Search Results
Abstract
A performance analysis of the three turbulence-measuring aircraft which participated in the GATE is presented. These aircraft were a Lockheed C-130 operated by the Meteorological Research Flight Centre of the U.K. Meteorological Office, a Douglas DC-6 operated by the Research Flight Facility of the National Oceanographic and Atmospheric Administration, and a Lockheed L-188 operated by the Research Aviation Facility of the National Center for Atmospheric Research.
The results are based on formal intercomparison flights and analysis of fair weather days on which two or more of the aircraft were flying. In the formal intercomparison flights, two or more of the aircraft flew side by side in the fair weather atmospheric mixed layer. In both cases, the aircraft flew L-shaped patterns, consisting of 30 km legs along and normal to the mixed layer wind direction.
Quantities compared include the variances of three wind components, potential temperature, moisture, and the vertical fluxes of horizontal momentum, temperature, and moisture. The analysis shows that when all components of the gust probe system are working properly, interaircraft biases are less than the expected atmospheric variability. Quirks of the three data sets are pointed out for the benefit of future GATE data users.
Abstract
A performance analysis of the three turbulence-measuring aircraft which participated in the GATE is presented. These aircraft were a Lockheed C-130 operated by the Meteorological Research Flight Centre of the U.K. Meteorological Office, a Douglas DC-6 operated by the Research Flight Facility of the National Oceanographic and Atmospheric Administration, and a Lockheed L-188 operated by the Research Aviation Facility of the National Center for Atmospheric Research.
The results are based on formal intercomparison flights and analysis of fair weather days on which two or more of the aircraft were flying. In the formal intercomparison flights, two or more of the aircraft flew side by side in the fair weather atmospheric mixed layer. In both cases, the aircraft flew L-shaped patterns, consisting of 30 km legs along and normal to the mixed layer wind direction.
Quantities compared include the variances of three wind components, potential temperature, moisture, and the vertical fluxes of horizontal momentum, temperature, and moisture. The analysis shows that when all components of the gust probe system are working properly, interaircraft biases are less than the expected atmospheric variability. Quirks of the three data sets are pointed out for the benefit of future GATE data users.
Abstract
Aircraft and airborne millimeter-wave radar observations are used to interpret the dynamics of radar echoes and radar-inferred updrafts within the well-developed, weakly sheared continental convective boundary layer. Vertically pointing radar reflectivity and Doppler velocity data collected above and below the aircraft, flying along fixed tracks in the central Great Plains during the International H2O Project (IHOP_2002), are used to define echo plumes and updraft plumes, respectively. Updraft plumes are generally narrower than echo plumes, but both types of plumes have the dynamical properties of buoyant eddies, especially at low levels. This buoyancy is driven both by temperature excess and water vapor excess over the ambient air. Plumes that are better defined in terms of reflectivity or updraft strength tend to be more buoyant.
Abstract
Aircraft and airborne millimeter-wave radar observations are used to interpret the dynamics of radar echoes and radar-inferred updrafts within the well-developed, weakly sheared continental convective boundary layer. Vertically pointing radar reflectivity and Doppler velocity data collected above and below the aircraft, flying along fixed tracks in the central Great Plains during the International H2O Project (IHOP_2002), are used to define echo plumes and updraft plumes, respectively. Updraft plumes are generally narrower than echo plumes, but both types of plumes have the dynamical properties of buoyant eddies, especially at low levels. This buoyancy is driven both by temperature excess and water vapor excess over the ambient air. Plumes that are better defined in terms of reflectivity or updraft strength tend to be more buoyant.
Abstract
During the 2010 Bio–Hydro–Atmosphere Interactions of Energy, Aerosols, Carbon, H2O, and Nitrogen (BEACHON) experiment in Colorado, nighttime temperatures over a site within the 2002 “Hayman” fire scar were considerably warmer than over the “Manitou” site that was located outside the fire scar. Temperature differences reached up to 7 K at the surface and extended to an average of 500 m AGL. Afternoon temperatures through the planetary boundary layer (PBL) were similar at the two locations. PBL growth during the day was more rapid at Manitou until 1300 local time, after which the two daytime PBLs had similar temperatures and depths. Observations were taken in fair weather, with weak winds. Runs of the Advanced Research version of the Weather Research and Forecasting model (ARW-WRF) coupled to the Noah-MP land surface model suggest that the fire-induced loss of surface and soil organic matter accounted for the 3–4-K warming at Hayman relative to its prefire state, more than compensating for the cooling due to the fire-induced change in vegetation from forest to grassland. Modeled surface fluxes and soil temperature and moisture changes were consistent with observational studies comparing several-year-old fire scars with adjacent unaffected forests. The remaining difference between the two sites is likely from cold-air pooling at Manitou. It was necessary to increase vertical resolution and replace terrain-following diffusion with horizontal diffusion in ARW-WRF to better capture nighttime near-surface temperature and winds. Daytime PBL growth and afternoon temperature profiles were reasonably reproduced by the basic run with postfire conditions. Winds above the surface were only fairly represented, and refinements made to capture cold pooling degraded daytime temperature profiles slightly.
Abstract
During the 2010 Bio–Hydro–Atmosphere Interactions of Energy, Aerosols, Carbon, H2O, and Nitrogen (BEACHON) experiment in Colorado, nighttime temperatures over a site within the 2002 “Hayman” fire scar were considerably warmer than over the “Manitou” site that was located outside the fire scar. Temperature differences reached up to 7 K at the surface and extended to an average of 500 m AGL. Afternoon temperatures through the planetary boundary layer (PBL) were similar at the two locations. PBL growth during the day was more rapid at Manitou until 1300 local time, after which the two daytime PBLs had similar temperatures and depths. Observations were taken in fair weather, with weak winds. Runs of the Advanced Research version of the Weather Research and Forecasting model (ARW-WRF) coupled to the Noah-MP land surface model suggest that the fire-induced loss of surface and soil organic matter accounted for the 3–4-K warming at Hayman relative to its prefire state, more than compensating for the cooling due to the fire-induced change in vegetation from forest to grassland. Modeled surface fluxes and soil temperature and moisture changes were consistent with observational studies comparing several-year-old fire scars with adjacent unaffected forests. The remaining difference between the two sites is likely from cold-air pooling at Manitou. It was necessary to increase vertical resolution and replace terrain-following diffusion with horizontal diffusion in ARW-WRF to better capture nighttime near-surface temperature and winds. Daytime PBL growth and afternoon temperature profiles were reasonably reproduced by the basic run with postfire conditions. Winds above the surface were only fairly represented, and refinements made to capture cold pooling degraded daytime temperature profiles slightly.
Abstract
In this paper, the characteristics of urban heat island (UHI) and boundary layer structures in the Beijing area, China, are analyzed using conventional and Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM) is used to simulate these urban weather features for comparison with observations. WRF is also used to test the sensitivity of model simulations to different urban land use scenarios and urban building structures to investigate the impacts of urbanization on surface weather and boundary layer structures. Results show that the coupled WRF/Noah/UCM modeling system seems to be able to reproduce the following observed features reasonably well: 1) the diurnal variation of UHI intensity; 2) the spatial distribution of UHI in Beijing; 3) the diurnal variation of wind speed and direction, and interactions between mountain–valley circulations and UHI; 4) small-scale boundary layer convective rolls and cells; and 5) the nocturnal boundary layer lower-level jet. The statistical analyses reveal that urban canopy variables (e.g., temperature, wind speed) from WRF/Noah/UCM compare better with surface observations than the conventional variables (e.g., 2-m temperature, 10-m wind speed). Both observations and the model show that the airflow over Beijing is dominated by mountain–valley flows that are modified by urban–rural circulations. Sensitivity tests imply that the presence or absence of urban surfaces significantly impacts the formation of horizontal convective rolls (HCRs), and the details in urban structures seem to have less pronounced but not negligible effects on HCRs.
Abstract
In this paper, the characteristics of urban heat island (UHI) and boundary layer structures in the Beijing area, China, are analyzed using conventional and Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM) is used to simulate these urban weather features for comparison with observations. WRF is also used to test the sensitivity of model simulations to different urban land use scenarios and urban building structures to investigate the impacts of urbanization on surface weather and boundary layer structures. Results show that the coupled WRF/Noah/UCM modeling system seems to be able to reproduce the following observed features reasonably well: 1) the diurnal variation of UHI intensity; 2) the spatial distribution of UHI in Beijing; 3) the diurnal variation of wind speed and direction, and interactions between mountain–valley circulations and UHI; 4) small-scale boundary layer convective rolls and cells; and 5) the nocturnal boundary layer lower-level jet. The statistical analyses reveal that urban canopy variables (e.g., temperature, wind speed) from WRF/Noah/UCM compare better with surface observations than the conventional variables (e.g., 2-m temperature, 10-m wind speed). Both observations and the model show that the airflow over Beijing is dominated by mountain–valley flows that are modified by urban–rural circulations. Sensitivity tests imply that the presence or absence of urban surfaces significantly impacts the formation of horizontal convective rolls (HCRs), and the details in urban structures seem to have less pronounced but not negligible effects on HCRs.
Abstract
A multiscale dataset that includes atmospheric, surface, and subsurface observations obtained from an observation network covering a region that has a scale order comparable to mesoscale and general circulation models is described and analyzed. The dataset is half-hourly time series of forcing and flux response data developed from the one-month Cooperative Atmosphere–Surface Exchange Study (CASES-97) experiment, located in the Walnut Watershed near Wichita, Kansas. The horizontal complexity of this dataset was analyzed by looking at the sensible and latent heat flux response of station data from the three main land surface types of winter wheat, grass/pastureland, and bare soil/sparse vegetation. The variability in the heat flux response at and among the different sites points to the need for a spatially distributed, time-varying atmospheric-forcing dataset for use in land surface modeling experiments. Such a dataset at horizontal spacings of 1, 5, and 10 km was developed from the station data and other remotely sensed platforms, and its spatial heterogeneity was analyzed.
Abstract
A multiscale dataset that includes atmospheric, surface, and subsurface observations obtained from an observation network covering a region that has a scale order comparable to mesoscale and general circulation models is described and analyzed. The dataset is half-hourly time series of forcing and flux response data developed from the one-month Cooperative Atmosphere–Surface Exchange Study (CASES-97) experiment, located in the Walnut Watershed near Wichita, Kansas. The horizontal complexity of this dataset was analyzed by looking at the sensible and latent heat flux response of station data from the three main land surface types of winter wheat, grass/pastureland, and bare soil/sparse vegetation. The variability in the heat flux response at and among the different sites points to the need for a spatially distributed, time-varying atmospheric-forcing dataset for use in land surface modeling experiments. Such a dataset at horizontal spacings of 1, 5, and 10 km was developed from the station data and other remotely sensed platforms, and its spatial heterogeneity was analyzed.
Abstract
This study will validate the S-band dual-polarization Doppler radar (S-Pol) radar refractivity retrieval using measurements from the International H2O Project conducted in the southern Great Plains in May–June 2002. The range of refractivity measurements during this project extended out to 40–60 km from the radar. Comparisons between the radar refractivity field and fixed and mobile mesonet refractivity values within the S-Pol refractivity domain show a strong correlation. Comparisons between the radar refractivity field and low-flying aircraft also show high correlations. Thus, the radar refractivity retrieval provides a good representation of low-level atmospheric refractivity. Numerous instruments that profile the temperature and moisture are also compared with the refractivity field. Radiosonde measurements, Atmospheric Emitted Radiance Interferometers, and a vertical-pointing Raman lidar show good agreement, especially at low levels. Under most daytime summertime conditions, radar refractivity measurements are representative of an ∼250-m-deep layer. Analyses are also performed on the utility of refractivity for short-term forecasting applications. It is found that the refractivity field may detect low-level boundaries prior to the more traditional radar reflectivity and Doppler velocity fields showing their existence. Data from two days on which convection initiated within S-Pol refractivity range suggest that the refractivity field may exhibit some potential utility in forecasting convection initiation. This study suggests that unprecedented advances in mapping near-surface water vapor and subsequent improvements in predicting convective storms could result from implementing the radar refractivity retrieval on the national network of operational radars.
Abstract
This study will validate the S-band dual-polarization Doppler radar (S-Pol) radar refractivity retrieval using measurements from the International H2O Project conducted in the southern Great Plains in May–June 2002. The range of refractivity measurements during this project extended out to 40–60 km from the radar. Comparisons between the radar refractivity field and fixed and mobile mesonet refractivity values within the S-Pol refractivity domain show a strong correlation. Comparisons between the radar refractivity field and low-flying aircraft also show high correlations. Thus, the radar refractivity retrieval provides a good representation of low-level atmospheric refractivity. Numerous instruments that profile the temperature and moisture are also compared with the refractivity field. Radiosonde measurements, Atmospheric Emitted Radiance Interferometers, and a vertical-pointing Raman lidar show good agreement, especially at low levels. Under most daytime summertime conditions, radar refractivity measurements are representative of an ∼250-m-deep layer. Analyses are also performed on the utility of refractivity for short-term forecasting applications. It is found that the refractivity field may detect low-level boundaries prior to the more traditional radar reflectivity and Doppler velocity fields showing their existence. Data from two days on which convection initiated within S-Pol refractivity range suggest that the refractivity field may exhibit some potential utility in forecasting convection initiation. This study suggests that unprecedented advances in mapping near-surface water vapor and subsequent improvements in predicting convective storms could result from implementing the radar refractivity retrieval on the national network of operational radars.
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.
