H24B-01 INVITED
Micro- and Macro- Dispersive Fluxes in Canopy Flows
Resolving every detail of the three-dimensional canopy morphology and its underlying topography remains untenable when modeling high Reynolds number geophysical flows. How to best represent the effects of such complex morphological variability and any concomitant topographic variability into one-dimensional bulk flow representation remains a fundamental challenge to be confronted. Theoretically, planar averaging to the scale of interest should be applied to the time-averaged mean momentum balance; however, such averaging gives rise to covariance or dispersive terms produced by spatial correlations of time-averaged quantities that remain "unclosed" or require parameterization. When the averaging scale is commensurate with few canopy heights, these covariances can be labeled as "micro- dispersive" fluxes. When averaging is intended to eliminate low-wavenumber topographic variations, we refer to these covariances as "macro-dispersive" terms. Two flume experiments were used to explore the magnitude and sign of both micro- and macro- dispersive fluxes relative to their conventional Reynolds stresses counterparts: a rod-canopy with variable roughness density and a dense rod canopy situated on gentle hilly terrain. When compared to the conventional momentum flux, the micro-dispersive fluxes in the lowest layers of sparse canopies can be significant (>30%). For dense canopies, the dispersive terms remain negligible when compared to the conventional momentum fluxes throughout. For the macro-dispersive fluxes, model calculations suggest that these terms can be neglected relative to the Reynolds stresses for a deep canopy situated on a narrow hill. For the region in which topographic variations can interact with the pressure, both model calculations and flume experiments here suggest that the macro-dispersive fluxes cannot be neglected, and their modeled value may be some 20% of the typical Reynolds stresses.
H24B-02 INVITED
Turbulent flow over isolated ridges; influence of vegetation
An investigation of turbulent flow over isolated rough ridges will be presented. Comparisons will be made between large-eddy simulations with 1) a specified roughness length and 2) resolving the influence of a forest canopy of equivalent roughness. In the latter case, the interplay between the pressure drag induced by the hill and that from the vegetation makes the hill appear more steep than reality. Wind tunnel simulations will provide support for the numerical results. In closing, the impact of this interplay on scalar transport and it's influence on point measurements in this regime will be emphasized.
H24B-03
Using large eddy simulation to evaluate source area contributions from aircraft flux measurements over heterogeneous land cover
The estimation of spatial patterns in surface fluxes from aircraft observations poses several challenges in presence of heterogeneous land cover, related to the effects of turbulence on scalars transport and the different behavior of passive (moisture) versus active (temperature) scalars. This in turn has significant implications associated with uncertainties in the source area/flux footprint estimation and comparison with spatial flux fields produced by land surface models. This may contribute to increased errors in both modeled and measured sensible (H) and latent (LE) heat fluxes. In fact, current footprint models do not adequately consider the role of atmospheric boundary layer (ABL) dynamics affecting source area contributions from aircraft-based flux measurements. This study provides some insight into the ABL processes that are likely to affect footprint/source- area contribution dynamics of H and LE to surface layer turbulent flux measurements from airborne sensors via Large Eddy Simulation (LES) of the land-atmosphere interactions. We focus on 30 m-level aircraft flux observations collected over a study site in central Oklahoma during the SGP97 experiment. Aircraft-model comparisons provide observational evidence of a difference in source area for turbulent H and LE fluxes. The LES correctly simulates the observed stationary eddies from updrafts originated from local surface discontinuities, and hence provide physical evidence of the observed anomalies in footprint estimation required for matching modeled and measured H and LE. A conceptual model for the interpretation of the aircraft observation will be described. The dependency of the LE/H partitioning and of their spatial correlation with respect to the filtering scale, the scale of the surface heterogeneity, and the distance from the ground will also be discussed. The results provide useful information for developing footprint models that consider differing source area/footprint contributions between active (H) and passive (LE) scalars by considering land surface heterogeneity and ABL dynamics.
H24B-04
Effects of soil moisture and snow cover initialization on simulations of atmospheric boundary layer flow over complex terrain
Soil moisture affects flow in the atmospheric boundary layer (ABL) through changes in surface heat fluxes. These fluxes are also affected by the presence or absence of snow cover. Standard surface boundary condition initialization procedures often rely on coarse grid data sets that are unable to capture variation over topography resolved by finer grids. To investigate this issue, high resolution simulations of ABL flow over Owens Valley, CA are carried out using the Advanced Regional Prediction System (ARPS). Owens Valley lies between the Sierra Nevada and White-Inyo mountain ranges, so that the simulation domain covers an area with large variation in land-surface characteristics. Simulations are performed first using standard surface boundary condition initialization procedures, then using field observations of soil moisture and temperature as well as adjusted snow cover. A quiescent and a strongly forced case are considered. Simulation results are compared to observations gathered during the Terrain- Induced Rotor Experiment in March and April, 2006. Preliminary results indicate that more accurate soil moisture and snow cover characterization changes flow evolution under quiescent conditions. The effects of soil moisture and snow cover initialization under strongly forced conditions are also examined.
H24B-05
On the Transient Correlation Structure of Water Vapor and Carbon Dioxide Time Series Over Low-Profile Vegetation
Much work has previously centered on temperature and water vapor similarity as a result of its application to the flux-variance method. Less attention, however, has been paid to similarity between carbon dioxide and water vapor. In this study, we analyze water vapor and carbon dioxide time series to infer causes of departure from correlation coefficients of +/-1. Photosynthesis and transpiration, both stomatal processes, have identical sources and sinks. Therefore, under hypothetical conditions in which carbon and water fluxes are comprised of only these two components, the effects of entrainment (which operates at a large scale) and differences in molecular diffusivities between water vapor and carbon dioxide (which operates at a small scale) can cause degradation in the correlation between the two time series. Under more realistic conditions in which the non- stomatal processes of respiration and bare evaporation are active, additional degradation is imparted upon the correlation. The correlation structure between water vapor and carbon dioxide time series is analyzed using high- frequency data collected over an early-growth corn field on the Eastern Shore of Virginia. We contrast the correlations observed following a long dry-down period with those observed shortly after a significant rainfall, when both bare soil evaporation and respiration fluxes became enhanced. Wavelet analysis is applied to the time series to distinguish between the effects of small-scale vs. large-scale process on the degradation of the correlation.
H24B-06
A Temperature and Water Vapor Scanning Raman Lidar for Observation of Land-Atmosphere Interaction
To understand the interaction and feedback between the terrestrial ecosystem and the atmosphere, rapid (every few seconds) and spatially resolved (every few meters) vertical measurements of temperature and water vapor concentration in the atmospheric boundary layer are highly desired. The Raman lidar technique applied in the solar blind region is a suitable approach for achieving the task: pure rotational Raman spectra of atmospheric nitrogen and oxygen molecules excited by laser radiation of 266-nm wavelength are used to measure the air temperature. Working in a solar blind region with no sky background noise gives an advantage of all day operation as well as an opportunity to use high field-of-view receiving telescopes that allows the operational range of the lidar starting from ten-fifteen meters. Multi-mirror facet telescope design of the lidar provides small dynamic range of the signals (less than three times within the range from 50 to 500 meters), and therefore allows nearly constant measurement accuracy within the whole operational range. A unique diffraction grating polychromator with capability of stray light suppression of 7-8 orders of magnitude provides sufficient spectral purity of the Raman signals even when working in clouds or in a dense haze condition. The polychromator of the lidar is designed to combine in one optical channel the light collected with four receiving telescopes, while using the same dispersion elements for all the telescopes. Equipped with elevation and azimuthal scanning drives the system is capable of 3D mapping of atmospheric parameters. Calibration and field-test experiments have demonstrated the capability of lidar to acquire temperature profiles with high spatial and temporal resolutions and reasonable accuracy. http://eflum.epfl.ch
H24B-07
Relation between Surface Flux Measurements and Hydrologic Conditions in a Subtropical Scrubland during the North American Monsoon
Land-atmosphere interactions are poorly understood in the North American monsoon region due to the paucity of field observations of the soil-plant-atmosphere continuum. Given the strong hydroclimatic seasonality and the variations induced by complex terrain, measuring and interpreting hydrological fluxes and states is a challenge. In this study, we describe recent efforts designed to reveal the interactions between surface conditions and turbulent fluxes in a subtropical scrubland experiencing dramatic greening during the monsoon season. Our analysis is based on the following data sets: (1) turbulent flux estimates from an eddy covariance tower, (2) vertical profiles of water vapor and carbon dioxide content, (3) spatial sampling of soil moisture and temperature in the tower footprint and (4) remotely-sensed surface characteristics. Based on these observations, we investigate the temporal variability in the relation between total evapotranspiration and footprint-averaged soil moisture, showing that vegetation greening, as captured by remote sensing, has a significant impact. We also relate the total evapotranspiration to CO2 and water vapor profile measurements to characterize diurnal patterns and their seasonal evolution. Finally, we discuss recent experiments utilizing stable isotopes to partition total evapotranspiration into evaporation and transpiration components in the subtropical scrubland. We also highlight how the field observations will aid numerical modeling efforts in the North American monsoon region. http://www.ees.nmt.edu/vivoni/sonora/www/
H24B-08
Inhomogeneous precipitation distribution and snow transport in steep terrain
The inhomogeneous snow distribution found in Alpine terrain is the result of wind and precipitation interacting with the (snow) surface over terrain. We introduce and explain preferential deposition of precipitation as the deposition process without erosion of previously deposited snow and thus in the absence of saltation. A numerical model is developed, describing on a physical basis the relevant processes saltation, suspension and preferential deposition. The model uses high resolution wind fields calculated with a meteorological model (ARPS). The model is used to simulate a 120 hour snow storm period over a steep Alpine ridge, for which also snow distribution measurements are made. The comparison to measurements shows that the model captures the larger scale snow distribution patterns and predicts the total additional lee slope loading well. However, the spatial resolution of 25 m is still insufficient to capture the smaller scale deposition features observed. The model suggests that the snow distribution on the ridge scale is primarily caused by preferential deposition and that this result is not sensitive to model parameters such as turbulent diffusivity, drift threshold, or concentration in the saltation layer.