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DOE OSTI2026

Investigation of onshore wind farm wake recovery with in situ aircraft measurements during AWAKEN

The share of wind power for electricity supply is increasing worldwide. This highly variable resource requires the improved prediction of power output for network stability. The interaction between wind farm wakes and the atmospheric boundary layer (ABL) introduces uncertainties in power production that warrant detailed investigation. The flow downwind of wind farms is characterized by a reduction in wind speed and an increase in turbulence, which both vary with atmospheric conditions. During the American WAKE experimeNt (AWAKEN), the Technische Universität Braunschweig conducted measurement flights with a research aircraft upwind and downwind of onshore wind farms in the southern Great Plains in Oklahoma in the USA. This study utilizes data from 20 flights conducted at approximately hub height in September 2023 to investigate the wind field variability downwind of the wind farms and vertical profiles to observe atmospheric stratification. The flights were aligned perpendicular to the main wind direction downwind of the King Plains and Armadillo Flats wind farms. Additionally, lidar data from both upwind and downwind ground-based measurement sites and sonic anemometer data were used for comprehensive analysis. Results indicate that under stable ABL conditions, the wake persists at greater downwind distances with a higher velocity deficit in the wake relative to the undisturbed flow compared to unstable stratification. In homogeneous terrain under stable conditions, wake recovery to 95 % occurs between a distance of 4.5 and 9 km downwind of the wind farm. In the semi-complex terrain characterized by shallow hills, slopes, and valleys, the wake exhibits a higher velocity deficit compared to homogeneous terrain, while in some cases the wake was amplified by the terrain resulting in higher velocity deficit 10 km downwind of the wind farm compared to the measurements closer to the wind farm. The turbulent kinetic energy (TKE) and “TKE difference” was found to be a valuable measure in understanding wakes in a semi-complex terrain, showing a clear wake recovery and formation depending on the stratification of the ABL.

17 WIND ENERGY↗

DOE OSTI2023

Turbulent Kinetic Energy Budgets over Gentle Topography Covered by Forests

Large-eddy simulations of flow over a “horizontally” uniform model forest are used to investigate the effects of gentle topography on the turbulent kinetic energy (TKE) budget within the canopy roughness sublayer. Despite significant differences between simulations using idealized sinusoidal topography and real topography of the Amazon forest, results indicate that the effects of topography are located predominantly in the upper canopy and above, and are mostly caused by mean advection of TKE. The “horizontally” averaged TKE budget from idealized and real gentle topographies are almost identical to that for flat terrain, including a clear inertial layer above the roughness sublayer in which shear production is balanced by local dissipation. At topography crests, where observational towers are usually located, mean vertical advection of TKE can be as important as horizontal advection. Here, we propose the use on an approximate TKE balance equation to estimate mean advection from single tower measurements, and introduce a new advection index that can be used as a proxy to quantify the importance of the topography on the TKE budget.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS2000

Spring and Summer Changes at the South Pole as Seen by the Mars Orbiter Camera

The Mars Orbiter Camera (MOC) on the Mars Global Surveyor (MGS) spacecraft has been able to follow individual features as the CO2 frost disappears and exposes the material underneath. Because the orbit of MGS is inclined at an angle of 93 degrees relative to the equator, the spacecraft gets especially good coverage of the ring at 87 degrees latitude. The following is a list of phenomena that have been seen during the spring and summer at the South Pole: (1) Circular depressions that are approximately ten meters deep and hundreds of meters in diameter. They are found only within the residual polar cap, the part that survives the summer. The high areas between the depressions are flat-topped mesas whose sides are concave circular arcs. In some places the depressions form patterns that exhibit north-south symmetry, suggesting some control by sunlight; (2) Dark layers that are exposed on the walls of the mesas. Each layer is at most a few meters thick. The dark layers might accumulate during climatic episodes of high atmospheric dust content, or they might accumulate during the annual cycling of dusty CO2; (3) Albedo differences that develop during the summer within the residual cap. These include subtle darkening of the floors of the depressions relative to the mesas and occasional major darkening of the floors, especially near the edge of the cap. The floors and mesas form a distinct stratum, suggesting they represent a distinct compositional boundary. For instance the floors may be water and the mesas may be CO2; (4) Small dark features that appear in spring on the seasonal frost outside the residual cap. Some of the features have parallel tails that are clearly shaped by the wind. Others are more symmetric, like dark snowflakes, with multiple branching arms. After the CO2 frost has disappeared the arms are seen as troughs and the centers as topographic lows; (5) Polygons whose sides are dark troughs. Those that are outside the residual cap seem to disappear when the frost disappears. The polygons and the dark snowflakelike structures may be related, and suggest that CO2 frost may form cohesive slabs; (6) Irregular depressions outside the residual cap. They look like degraded versions of the circular depressions inside the residual cap, and may be a remnant of the cap's changing location; and (7) Areas of burial and exhumation of circular depressions. Thomas et al. give an example with a sharp boundary: On one side the depressions are buried and on the other side they are exposed. In other cases there are rounded troughs up to one kilometer wide, which are dark in summer and appear to have eroded down below the floor of the circular depressions.

Ingersoll, A. P.↗

NASA NTRS1989

Daytime turbulent exchange between the Amazon forest and the atmosphere

Detailed observations of turbulence just above and below the crown of the Amazon rain forest during the wet season are presented. The forest canopy is shown to remove high frequency turbulent fluctuations while passing lower frequencies. Filter characteristics of turbulent transfer into the Amazon rain forest canopy are quantified. Simple empirical relations that relate observed turbulent heat fluxes to horizontal wind variance are presented. Changes in the amount of turbulent coupling between the forest and the boundary layer associated with deep convective clouds are presented both as statistical averages and as a series of case studies. These convective processes during the rainy season are shown to alter the diurnal course of turbulent fluxes. In wake of giant coastal systems, no significant heat or moisture fluxes occur for up to a day after the event. Radar data is used to demonstrate that even small raining clouds are capable of evacuating the canopy of substances normally trapped by persistent static stability near the forest floor. Recovery from these events can take more than an hour, even during mid-day. In spite of the ubiquitous presence of clouds and frequent rain during this season, the average horizontal wind speed spectrum is well described by dry CBL similarity hypotheses originally found to apply in flat terrain.

Fitzjarrald, David R.↗

NASA NTRS1990

Daytime turbulent exchange between the Amazon forest and the atmosphere

Fitzjarrald, David R.↗

NASA NTRS1993

Control of unsteady shock-induced turbulent boundary layer separation upstream of blunt fins

Fluctuating wall pressure measurements have been made on centerline upstream of blunt fins in a Mach 5 turbulent boundary layer. Standard time series analysis and conditional sampling algorithms have been used to examine the effects of leading edge sweepback, leading edge shape, and fin root modifications on the fluctuating pressures. Leading edge sweep considerably reduces the mean and rms pressure loading at the fin root, the extent of the region of unsteady separation shock motion, and the separation length. The frequency of pressure fluctuations in the intermittent region increases with leading edge sweepback, while the spectral content of pressure fluctuations in the separated region is virtually unchanged by leading edge sweep. A swept hemicylindrically blunted root fillet reduces the centerline upstream influence and intermittent region length by 50 percent, and reduces the mean and rms pressure loading at the fin root by about 75 percent and 95 percent respectively. Experiments using hemicylindrical, wedge shaped and flat leading edges show that while separated flow scales increase with increasing 'bluntness', intermittent region length and root loading decrease, and separation shock frequency increases.

Kleifges, K.↗

NASA NTRS1977

The low-latitude circulation of Mars

A steady-state two-layer atmospheric model is used in an attempt to simulate the circulation of the equatorial regions of Mars, with consideration of both thermal and mechanical orographic effects. A steady-state solution is sought with the aim of producing mean seasonal circulations upon which tides can be superimposed, and the relative importance of orographic forcing at low latitudes is evaluated. It is assumed that the atmospheres over a topographic feature and over flat terrain possess the same radiative and convective properties, simple dissipation laws are introduced which represent a surface drag effect, and six first-order difference equations are obtained and then reduced to two linear complex second-order coupled difference equations. Analysis of the response of the equinoctial basic wind field to a Gaussian orographic feature situated at the equator and zero deg longitude reveals a number of similarities and differences between the Martian and terrestrial responses. Most of the similarities are explained in terms of the dynamics of a rotating atmosphere, while the major differences are at least partially explained by the different optical properties, lower boundaries, and smaller mass of the Martian atmosphere.

Webster, P. J.↗

DOE OSTI2026

Characterizing Turbulence at a Forest Edge: Comparing Sub-Filter Scale Turbulence Models in Simulations of Flow over a Canopy

In wildfires, atmospheric turbulence plays a major role in the transfer of turbulent kinetic energy. Understanding how turbulence feeds back into a dynamical system is important, down to the varying small scales of fuel structures (i.e. pine needles, grass). Large eddy simulations (LES) are a common way of numerically representing turbulence. The Smagorinsky model (1963) serves as one of the most studied sub-grid scale representations in LES. In this investigation, the Smagorinsky model was implemented in HIGRAD/FIRETEC, LANL’s coupled fire-atmosphere model. This study was motivated by the need to quantitatively investigate the vorticity budget equation in HIGRAD/FIRETEC. The Smagorinsky turbulent kinetic energy (TKE) was compared to FIRETEC’s 1.5-order TKE eddy-viscosity subgrid-scale model, known as the Linn turbulence model. This was done in simulations of flow over flat terrain with a homogeneous, cuboidal canopy in the center of the domain. Examinations of the modeled vertical TKE profile and turbulent statistics at the leading edge, and throughout the canopy, show that the Smagorinsky model provides comparable results to that of the original closure model posed in FIRETEC.

58 GEOSCIENCES↗

NASA NTRS1990

The diffusive boundary layer of sediments: oxygen microgradients over a microbial mat

Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.

NASA Center ARC↗

NASA NTRS2006

Layered Deposits of Arabia Terra and Meridiani Planum: Keys to the Habitability of Ancient Mars

Understanding the habitability of ancient Mars is a key goal in the exploration of that planet. Evidence for conditions favorable to early life must be sought in ancient sedimentary rocks, such as those of Arabia Terra and Meridiani Planum. Arabia Terra, the northernmost extension of the ancient highlands, is dominated by cratered plains and minor ridged units. These plains extend south into the adjacent Meridiani Planum. The Opportunity rover landed in northern Meridiani, close to the border with Arabia. High resolution MOC images reveal extensive layered sequences across much of the Arabia and Meridiani region. These layers have been interpreted as eroded remnants of sedimentary rock deposits (Edgett, 2005). The layered sequences are concentrated in the SW quadrant of Arabia and in northern Meridiani. Preliminary mapping by Edgett (2005) distinguished four large scale layered sequences in the Arabia and Meridiani region. These have dimensions of hundreds to more than 1,000 km. MOLA altimetry shows that each of the sequences can attain a thickness of 200 to 400 m, with a total thickness greater than 1 km. The sequences are generally flat lying, with regional slopes of a few degrees. Much finer layering is evident within a number of craters. The plains and ridged units of the Arabia and Meridiani region were originally mapped as Noachian based on crater statistics, particularly the number of large craters (Scott and Carr, 1978). The layered sequences in the current study postdate many, but not all, of these large craters. The layered sequences have partially or totally filled a number of craters with diameters ranging from 20 to over 50 km. The topmost layered sequence, as well as the lower two sequences, have intermediate thermal inertia, as derived from THEMIS, indicative of moderate induration. The TES spectra from the lower sequences include features indicative of basalt. Some areas of the topmost sequence, which includes the Opportunity landing site, have TES spectra dominated by hematite. Just below this topmost sequence lies a sequence with higher thermal inertia, indicative of more indurated or coarser grained material. The TES spectra of this sequence lack distinctive mineral features, and the rocks may be obscured by a thin coating of dust. The layers have been extensively eroded. The uppermost sequences are characterized by deeply scalloped boundaries. Filled craters have been partially exhumed. Finely layered deposits within craters have been strongly dissected. Landforms uniquely attributable to wind erosion are rare, but erosive styles and geomorphology characteristic of water and possibly ice are present. The layered sequences in Arabia Terra and Meridiani Planum likely reflect an epoch when the planet was much more habitable than it is today. Several areas in these layered sequences are under intensive study as candidate landing sites for the 2009 Mars Science Laboratory.

Allen, Carlton C.↗

DOE OSTI2020

Evaluating Implementations of the Immersed Boundary Method in the Weather Research and Forecasting Model

The terrain-following coordinate system used by many atmospheric models can cause numerical instabilities due to discretization errors as resolved terrain slopes increase and the grid becomes highly skewed. The immersed boundary (IB) method, which does not require the grid to conform to the terrain, has been shown to alleviate these errors, and has been used successfully for high-resolution atmospheric simulations over steep terrain, including vertical building surfaces. Since many previous applications of IB methods to atmospheric models have used very fine grid resolution (5 m or less), the present study seeks to evaluate IB method performance over a range of grid resolutions and aspect ratios. Two classes of IB algorithms, velocity reconstruction and shear stress reconstruction, are tested within the common framework of the Weather Research and Forecasting (WRF) Model. Performance is evaluated in two test cases, one with flat terrain and the other with the topography of Askervein Hill, both under neutrally stratified conditions. WRF-IB results are compared to similarity theory, observations, and native WRF results. Despite sensitivity to the location at which the IB intersects the model grid, the velocity reconstruction IB method shows consistent performance when used with a hybrid RANS/LES surface scheme. The shear stress reconstruction IB method is not sensitive to the grid intersection, but is less consistent and near-surface velocity errors can occur at coarse resolutions. This study represents an initial investigation of IB method variability across grid resolutions in WRF. Future work will focus on improving IB method performance at intermediate to coarse resolutions.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS1961

Effect of Nose Bluntness on Transition for a Cone and a Hollow Cylinder at Mach Numbers 1.41 and 2.01

An investigation has been made to determine the effects of nose bluntness on boundary-layer transition for a cone with an included angle of 10 degrees and for a hollow cylinder. The tests were conducted at Mach numbers of 1.41 and 2.01 for free-stream Reynolds numbers per foot ranging from 1 x 10(exp 6) to 9 x 10(exp 6). The investigation was made with the use of schlieren photography for which the models were aligned with the free stream. For the 10 degree cone, the favorable effects of nose blunting were so small at both test Mach numbers as to be lost within the experimental accuracy. For small amounts of nose blunting on the hollow cylinder, for which the ratio of bluntness height to transition distance for the sharp-leading-edge cylinder was relatively small, there was little, if any, effect of blunting on transition. For somewhat larger values of this ratio, nose blunting had a favorable effect on transition. The magnitude of the favorable effect was dependent upon the size and the shape of the bluntness, and the maximum increase in transition distance relative to the sharp-leading-edge cylinder is in good agreement with the theoretical predictions of NACA Technical Report 1312. For relatively large values of the ratio of nose bluntness to transition distance, the effects of nose blunting were adverse for both the cone and the cylinder. In general, adverse effects due to blunting were larger for the flat bluntness than for the hemispherical or the round bluntness of equal bluntness height. Increasing the Mach number increased the size of bluntness required to induce adverse effects at constant free-stream Reynolds number per foot, delayed the adverse effects to higher values of Reynolds number per foot for constant nose bluntness, and reduced the abruptness of the transition decrease.

Monta, William J.↗

DOE OSTI2022

Impacts of the Thermal Gradient on Inland Advecting Sea Breezes in the Southeastern United States

Sea breezes are frequently observed in the South Carolina/Georgia region of the Southeastern United States (SEUS) and can reach upwards of 150km inland. This region is unique among the places frequently affected by sea breeze due to it being a continental location with relatively flat topography. The thermal gradient between land and water environments is a factor in introducing the sea breeze, but its role in the inland extent of sea breeze propagation isn’t as well known. We investigate the role of the thermal gradient in previously catalogued sea breeze events observed at the Savannah River Site (SRS) by taking differences of temperature measurements at inland and coastal weather stations for the days that the events occurred. We saw that the temperature differences for those days were much higher than in the non-sea breeze days during the mornings and afternoon. Numerical models were also used to conduct a sensitivity study on a sea breeze case, using simple modifications of the temperature gradient. We found that while the modifications did not stop the generation of a sea breeze circulation, the extent of the inland propagation was dependent on the magnitude of the thermal gradient.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS2021

Evaluation and Intercomparison of Wildfire Smoke Forecasts from Multiple Modeling Systems for the 2019 Williams Flats Fire

Wildfire smoke is one of the most significant concerns of human and environmental health, associated with its substantial impacts on air quality, weather, and climate. However, biomass burning emissions and smoke remain among the largest sources of uncertainties in air quality forecasts. In this study, we evaluate the smoke emissions and plume forecasts from 12 state-of-the-art air quality forecasting systems during the Williams Flats fire in Washington State, US, August 2019, which was intensively observed during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign. Model forecasts with lead times within 1 d are intercompared under the same framework based on observations from multiple platforms to reveal their performance regarding fire emissions, aerosol optical depth (AOD), surface PM2.5, plume injection, and surface PM2.5 to AOD ratio. The comparison of smoke organic carbon (OC) emissions suggests a large range of daily totals among the models, with a factor of 20 to 50. Limited representations of the diurnal patterns and day-to-day variations of emissions highlight the need to incorporate new methodologies to predict the temporal evolution and reduce uncertainty of smoke emission estimates. The evaluation of smoke AOD (sAOD) forecasts suggests overall underpredictions in both the magnitude and smoke plume area for nearly all models, although the high-resolution models have a better representation of the fine-scale structures of smoke plumes. The models driven by fire radiative power (FRP)-based fire emissions or assimilating satellite AOD data generally outperform the others. Additionally, limitations of the persistence assumption used when predicting smoke emissions are revealed by substantial underpredictions of sAOD on 8 August 2019, mainly over the transported smoke plumes, owing to the underestimated emissions on 7 August. In contrast, the surface smoke PM2.5 (sPM2.5) forecasts show both positive and negative overall biases for these models, with most members presenting more considerable diurnal variations of sPM2.5. Overpredictions of sPM2.5 are found for the models driven by FRP-based emissions during nighttime, suggesting the necessity to improve vertical emission allocation within and above the planetary boundary layer (PBL). Smoke injection heights are further evaluated using the NASA Langley Research Center's Differential Absorption High Spectral Resolution Lidar (DIAL-HSRL) data collected during the flight observations. As the fire became stronger over 3–8 August, the plume height became deeper, with a day-to-day range of about 2–9 km a.g.l. However, narrower ranges are found for all models, with a tendency of overpredicting the plume heights for the shallower injection transects and underpredicting for the days showing deeper injections. The misrepresented plume injection heights lead to inaccurate vertical plume allocations along the transects corresponding to transported smoke that is 1 d old. Discrepancies in model performance for surface PM2.5 and AOD are further suggested by the evaluation of their ratio, which cannot be compensated for by solely adjusting the smoke emissions but are more attributable to model representations of plume injections, besides other possible factors including the evolution of PBL depths and aerosol optical property assumptions. By consolidating multiple forecast systems, these results provide strategic insight on pathways to improve smoke forecasts.

AOD↗