Search Results
Abstract
Methods of eliminating or reducing three types of errors found in the Gill UVW anemometer have been investigated by utilizing field experiments comparing this sensor with a three-component sonic anemometer. The non-cosine response of each of the three orthogonal propellers to a wind which is not parallel to the propeller axis was adequately corrected during computer processing of the data, using the manufacturer's wind-tunnel-measured calibrations. The accepted theory describing a propeller as a first-order system with a time constant τ = L/ū (where L is a distance constant characterizing the propeller inertia and ū is the mean wind) was found to be only a fair description of the frequency response, probably due to dependence of L on properties of the flow, but was used to qualitatively delineate proper applications for this sensor. The threshold response was improved for the U and V components by orienting the anemometer so that the mean wind direction bisects the angle between the horizontal axis propellers. Improvement was also achieved for the vertical component of the wind by rotating the formerly vertical W propeller 45° into the horizontal, in the plane bisecting the horizontal propeller axes. The orthogonal components of the wind must then be calculated during computer processing of the data. Since for many applications the finite, but small, response threshold of the vertical component was not found to be a serious problem, the additional complication of this modification may be unnecessary.
Abstract
Methods of eliminating or reducing three types of errors found in the Gill UVW anemometer have been investigated by utilizing field experiments comparing this sensor with a three-component sonic anemometer. The non-cosine response of each of the three orthogonal propellers to a wind which is not parallel to the propeller axis was adequately corrected during computer processing of the data, using the manufacturer's wind-tunnel-measured calibrations. The accepted theory describing a propeller as a first-order system with a time constant τ = L/ū (where L is a distance constant characterizing the propeller inertia and ū is the mean wind) was found to be only a fair description of the frequency response, probably due to dependence of L on properties of the flow, but was used to qualitatively delineate proper applications for this sensor. The threshold response was improved for the U and V components by orienting the anemometer so that the mean wind direction bisects the angle between the horizontal axis propellers. Improvement was also achieved for the vertical component of the wind by rotating the formerly vertical W propeller 45° into the horizontal, in the plane bisecting the horizontal propeller axes. The orthogonal components of the wind must then be calculated during computer processing of the data. Since for many applications the finite, but small, response threshold of the vertical component was not found to be a serious problem, the additional complication of this modification may be unnecessary.
Abstract
A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the ground cools and on how large the vertical temperature gradient is within the layer. A radiative flux difference of more than 10 W m−2 over 46 m of height was observed under weak-wind and clear-sky conditions after hot days. Wind speed variation can change not only the sensible heat transfer but also the surface longwave radiation because of variations of the area exposure of the warmer grass stems and soil surfaces versus the cooler grass blade tips, leading to fluctuations of the radiative flux divergence throughout the night.
Abstract
A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the ground cools and on how large the vertical temperature gradient is within the layer. A radiative flux difference of more than 10 W m−2 over 46 m of height was observed under weak-wind and clear-sky conditions after hot days. Wind speed variation can change not only the sensible heat transfer but also the surface longwave radiation because of variations of the area exposure of the warmer grass stems and soil surfaces versus the cooler grass blade tips, leading to fluctuations of the radiative flux divergence throughout the night.
Abstract
This paper describes important characteristics of an uncoupled high-resolution land data assimilation system (HRLDAS) and presents a systematic evaluation of 18-month-long HRLDAS numerical experiments, conducted in two nested domains (with 12- and 4-km grid spacing) for the period from 1 January 2001 to 30 June 2002, in the context of the International H2O Project (IHOP_2002). HRLDAS was developed at the National Center for Atmospheric Research (NCAR) to initialize land-state variables of the coupled Weather Research and Forecasting (WRF)–land surface model (LSM) for high-resolution applications. Both uncoupled HRDLAS and coupled WRF are executed on the same grid, sharing the same LSM, land use, soil texture, terrain height, time-varying vegetation fields, and LSM parameters to ensure the same soil moisture climatological description between the two modeling systems so that HRLDAS soil state variables can be used to initialize WRF–LSM without conversion and interpolation. If HRLDAS is initialized with soil conditions previously spun up from other models, it requires roughly 8–10 months for HRLDAS to reach quasi equilibrium and is highly dependent on soil texture. However, the HRLDAS surface heat fluxes can reach quasi-equilibrium state within 3 months for most soil texture categories. Atmospheric forcing conditions used to drive HRLDAS were evaluated against Oklahoma Mesonet data, and the response of HRLDAS to typical errors in each atmospheric forcing variable was examined. HRLDAS-simulated finescale (4 km) soil moisture, temperature, and surface heat fluxes agreed well with the Oklahoma Mesonet and IHOP_2002 field data. One case study shows high correlation between HRLDAS evaporation and the low-level water vapor field derived from radar analysis.
Abstract
This paper describes important characteristics of an uncoupled high-resolution land data assimilation system (HRLDAS) and presents a systematic evaluation of 18-month-long HRLDAS numerical experiments, conducted in two nested domains (with 12- and 4-km grid spacing) for the period from 1 January 2001 to 30 June 2002, in the context of the International H2O Project (IHOP_2002). HRLDAS was developed at the National Center for Atmospheric Research (NCAR) to initialize land-state variables of the coupled Weather Research and Forecasting (WRF)–land surface model (LSM) for high-resolution applications. Both uncoupled HRDLAS and coupled WRF are executed on the same grid, sharing the same LSM, land use, soil texture, terrain height, time-varying vegetation fields, and LSM parameters to ensure the same soil moisture climatological description between the two modeling systems so that HRLDAS soil state variables can be used to initialize WRF–LSM without conversion and interpolation. If HRLDAS is initialized with soil conditions previously spun up from other models, it requires roughly 8–10 months for HRLDAS to reach quasi equilibrium and is highly dependent on soil texture. However, the HRLDAS surface heat fluxes can reach quasi-equilibrium state within 3 months for most soil texture categories. Atmospheric forcing conditions used to drive HRLDAS were evaluated against Oklahoma Mesonet data, and the response of HRLDAS to typical errors in each atmospheric forcing variable was examined. HRLDAS-simulated finescale (4 km) soil moisture, temperature, and surface heat fluxes agreed well with the Oklahoma Mesonet and IHOP_2002 field data. One case study shows high correlation between HRLDAS evaporation and the low-level water vapor field derived from radar analysis.
