A scalable plant-resolving radiative transfer model based on optimized GPU ray tracing

A new model for radiative transfer in participating media and its application to complex plant canopies is presented. The goal was to be able to efficiently solve complex canopy-scale radiative transfer problems while also representing sub-plant heterogeneity. In the model, individual leaf surfaces are not resolved, but rather vegetation is aggregated into isothermal volumes. Using the leaf angle distribution and leaf area density functions, the volumes realistically augment the radiation field through absorption and anisotropic scattering and re-emission. The volumes are grouped to form individual plants, and individual plants are grouped to form entire canopies. The model increases efficiency by performing ray tracing calculations on graphics processing units (GPUs) using the NVIDIA® OptiXTM and CUDATM frameworks, and through efficient algorithms for radiation reflection, scattering, and emission. This efficiency allows for realistic representation of heterogeneity, while also allowing for the solution of problems with very large domains (∼105 trees) quickly on an inexpensive desktop workstation. Problem execution time scaled nearly linearly with the number of discrete elements in the domain. Model results are compared with experimental data collected from an array of radiation sensors within and above a grapevine canopy and an isolated tree. Agreement between simulated and measured values of shortwave and longwave radiation were very good, with model predictions generally within the expected measurement accuracy

Countergradient heat flux observations during the evening transition period.

International audienceGradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several Intensive Observational Periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower instrumented with high-speed temperature and velocity sensors. The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are ~30-80 min. The existence of the delay and its duration can be explained by considering the convective time scale and the competition of forces associated with the classical Rayleigh-Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turn - over time - scale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover time-scale

Structure of Turbulence in Katabatic Flows Below and Above the Wind-Speed Maximum

Measurements of small-scale turbulence made in the atmospheric boundary layer over complex terrain during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to describe the structure of turbulence in katabatic flows. Turbulent and mean meteorological data were continuously measured on four towers deployed along the east lower slope (2\u20134 18) of Granite Mountain near Salt Lake City in Utah, USA. The multi-level (up to seven) observations made during a 30-day long MATERHORN field campaign in September\u2013October 2012 allowed the study of temporal and spatial structure of katabatic flows in detail, and herein we report turbulence statistics (e.g., fluxes, variances, spectra, and cospectra) and their variations in katabatic flow. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along-slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The momentum flux is directed downward (upward) whereas the along-slope heat flux is downslope (upslope) below (above) the wind maximum. This suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the along-slope heat flux) to derive the height where the flux becomes zero. It is shown that the standard deviations of all wind-speed components (and therefore of the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases when the destructive effect of vertical heat flux is completely cancelled by the generation of turbulence due to the along-slope heat flux. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for the stably stratified boundary laye

An Overview of the MATERHORN Fog Project: Observations and Predictability

A field campaign design to study fog processes in complex terrain was a component of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. The experiment was conducted in the Wasatch Mountains during January 5\u2013February 15, 2015. Fog and in particular, Ice fog (IF), defined as fog composed of only ice crystals, was studied during a part of the campaign, and this component of the program was dubbed MATERHORN-Fog. Ice fog often occurs in mountainous regions due do rapid cooling, such as radiative cooling, advective cooling, and cooling associated with mountain circulations (e.g., slope and valley winds). A variety of major instrument platforms were deployed, which included meteorological towers, a SODAR, a LiDAR, ceilometers, and a tethersonde profiler. In addition, in situ measurements took place at several locations surrounding Salt Lake City and Heber City. During the campaign, ice fog occurred at temperatures below 125 \ub0C down to 1213 \ub0C and lasted for several hours until radiative heating became significant. The visibility (Vis) during ice fog events ranged from 100 m up to 10 km. At the Heber City site an array of sensors for measuring microphysical, radiative, and dynamical characteristics of IF events were deployed. Some local effects such as upslope advection were observed to affect the IF conditions. As expected during these events, ice water content (IWC) varied from 0.01 up to 0.2 g m 123, with radiative cooling fluxes as strong as 200 W m 122; turbulent heat and moisture fluxes were significantly lower during fog events than those of fog dissipation. At times, the measured ice crystal number concentration was as high as 100 cm 123 during periods of saturation with respect to ice. Ni was not a constant as usually assumed in forecasting simulations, but rather changed with increasing IWC. Measurement based statistics suggested that the occurrence of IF events in the region was up to 30 % during the study period in the winter of 2015. Temperature profiles suggested that an inversion layer contributed significantly to IF formation at Heber. Ice fog forecasts via Weather Research and Forecasting (WRF) model indicated the limitations of IF predictability. Results suggest that IF predictions need to be improved based on ice microphysical parameterizations and ice nucleation processes

The materhorn : Unraveling the intricacies of mountain weather

Emerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort\u2014the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program\u2014that draws the expertise of a multidisciplinary, multi-institutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms, has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent submesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting