to study convective boundary layer (CBL) flow through suburban regions with both large and small scale heterogeneities in surface temperature. Constant remotely sensed surface temperatures were applied at the surface boundary at resolutions of 10 m, 90 m, 200 m, and 1 km. Increasing the surface resolution from 1 km to 200 m had the most significant impact on the mean and turbulent flow characteristics as the larger scale heterogeneities became resolved. While previous studies concluded that scales of heterogeneity much smaller than the CBL inversion height have little impact on the CBL characteristics, we found that further increasing the surface resolution (resolving smaller scale heterogeneities) results in an increase in mean surface heat flux, thermal blending height, and potential temperature profile. The results of this study will help to better inform sub-grid parameterization for meso-scale meteorological models. The simulation tool developed through this study (combining LES and high resolution remotely sensed surface conditions) is a significant step towards future studies on the micro-scale meteorology in urban areas.

Dominguez, Anthony↗

NASA NTRS1987

Conserved variable analysis of the convective boundary layer thermodynamic structure over the tropical oceans

An analysis of FGGE dropwindsonde data using conserved thermodynamic variables shows mixing line structures for the convective boundary layer over the equatorial Pacific. Deeper boundary layers show a double structure. Reversals of the gradients of mixing ratio and equivalent potential temperature above the boundary-layer top are present in all the averages and suggest that the origin of the air sinking into the boundary layer needs further study.

Betts, Alan K.↗