Cloud And Radiation Testbed Research Articles

Variability in surface BRDF at different spatial scales (30 m–500 m) over a mixed agricultural landscape as retrieved from airborne and satellite spectral measurements

Over the past decade, the role of multiangle remote sensing has been central to the development of algorithms for the retrieval of global land surface properties including models of the bidirectional reflectance distribution function (BRDF), albedo, land cover/dynamics, burned area extent, as well as other key surface biophysical quantities impacted by the anisotropic reflectance characteristics of vegetation. In this study, a new retrieval strategy for fine-to-moderate resolution multiangle observations was developed, based on the operational sequence used to retrieve the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 5 reflectance and BRDF/albedo products. The algorithm makes use of a semiempirical kernel-driven bidirectional reflectance model to provide estimates of intrinsic albedo (i.e., directional-hemispherical reflectance and bihemispherical reflectance), model parameters describing the BRDF, and extensive quality assurance information. The new retrieval strategy was applied to NASA's Cloud Absorption Radiometer (CAR) data acquired during the 2007 Cloud and Land Surface Interaction Campaign (CLASIC) over the well-instrumented Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site in Oklahoma, USA. For the case analyzed, we obtained ~ 1.6 million individual surface bidirectional reflectance factor (BRF) retrievals, from nadir to 75° off-nadir, and at spatial resolutions ranging from 3 m to 500 m. This unique dataset was used to examine the interaction of the spatial and angular characteristics of a mixed agricultural landscape; and provided the basis for detailed assessments of: (1) the use of a land cover type-specific a priori knowledge in kernel-driven BRDF model inversions; (2) the interaction between surface reflectance anisotropy and instrument spatial resolution; and (3) the uncertainties that arise when sub-pixel differences in the BRDF are aggregated to a moderate resolution satellite pixel. Results offer empirical evidence concerning the influence of scale and spatial heterogeneity in kernel-driven BRDF models; providing potential new insights into the behavior and characteristics of different surface radiative properties related to land/use cover change and vegetation structure.

Synergistic use of AIRS and MODIS radiance measurements for atmospheric profiling

Retrieval of atmospheric profiles from combined radiance measurements of the Atmospheric InfraRed Sounder (AIRS) and the MODerate resolution Imaging Spectroradiometer (MODIS) onboard the NASA Aqua satellite is investigated. The collocated operational MODIS cloud mask product and the clear‐sky InfraRed (IR) radiance measurements are used to characterize the AIRS sub‐pixel cloud fraction and improve the atmospheric sounding and the surface parameters at the AIRS single field‐of‐view (SFOV) resolution. The synergistic algorithm employs an eigenvector based regression with additional information from MODIS measurements. The regression coefficients are derived from a global dataset containing atmospheric temperature, moisture, and ozone profiles. Evaluation of the retrieved profiles is performed by comparisons between AIRS soundings, European Centre for Medium‐Range Weather Forecasts (ECMWF) analysis fields and radiosonde observations from the Cloud And Radiation Testbed (CART) in Oklahoma. Spectral radiance calculations from the sounding retrievals agree with the observations from AIRS adjacent clear neighbor footprint quite well. These comparisons demonstrate the potential advantage of synergistic use of high‐spectral sounder and high‐spatial imager over either system alone.

GOES sounding improvement and applications to severe storm nowcasting

An improved clear‐sky physical retrieval algorithm for atmospheric temperature and moisture is applied to the Geostationary Operational Environmental Satellite‐12 (GOES‐12) Sounder. A comparison with the microwave radiometer (MWR) measured total precipitable water (TPW) at the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site from June 2003 to May 2005 shows that the TPW retrievals are improved by 0.4 mm over the legacy GOES Sounder TPW product. The Lifted Index (LI) derived product imagery (DPI) from the improved soundings better depicts the pre‐convective environment surrounding a tornadic supercell at Eagle Pass, Texas on 24 April 2007. Another severe storm case from 13 April 2006 demonstrates that the improved physical algorithm successfully detects low‐level moisture. Both cases show the new retrievals along with the derived products will help the forecasters with short‐term severe storm nowcasting.

Atmospheric profile retrieval with AIRS data and validation at the ARM CART site

The physical retrieval algorithm of atmospheric temperature and moisture distribution from the Atmospheric InfraRed Sounder (AIRS) radiances is presented. The retrieval algorithm is applied to AIRS clearsky radiance measurements. The algorithm employs a statistical retrieval followed by a subsequent nonlinear physical retrieval. The regression coefficients for the statistical retrieval are derived from a dataset of global radiosonde observations (RAOBs) comprising atmospheric temperature, moisture, and ozone profiles. Evaluation of the retrieved profiles is performed by a comparison with RAOBs from the Atmospheric Radiation Measurement (ARM) Program Cloud And Radiation Testbed (CART) in Oklahoma, U. S. A. Comparisons show that the physicallybased AIRS retrievals agree with the RAOBs from the ARM CART site with a Root Mean Square Error (RMSE) of 1 K on average for temperature profiles above 850 hPa, and approximately 10% on average for relative humidity profiles. With its improved spectral resolution, AIRS depicts more detailed structure than the current Geostationary Operational Environmental Satellite (GOES) sounder when comparing AIRS sounding retrievals with the operational GOES sounding products.

Measurement of Low Amounts of Precipitable Water Vapor Using Ground-Based Millimeterwave Radiometry

Abstract Extremely dry conditions characterized by amounts of precipitable water vapor (PWV) as low as 1–2 mm commonly occur in high-latitude regions during the winter months. While such dry atmospheres carry only a few percent of the latent heat energy compared to tropical atmospheres, the effects of low vapor amounts on the polar radiation budget—both directly through modulation of longwave radiation and indirectly through the formation of clouds—are considerable. Accurate measurements of PWV during such dry conditions are needed to improve polar radiation models for use in understanding and predicting change in the climatically sensitive polar regions. To this end, the strong water-vapor absorption line at 183.310 GHz provides a unique means of measuring low amounts of PWV. Weighting function analysis, forward model calculations based upon a 7-yr radiosonde dataset, and retrieval simulations consistently predict that radiometric measurements made using several millimeter-wavelength (MMW) channels near the 183-GHz line, together with established microwave (MW) measurements near the 22.235-GHz water-vapor line and ∼31-GHz atmospheric absorption window can be used to determine within 5% uncertainty the full range of PWV expected in the Arctic. This combined capability stands in spite of accuracy limitations stemming from uncertainties due to the sensitivity of the vertical distribution of temperature and water vapor at MMW channels. In this study the potential of MMW radiometry using the 183-GHz line for measuring low amounts of PWV is demonstrated both theoretically and experimentally. The study uses data obtained during March 1999 as part of an experiment conducted at the Department of Energy’s Cloud and Radiation Testbed (CART) site near Barrow, Alaska. Several radiometers from both NOAA and NASA were deployed during the experiment to provide the first combined MMW and MW ground-based dataset during dry Arctic conditions. Single-channel retrievals of PWV were performed using the MW and MMW data. Discrepancies in the retrieved values were found to be consistent with differences observed between measured brightness temperatures (TBs) and forward-modeled TBs based on concurrent radiosonde profiles. These discrepancies are greater than can be explained by radiometer measurement error alone; errors in the absorption models and uncertainty in the radiosonde measurements contribute to the discrepancies observed. The measurements, retrieval technique, and line model discrepancies are discussed, along with difficulties and potential of MMW/MW PWV measurement.

Characterization of Upper-Troposphere Water Vapor Measurements during AFWEX Using LASE

AbstractWater vapor mass mixing ratio profiles from NASA's Lidar Atmospheric Sensing Experiment (LASE) system acquired during the Atmospheric Radiation Measurement (ARM)–First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Water Vapor Experiment (AFWEX) are used as a reference to characterize upper-troposphere water vapor (UTWV) measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors over the Department of Energy (DOE) ARM Southern Great Plains (SGP) site in northern Oklahoma. LASE was deployed from the NASA DC-8 aircraft and measured water vapor over the ARM SGP Central Facility (CF) site during seven flights between 27 November and 10 December 2000. Initially, the DOE ARM SGP Cloud and Radiation Testbed (CART) Raman lidar (CARL) UTWV profiles were about 5%–7% wetter than LASE in the upper troposphere, and the Vaisala RS80-H radiosonde profiles were about 10% drier than LASE between 8 and 12 km. Scaling the Vaisala water vapor profiles to match the precipitable water vapor (PWV) measured by the ARM SGP microwave radiometer (MWR) did not change these results significantly. By accounting for an overlap correction of the CARL water vapor profiles and by employing schemes designed to correct the Vaisala RS80-H calibration method and account for the time response of the Vaisala RS80-H water vapor sensor, the average differences between the CARL and Vaisala radiosonde upper-troposphere water vapor profiles are reduced to about 5%, which is within the ARM goal of mean differences of less than 10%. The LASE and DC-8 in situ diode laser hygrometer (DLH) UTWV measurements generally agreed to within about 3%–4%. The DC-8 in situ frost point cryogenic hygrometer and Snow White chilled-mirror measurements were drier than the LASE, Raman lidars, and corrected Vaisala RS80H measurements by about 10%–25% and 10%–15%, respectively. Sippican (formerly VIZ Manufacturing) carbon hygristor radiosondes exhibited large variabilities and poor agreement with the other measurements. PWV derived from the LASE profiles agreed to within about 3% on average with PWV derived from the ARM SGP microwave radiometer. The agreement between the LASE and MWR PWV and the LASE and CARL UTWV measurements supports the hypotheses that MWR measurements of the 22-GHz water vapor line can accurately constrain the total water vapor amount and that the CART Raman lidar, when calibrated using the MWR PWV, can provide an accurate, stable reference for characterizing upper-troposphere water vapor.

Spatiotemporal Variations in Soil Water: First Results from the ARM SGP CART Network

Abstract A network of automated soil water and temperature systems, installed at 21 locations in Oklahoma and Kansas in 1996 and 1997, is providing hourly profiles of soil temperature and water at eight depths, from 0.05 to 1.75 m below the surface, in twin profiles 1 m apart. Dubbed the Soil Water and Temperature System (SWATS), these systems are an addition to the extended facilities of the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site. Average spacing between SWATS systems is about 75 km. The SWATS network is one of three overlapping soil water networks in the region but is unique in depth of deployment, providing observations of available soil water through most of the rooting zone of SGP pastures and prairies. A description of the SWATS sensor and network, calibration and data verification, and example time series from the first 3 yr of operation are presented. Perusal of the time series reveals system...

Retrieval of atmospheric-temperature and water-vapor profiles by use of combined satellite and ground-based infrared spectral-radiance measurements.

A nonlinear sounding retrieval algorithm is used to produce vertical-temperature and water-vapor profiles from coincident observations taken by the airborne High-resolution Interferometer Sounder (HIS) and the ground-based Atmospheric Emitted Radiance Interferometer (AERI) during the SUbsonic Contrails and Clouds Effects Special Study (SUCCESS). Also, clear sky Geostationary Operational Environmental Satellite (GOES) and AERI radiance measurements, achieved on a daily real-time basis at the Department of Energy's Oklahoma CART (Cloud and Radiation Testbed) site, are used to demonstrate the current profiling capability by use of simultaneous geostationary satellite and ground-based remote sensing observations under clear-sky conditions. The discrepancy principle, a method to find the proper smoothing parameters from the minimum value between the normalized spectral residual norm and the a priori upper bound, is used to demonstrate the feasibility and effectiveness of on-line simultaneous tuning of the multiple weighting and smoothing parameters from the combined satellite/airborne and ground-based measurements for the temperature and water-vapor retrieval in this nonlinear-retrieval process. An objective method to determine the degrees of freedom (d.f.) of the observation signal is derived. The d.f. of the radiance signal for the combined GOES and AERI measurements is larger than that for either instrument alone; while the d.f. of the observation signal for the combined GOES and AERI measurements is larger than that for either instrument alone and of the combined GOES and AERI measurements. The use of simultaneous clear-sky AERI and GOES data now provides improved vertical temperature and moisture soundings on an hourly basis for use in the Atmospheric Radiation Measurement program [J. Appl. Meteorol. 37, 875 (1998)].

WVR-GPS comparison measurements and calibration of the 20-32 GHz tropospheric water vapor absorption model

Collocated measurements of opacity (from water vapor radiometer brightness temperatures) and wet path delay (from ground-based tracking of global positioning satellites) are used to constrain the model of atmospheric water vapor absorption in the 20-32 GHz band. A differential approach is presented in which the slope of opacity-versus-wet delay data is used as the absorption model constraint. This technique minimizes the effects of radiometric calibration errors and oxygen model uncertainties in the derivation of a best-fit vapor absorption model. A total of approximately five months of data was obtained from two experiment sites. At the Cloud and Radiation Testbed (CART) site near Lamont, Oklahoma, three independent water vapor radiometers (WVRs) provided near-continuous opacity measurements over the interval July-September 1998. At the NASA/Goldstone tracking station in the California desert two WVRs; obtained opacity data over the September-October 1997 interval. At both sites a Global Positioning Satellite (GPS) receiver and surface barometer obtained the data required for deriving the zenith wet delays over the same time frames. Measured values of the opacity-versus-wet delay slope parameter were obtained at four WVR frequencies (20.7, 22.2, 23.8, and 31.4 GHz) and compared with predictions of four candidate absorption models referenced in the literature. With one exception, all three models provide agreement within 5% of the opacity-versus-wet delay slope measurements at all WVR frequencies at both sites. One model provides agreement for all channels at both sites to the 2-3% level. This absorption model accuracy level represents a significant improvement over that attainable using radiosondes.

Comments on “Atmospheric Disturbance Caused by Human Modification of the Landscape”

In a recent article published in the February 2001 issue of the Bulletin of American Meteorological Society, Weaver and Avissar (hereafter WA) investigated the response of the atmosphere to landscape heterogeneity to assess the importance of mesoscale fluxes of sensible and latent heat arising from differences in land use in the southern Great Plains. To do so they used the RAMS mesoscale model and a surface flux data set derived from observations and the Simple Biosphere Model. They selected six days for analysis based on the criterion that the study region located at the Department of Energy's Atmospheric Radiation Measurement Program's Cloud and Radiation Testbed (CART) was not obscured by clouds with obvious non-local origins. WA noted that mesoscale fluxes of latent heat on those six days were large, especially near and above the top of the boundary layer. They argued that regions of strong vertical motion that they associated with mesoscale fluxes were closely correlated with regions of precipitation on one of the six days they studied and with cloudy regions on two other days. From these findings, WA concluded that landscape heterogeneity arising from human influences can''significantly affect local weather and climate.'' They further concluded that the inabilitymore » to predict mesoscale fluxes is a serious failing of general circulation models (GCMs) used to predict climate change and that further studies in parameterizing such fluxes for use in GCMs is therefore necessary. In this paper we offer a somewhat different interpretation and perspective on the findings of WA.« less

Satellite remote sensing of land surface temperatures: Application of the atmospheric correction method and split-window technique to data of ARM-SGP site

The performance of algorithms for the satellite remote sensing of land surface temperatures based on an atmospheric correction method and a split-window technique was examined. For this purpose, we employed comprehensive data collected over the Atmospheric Radiation Measurement Program (ARM) Southern Great Plain (SGP) Cloud and Radiation Testbed (CART) site for a number of manually determined clear and mostly clear dates during 1997 and 1999 with cloud coverage of less than 10%. The data sources included brightness temperature data measured by the Advanced Very High Resolution Radiometer (AVHRR) on-board National Oceanic and Atmospheric Administration (NOAA)-12 and NOAA-14, collocated and coincident observed surface temperatures derived from the solar and IR radiation observing system (SIROS) and solar and IR station (SIRS) data, and balloon-borne atmospheric sounding profiles. For the application of the atmospheric correction method, we used the Moderate Resolution Transmittance Model (MODTRAN) 3.7 to compute transmittances and atmospheric radiances. In the present study, comparison of the retrieved surface temperatures with observations obtained at the central and external facility sites over the SGP area shows that the accuracy of the atmospheric correction scheme is closer to the requirement that is acceptable for estimating surface latent heat fluxes than the split-window method.

Four years of continuous surface aerosol measurements from the Department of Energy's Atmospheric Radiation Measurement Program Southern Great Plains Cloud and Radiation Testbed site

Continuous measurements of the optical and microphysical properties of aerosol particles have been made at the Department of Energy's Atmospheric Radiation Measurement Program Southern Great Plains Cloud and Radiation Testbed (CART) site covering the 4‐year period from July 1996 through June 2000. Hourly, daily, and monthly statistics have been calculated that illustrate aerosol variability over a range of timescales. A pronounced peak in total particle number, centered on the midafternoon hours (local time), is evident in the hourly statistics. A broad early morning peak in the concentration of particles >0.1‐μm aerodynamic diameter corresponds with a similar peak in aerosol light‐scattering coefficient, σsp. No strong cycles were observed in the daily statistics, suggesting that day of the week has only a minor influence on the observed aerosol variability. The σsp at a wavelength of 550 nm for the 4‐year period showed a median value of 33 Mm−1 and was highest in February and August. The median fraction of aerosol light scattering at 550 nm due to particles <1‐μm aerodynamic diameter was 0.85 over the entire record. The median aerosol light absorption coefficient, σap, for the 4‐year period was ∼1.5 Mm−1 and was observed to be highest in late summer and autumn. The σap showed an increasing trend of nearly 0.5 Mm−1 yr, possibly due to increased agricultural field burning in the area. The occurrence of an autumn decrease in single‐scattering albedo, ω0, was observed and may be caused by regional‐scale agricultural or transportation activities or seasonal changes in atmospheric flow patterns. The median value for ω0 over the 4‐year period was 0.95, but this value has decreased ∼1–2% yr−1 presumably due to increased agricultural burning. Numerous field fires during the second half of 1999 influenced the surface aerosol at the CART site causing substantial variability of aerosol optical properties. The aerosol hygroscopic growth factor (f(RH)), corresponding to a relative humidity increase of 40–85%, showed a median value of 1.83 for 1999, although much lower values were observed during periods that were probably influenced by locally generated smoke and dust aerosols (median f(RH) = 1.55 and 1.59, respectively).

A study of the effects of sub‐grid‐scale land use differences on atmospheric stability in prestorm environments

The effects of sub‐grid‐scale land use differences in modifying the stability of prestorm environments has been studied using data from the Department of Energy Cloud and Radiation Testbed (CART) in Kansas and Oklahoma and a mesoscale model. To quantify the atmospheric instability, three indices were used: the lifted index, the modified K index, and convective available potential energy. The spatial variations of these indices were calculated from simulations using spatially varying and spatially uniform surface fluxes as lower boundary conditions. The CART is approximately 105 km2 in area and is characterized by large areas of contrasting vegetation cover and surface sensible and latent heat fluxes. The spatially varying fluxes were calculated with the SiB2 model using data from the CART. Six days, during which isolated thunderstorms developed, were chosen for the study. The results suggest that sub‐grid‐scale variations in land use differences do not modify the spatial distribution of the stability indices in the southern Great Plains to any significant degree. Predictions of areas of preferred development of deep convection, based on changes in the indices, are not improved by accounting for sub‐grid‐scale variations in land use. The indices were found to be potentially useful but imperfect indicators of the occurrence of deep convective precipitation at scales smaller than that of the CART. At the model resolution used (grid spacing of 2.08 km) a close correlation was not found between regions of precipitation and regions with enhanced simulated vertical updrafts related to land‐use differences, but a detailed study of local triggering has not been carried out.

Enhancement of ARM Surface Meteorological Observations during the Fall 1996 Water Vapor Intensive Observation Period

Abstract This work describes in situ moisture sensor comparisons that were performed in conjunction with the first Water Vapor Intensive Observation Period (IOP) conducted at the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site during September of 1996. Two Raman lidars, two Atmospheric Emitted Radiance Interferometers, (AERIs), and a suite of 13 microwave radiometers were assembled at the CART site during the IOP, and in situ measurements were used for calibration and verification. In addition, this work was meant to help assess the current observing strategy in an effort to make improvements to the routine continuous measurements. To accomplish these goals, verification of the in situ measurements was required. Therefore, a laboratory intercomparison of the in situ moisture sensors (nine capacitive chip relative humidity sensors and four chilled mirror sensors) was performed at the Oklahoma Mesonet temperature and relative humidity testi...

The Argonne Boundary Layer Experiments Facility: Using Minisodars to Complement a Wind Profiler Network

The Argonne Boundary Layer Experiments (ABLE) facility, located in south central Kansas, east of Wichita, is devoted primarily to investigations of and within the planetary boundary layer (PBL), including the dynamics of the mixed layer during both day and night; effects of varying land use and land form; the interactive role of precipitation, runoff, and soil moisture; storm development; and energy budgets on scales of 10 to 100 km. Located entirely within the Walnut River watershed, ABLE provides intense measurements within the northeast quadrant (Fig. 1) of the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) of the Atmospheric Radiation Measurement (ARM) Program (Stokes and Schwarz, 1994). By combining the continuous measurements of ABLE with ancillary continuous measurements of, for example, the ARM and the Global Energy Water cycle Experiment (GEWEX) (Kinster and Shukla, 1990) programs, ABLE provides a platform within which shorter, more intensive studies, such as those conducted by the Cooperative Atmosphere-Surface Exchange Studies (CASES) Program, can realize the full benefit of a wide variety of atmospheric measurements on many scales; this allows the study of hypothesized features of PBL development and dynamics, including frontal dynamics, nocturnal boundary development and breakdown, urban heat island effects, precipitation enhancement, and modification of the surface energy budget partitioning.

Computing Surface Fluxes from Mesonet Data

Abstract By using air–vegetation–soil layer coupled model equations as weak constraints, a variational method is developed to compute sensible and latent heat fluxes from conventional observations obtained at meteorological surface stations. This method also retrieves the top soil layer water content (daytime only) and the surface skin temperature as by-products. The method is applied to Oklahoma Mesonet data collected in the summer of 1995. Fluxes computed for selected Mesonet stations are verified against those obtained by the surface energy and radiation balance system at Atmospheric Radiation Measurement (ARM) Cloud and Radiation Testbed (CART) sites closest to the selected Mesonet stations. The retrieved values of soil water content are also compared with the direct measurements at the closest ARM CART stations. With data provided by the dense Mesonet, the method is shown to be useful in deriving the mesoscale distributions and temporal variabilities of surface fluxes, soil water content, and skin te...

Review of Science Issues, Deployment Strategy, and Status for the ARM North Slope of Alaska–Adjacent Arctic Ocean Climate Research Site

Abstract Recent climate modeling results point to the Arctic as a region that is particularly sensitive to global climate change. The Arctic warming predicted by the models to result from the expected doubling of atmospheric carbon dioxide is two to three times the predicted mean global warming, and considerably greater than the warming predicted for the Antarctic. The North Slope of Alaska–Adjacent Arctic Ocean (NSA–AAO) Cloud and Radiation Testbed (CART) site of the Atmospheric Radiation Measurement (ARM) Program is designed to collect data on temperature-ice-albedo and water vapor–cloud–radiation feedbacks, which are believed to be important to the predicted enhanced warming in the Arctic. The most important scientific issues of Arctic, as well as global, significance to be addressed at the NSA–AAO CART site are discussed, and a brief overview of the current approach toward, and status of, site development is provided. ARM radiometric and remote sensing instrumentation is already deployed and taking data in the perennial Arctic ice pack as part of the SHEBA (Surface Heat Budget of the Arctic Ocean) experiment. In parallel with ARM’s participation in SHEBA, the NSA–AAO facility near Barrow was formally dedicated on 1 July 1997 and began routine data collection early in 1998. This schedule permits the U.S. Department of Energy’s ARM Program, NASA’s Arctic Cloud program, and the SHEBA program (funded primarily by the National Science Foundation and the Office of Naval Research) to be mutually supportive. In addition, location of the NSA–AAO Barrow facility on National Oceanic and Atmospheric Administration land immediately adjacent to its Climate Monitoring and Diagnostic Laboratory Barrow Observatory includes NOAA in this major interagency Arctic collaboration.

Momentum roughness and view-angle dependent heat roughness at a Southern Great Plains test-site

Surface roughness parameters were determined for harvested wheat fields over level terrain at the US Department of Energy's Cloud and Radiation Testbed (CART) site in north-central Oklahoma. Measurements of wind speed and temperature were made by radiosondes and instruments mounted on 2 and 10m towers during neutral and unstable atmospheric conditions in the atmospheric surface layer. Surface temperatures were measured radiatively over 750m trajectories. Roughness heights were calculated for the region using the Monin–Obukhov similarity theory. The scalar roughness and the local momentum roughness were determined using wind speed measurements at 10m and temperature measurements at 2m combined with eddy correlation measurements for u∗. The scalar roughness zoh was determined to be 0.0021 and 0.0038m for the nadir and the off-nadir viewing angle, respectively. It was estimated that the displacement height d is negligible. A regional momentum surface roughness of zo=0.15m was determined by means of the radiosonde profiles. Good agreement (r=0.92) between measured and calculated sensible heat flux values was found using an independent data set of radiosonde profiles.

An Unattended Cloud-Profiling Radar for Use in Climate Research

A new millimeter-wave cloud radar (MMCR) has been designed to provide detailed, long-term observations of nonprecipitating and weakly precipitating clouds at Cloud and Radiation Testbed (CART) sites of the Department of Energy's Atmospheric Radiation Measurement (ARM) program. Scientific requirements included excellent sensitivity and vertical resolution to detect weak and thin multiple layers of ice and liquid water clouds over the sites and long-term, unattended operations in remote locales. In response to these requirements, the innovative radar design features a vertically pointing, single-polarization, Doppler system operating at 35 GHz (Ka band). It uses a low-peak-power transmitter for long-term reliability and high-gain antenna and pulse-compressed waveforms to maximize sensitivity and resolution. The radar uses the same kind of signal processor as that used in commercial wind profilers. The first MMCR began operations at the CART in northern Oklahoma in late 1996 and has operated continuously there for thousands of hours. It routinely provides remarkably detailed images of the ever-changing cloud structure and kinematics over this densely instrumented site. Examples of the data are presented. The radar measurements will greatly improve quantitative documentation of cloud conditions over the CART sites and will bolster ARM research to understand how clouds impact climate through their effects on radiative transfer. Millimeter-wave radars such as the MMCR also have potential applications in the fields of aviation weather, weather modification, and basic cloud physics research.

Estimating Clear-Sky Regional Surface Fluxes in the Southern Great Plains Atmospheric Radiation Measurement Site with Ground Measurements and Satellite Observations

Abstract The authors compared methods for estimating surface fluxes under clear-sky conditions over a large heterogeneous area from a limited number of ground measurements and from satellite observations using data obtained from the southern Great Plains Cloud and Radiation Testbed (CART) site, an area of approximately 350 km × 400 km located in Kansas and Oklahoma. In situ measurements from 10 energy balance Bowen ratio (EBBR) stations showed large spatial variations in latent and sensible heat fluxes across the site because of differences in vegetation and soil conditions. This variation was reproduced by a model for parameterization of subgrid- scale (PASS) surface fluxes that was developed previously and extended in the present study to include a distribution of soil moisture inferred from combined visible and thermal infrared remote sensing data. In the framework of the PASS model, the satellite-derived normalized difference vegetation index and surface temperature were used to derive essential surfa...