More Like this

1. The Influence of Heterogeneous Surface Heating on Organized Vertical Motions within and above a Sheared, Unstable Atmospheric Boundary Layer

   https://doi.org/10.1175/JAS-D-24-0116.1  

   Warner, Garett R_T; Pan, Ying; Peters, John (September 2025, Journal of the Atmospheric Sciences)

   Bou-Zeid, Elie (Ed.)

   Abstract Large-eddy simulation (LES) runs are performed to understand the influence of a one-dimensional (1D) surface heating heterogeneity on organized vertical motions within and above the atmospheric boundary layer (ABL). Two knowledge gaps are of interest: (i) how updrafts develop in the low free troposphere and (ii) what parameters control updraft location and strength within the ABL? LES runs are performed for a sheared, unstable ABL driven by geostrophic winds of the same magnitude but in various directions relative to a 1D surface-heat-flux heterogeneity. Quasi-steady-state LES results are phase-averaged over time and the horizontal dimension perpendicular to the surface-heat-flux gradient to quantify secondary circulations. Regarding the first knowledge gap, the results show that organized vertical motions in the low free troposphere can be modeled as two-dimensional (2D), stationary gravity waves, whose amplitudes depend on ABL updraft strength and instability development within the free troposphere. For the second gap, the results show that organized updrafts within the ABL may form above warm surfaces or downwind of warm-to-cool transitions. These different locations are well explained by both the relative contributions of horizontal and vertical velocities to the phase-averaged vorticity fluctuations tied to secondary circulations, and the relative importance of horizontal advection and turbulent transport in the phase-averaged internal energy fluctuation equation. The main balances associated with each updraft location are used to propose empirical models of updraft strength, and it is shown that the presence of sufficiently strong organized vertical motions can potentially change parameters used by atmospheric models that do not resolve ABL turbulence. Significance StatementThe purpose of this study is to better understand how heterogeneous surface heating affects updraft location and strength in the lowest kilometers of the atmosphere. We focus on horizontal heterogeneity scales comparable to the most frequently observed cloud size, a necessary step toward the parameterization of cloud shadow effects in weather and climate models. The results show that persistent updrafts may form above either warm or cool surfaces, with their location depending on the relative importance of terms in certain budget equations. Near-surface updrafts become stronger as the background mean wind becomes more perpendicular to the surface-heat-flux gradient, but their potential to influence clouds peaks when the background mean wind is neither parallel nor perpendicular to the surface-heat-flux gradient. 

   more » « less
2. Impact of Surface Heterogeneity Induced Secondary Circulations on the Atmospheric Boundary Layer

   https://doi.org/10.1007/s10546-024-00893-7  

   Paleri, Sreenath; Wanner, Luise; Sühring, Matthias; Desai, Ankur R; Mauder, Matthias; Metzger, Stefan (January 2025, Boundary-Layer Meteorology)

   We investigate how effective surface length scales (Le f f ) and atmospheric boundary layer stability modulate surface-induced secondary circulations over a realistic heterogeneous sur- face. The evolution of the circulations and their impact on surface-atmosphere fluxes are studied using coupled large eddy simulations of the CHEESEHEAD19 field campaign. The heterogeneity-induced circulations were diagnosed using time and ensemble averaging of the atmospheric fields. Simulations were performed for summer (August) and autumn (Septem- ber) Intensive Observation Periods of the field campaign, characterised differently in terms of normalised surface length scales and ABL stability. Quasi-stationary and persistent cir- culations were diagnosed in the daytime ABL that span the entire mixed layer height (zi ). Their variation in time and space are presented. Homogeneous control runs were also per- formed to compare and contrast spatial organisation and validate the time-ensemble averaging operation. In the convective boundary layers simulated during the summer time simulations, wavelengths that scale as the effective surface heterogeneity length scales contribute the most to the heterogeneity-induced transport. Contributions from surface-induced circulations were lower in the simulated near-neutral BL for the autumn simulations. We find that both Le f f /zi and ABL static stability control the relative contribution of surface-induced circulations to the area averaged vertical transport. This scale analysis supports prior work over the study domain on scaling tower measured fluxes by including low frequency contribution. We believe that the conceptual framework presented here can be extended to include the effects of sub-grid land surface heterogeneity in numerical weather prediction and climate models and also to further explore scale-aware scaling methodologies for near surface-atmosphere exchanges. 

   more » « less
3. Statistical Analysis of Convective Updrafts in Tropical Cyclone Rainbands Observed by Airborne Doppler Radar

   https://doi.org/10.1029/2021JD035718  

   Barron, Nicholas R.; Didlake, Jr., Anthony C.; Reasor, Paul D. (March 2022, Journal of Geophysical Research: Atmospheres)

   Abstract Ten years of airborne Doppler radar observations are used to study convective updrafts' kinematic and reflectivity structures in tropical cyclone (TC) rainbands. An automated algorithm is developed to identify the strongest rainband updrafts across 12 hurricane‐strength TCs. The selected updrafts are then collectively analyzed by their frequency, radius, azimuthal location (relative to the 200–850 hPa environmental wind shear), structural characteristics, and secondary circulation (radial/vertical) flow pattern. Rainband updrafts become deeper and stronger with increasing radius. A wavenumber‐1 asymmetry arises, showing that in the downshear (upshear) quadrants of the TC, updrafts are more (less) frequent and deeper (shallower). In the downshear quadrants, updrafts primarily have in‐up‐out or in‐up‐in secondary circulation patterns. The in‐up‐out circulation is the most frequent pattern and has the deepest updraft and reflectivity tower. Upshear, the updrafts generally have out‐up‐in or in‐up‐in patterns. The radial flow of the updraft circulations largely follows the vortex‐scale radial flow shear‐induced asymmetry, being increased low‐level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) quadrants. It is hypothesized that the convective‐scale circulations are significantly influenced by the vortex‐scale radial flow at the updraft base and top altitudes. Other processes of the convective life cycle, such as bottom‐up decay of aging convective updrafts due to increased low‐level downdrafts, can influence the base altitude and, thus, the base radial flow of the updraft circulation. The findings presented in this study support previous literature regarding convective‐scale patterns of organized rainband convection in a mature, sheared TC. 

   more » « less
4. Vertical Motions in Orographic Cloud Systems over the Payette River Basin. Part 2: Fixed and Transient Updrafts and their Relationship to Forcing

   https://doi.org/10.1175/JAMC-D-21-0229.1  

   Troy J. Zaremba, Kaylee Heimes (July 2022, Journal of applied meteorology and climatology)

   Rapp, Anita (Ed.)

   Updrafts in wintertime cloud systems over mountainous regions can be described as fixed, mechanically driven by the terrain under a given ambient wind and stability profile (i.e., vertically propagating gravity waves), and transient, related to vertical wind shear and conditional instability within passing weather systems. This analysis quantifies the magnitude of fixed and transient updraft structures over the Payette River Basin sampled during the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE). Vertical motions were retrieved from Wyoming Cloud Radar measurements of radial velocity using the algorithm presented in Part 1. Transient circulations were removed and fixed orographic circulations were quantified by averaging vertical circulations along repeated cross sections over the same terrain during the campaign. Fixed orographic vertical circulations had magnitudes of 0.3-0.5 m s-1. These fixed vertical circulations comprised a background circulation in which transient circulations were embedded. Transient vertical circulations are shown to be associated with transient wave motions, cloud top generating cells, convection, and turbulence. Representative transient vertical circulations are illustrated and data from rawinsondes over the Payette River Basin are used to infer the relationship of the vertical circulations to shear and instability. Maximum updrafts are shown to exceed 5 m s 1 within Kelvin-Helmholtz waves, 4 m s-1 associated with transient gravity waves, 3 m s-1 in generating cells, 6 m s-1 in elevated convection, 4 m s-1 in surface-based deep convection, 5 m s-1 in boundary layer turbulence, and 9 m s-1 in shear-induced turbulence. 

   more » « less
5. Are Supercells Resistant to Entrainment because of Their Rotation?

   https://doi.org/10.1175/JAS-D-19-0316.1  

   Peters, John M.; Nowotarski, Christopher J.; Mullendore, Gretchen L. (April 2020, Journal of the Atmospheric Sciences)

   This research investigates a hypothesis posed by previous authors, which argues that the helical nature of the flow in supercell updrafts makes them more resistant to entrainment than nonsupercellular updrafts because of the suppressed turbulence in purely helical flows. It was further supposed that this entrainment resistance contributes to the steadiness and longevity of supercell updrafts. A series of idealized large-eddy simulations were run to address this idea, wherein the deep-layer shear and hodograph shape were varied, resulting in supercells in the strongly sheared runs, nonsupercells in the weakly sheared runs, and variations in the percentage of streamwise vorticity in updrafts among runs. Fourier energy spectrum analyses show well-developed inertial subranges in all simulations, which suggests that the percentages of streamwise and crosswise vorticity have little effect on turbulence in convective environments. Additional analyses find little evidence of updraft-scale centrifugally stable flow within updrafts, which has also been hypothesized to limit horizontal mass flux across supercell updrafts. Results suggest that supercells do have smaller fractional entrainment rates than nonsupercells, but these differences are consistent with theoretical dependencies of entrainment on updraft width, and with supercells being wider than nonsupercells. Thus, while supercells do experience reduced fractional entrainment rates and entrainment-driven dilution, this advantage is primarily attributable to increased supercell updraft width relative to ordinary convection, and has little to do with updraft helicity and rotation. 

   more » « less