Southern Great Plains Site Research Articles

Mesoscale Simulation of Rapid Soil Drying and Its Implications for Predicting Daytime Temperature

Abstract Rapid soil-surface drying, which is called “decoupling,” accompanied by an increase in near-surface air temperature and sensible heat flux, is typically confined to the top 1–2 cm of the soil, while the deeper layers remain relatively moist. Because decoupling depends also on a precise knowledge of fractional vegetation cover, soil properties, and soil water content, an accurate knowledge of these parameters is essential for making good predictions of temperature and humidity. Accordingly, some simulations centered on the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed Southern Great Plains site in Kansas and Oklahoma using a high-resolution substrate layer (Simulator for Hydrology and Energy Exchange at the Land Surface), the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, and derived and default values for soil water content and fractional vegetation cover are presented. In so doing, the following points are made: 1...

A New Normalized Difference Cloud Retrieval Technique Applied to Landsat Radiances over the Oklahoma ARM Site

The authors propose a new cloud property retrieval technique that accounts for cloud side illumination and shadowing effects present at high solar zenith angles. The technique uses the normalized difference of nadir reflectivities (NDNR) at a conservative and an absorbing (with respect to liquid water) wavelength. It can be further combined with the inverse nonlocal independent pixel approximation (NIPA) of Marshak et al. that corrects for radiative smoothing, thus providing a retrieval framework where all 3D cloud effects can potentially be accounted for. The effectiveness of the new technique is demonstrated using Monte Carlo simulations. Realworld application is shown to be feasible using Thematic Mapper (TM) radiance observations from Landsat-5 over the Southern Great Plains (SGP) site of the Atmospheric Radiation Measurement (ARM) Program. For the moderately oblique (458) solar zenith angle of the available Landsat scene, NDNR gives similar regional statistics and histograms when compared with standard independent pixel approximation (IPA), but significant differences at the pixel level. Inverse NIPA is also applied for the first time on observed high-resolution radiances of overcast Landsat subscenes. The dependence of the NIPA-retrieved cloud fields on the parameters of the method is illustrated and practical issues related to the optimal choice of these parameters are discussed. It is natural to compare novel cloud retrieval techniques with standard IPA retrievals. IPA is useful in revealing the inadequacy of plane parallel theory in certain situations and in demonstrating sensitivities to parameter choices, parameterizations, and assumptions. For example, it is found that IPA has problems in matching modeled and observed band-7 (2.2 mm) reflectance values for ;6% of the pixels, most of which are at cloud edges. For simultaneous cloud optical depth‐droplet effective radius retrievals (where a conservative and an absorptive TM band are needed), it is found that the band-4 (0.83 mm)‐band-7 pair was the most well behaved, having less saturation, smaller changes in nominal calibration, and better overall consistency with modeled values than other bands. Mean values of optical depth, effective radius, and liquid water path (LWP) for typical IPA retrievals using this pair are t 5 22, re 5 11 mm, and LWP 5 157 gm 22, respectively. Inclusion of aerosol scattering above clouds results in ;8% decrease in mean cloud optical depth for an aerosol optical depth of 0.2. Degradation of instrument resolution up to ;2 km has small effects on the optical property means and histograms, suggesting small actual cloud variability at these scales and/or radiative smoothing. Comparisons with surface instruments (microwave radiometer, pyranometer, and pyrgeometer) verify the statisitical adequacy of the IPA retrievals. Last, cloud fractions derived with a simple threshold method are compared with those from an automated procedure called Automatic Cloud Cover Assessment now in operational use for Landsat-7. For the northernmost 2000 scanlines of the scene, the cloud fraction Ac is 0.585 from thresholding, as compared with Ac 5 0.563 for the automated procedure, and the full scene values are Ac 5 0.870 and Ac 5 0.865, respectively. This suggests that the Landsat-7 automated procedure will likely give reliable scene-averaged cloud fractions for moderately thick clouds over continental U.S. scenes similar to SGP.

ARM Southern Great Plains Site Observations of the Smoke Pall Associated with the 1998 Central American Fires

Drought-stricken areas of Central America and Mexico were victimized in 1998 by forest and brush fires that burned out of control during much of the first half of the year. Wind currents at various times during the episode helped transport smoke from these fires over the Gulf of Mexico and into portions of the United States. Visibilities were greatly reduced during favorable flow periods from New Mexico to south Florida and northward to Wisconsin as a result of this smoke and haze. In response to the reduced visibilities and increased pollutants, public health advisories and information statements were issued by various agencies in Gulf Coast states and in Oklahoma. This event was also detected by a unique array of instrumentation deployed at the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program Southern Great Plains Cloud and Radiation Testbed and by sensors of the Oklahoma Department of Environmental Quality/Air Quality Division. Observations from these measurement devices sugg...

Explicit Simulation of Midlatitude Cumulus Ensembles: Comparison with ARM Data

Abstract This study describes some results from several simulations of cumulus ensembles at the Southern Great Plains site of the Atmospheric Radiation Measurement (ARM) program during the July 1995 Intensive Observation Period (IOP). A 2D cloud ensemble model (CEM) is used to simulate the macroscopic properties of midlatitude cumulus ensembles. The observed large-scale, horizontal advective tendencies and large-scale vertical velocity or the total advective tendencies are used to drive the CEM, in addition to nudging of the simulated, domain-averaged horizontal wind components toward the observed winds. A detailed comparison with available observations and tropical convection is made in this study. In general, the CEM-simulated results agree reasonably well with the available observations from the July 1995 IOP. The differences between simulations and observations are, however, much larger than those obtained in tropical cases, especially those based on the Global Atmospheric Research Program Atlantic Tr...

Temporal Variations of Land Surface Microwave Emissivities over the Atmospheric Radiation Measurement Program Southern Great Plains Site

Land surface microwave emissivities are important geophysical parameters for atmospheric, hydrological, and biospheric studies. This study estimates land surface microwave emissivity using an atmospheric microwave radiative transfer model and a combination of the Special Sensor Microwave Imager (SSM/I) satellite observations and data from the Atmospheric Radiation Measurement Program southern Great Plains (SGP) site during October of 1995. Emissivities are retrieved for both clear and cloudy conditions. Emissivity standard deviations of ;0.035 were found at the SGP site. Much of the variability is produced by a distinct diurnal cycle. The emissivity variability at each SSM/I overpass time (0630, 1100, 1730, and 1000 local time) is about half that for all four times combined. Early morning emissivities are ;0.06 less than those at other times, and the polarization differences at the four times are similar. This behavior is likely the result of dew and surface rewetting effects. Ground observations of dewpoint and temperature difference between air and skin support this theory. The surface emissivities have a significant negative correlation with soil moisture, which can explain about 60%‐ 80% of the emissivity variance when pentad running means are used. Strong correlations among all seven SSM/I channels indicate that the emissivities need to be determined directly for only two or three channels.

A Microphysical Retrieval Scheme for Continental Low-Level Stratiform Clouds: Impacts of the Subadiabatic Character on Microphysical Properties and Radiation Budgets

Abstract Using measurements from the Department of Energy’s Atmospheric Radiation Measurement Program, a modified ground-based remote sensing technique is developed and evaluated to study the impacts of the subadiabatic character of continental low-level stratiform clouds on microphysical properties and radiation budgets. Airborne measurements and millimeter-wavelength cloud radar data are used to validate retrieved microphysical properties of three stratus cloud systems occurring in the April 1994 and 1997 intensive observation periods at the Southern Great Plains site. The addition of the observed cloud-top height into the Han and Westwater retrieval scheme eliminates the need to invoke the adiabatic assumption. Thus, the retrieved liquid water content (LWC) profile is represented as the product of an adiabatic LWC profile and a weighting function. Based on in situ measurements, two types of weighting functions are considered in this study: one is associated with a subadiabatic condition involving cloud...

Comparison of aerosol optical depth inferred from surface measurements with that determined by Sun photometry for cloud‐free conditions at a continental U.S. site

Evaluation of the forcing of climate by aerosol scattering of shortwave radiation in cloud‐free conditions (direct aerosol forcing) requires knowledge of aerosol optical properties on relevant spatial and temporal scales. It is convenient to measure these properties at the surface. However, before these measurements can be used to quantitatively estimate direct climate forcing, it is necessary to determine the extent to which these properties are representative of the entire atmospheric column. In this paper we compare aerosol optical depth (AOD) determined by Sun photometry at the Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site in north central Oklahoma for several cloud‐free days with estimates of AOD based on two methods. First, the aerosol extinction measured at the surface (taken as the sum of the aerosol scattering and absorption coefficients at instrumental relative humidity of∼20%) is multiplied by the mixing height determined from temperature profiles from radiosonde measurements. Even under conditions of vigorous midday mixing this approach underestimates AOD by as much as 70% using dry aerosol measurements and by roughly 40% when hygroscopic growth of aerosol under ambient relative humidity is taken into account. This discrepancy is attributed primarily to underestimation of aerosol column extinction, as confirmed by examination of normalized aerosol backscatter profiles obtained from micropulse lidar (MPL), which show substantial contributions of aerosol loading above the atmospheric boundary layer. The second approach uses MPL profiles of normalized aerosol backscatter to estimate the vertical profile of aerosol extinction using surface values. The resulting AOD's are on average 30% less than measured values. This discrepancy is attributed to hygroscopic growth of aerosols in the atmospheric column. The results show that at the SGP site even under conditions of vigorous mixing in the atmospheric boundary layer the aerosol optical depth cannot be estimated with surface measurements of aerosol extinction unless information on the vertical profile of aerosol extinction is taken into account.

A 25‐month database of stratus cloud properties generated from ground‐based measurements at the Atmospheric Radiation Measurement Southern Great Plains Site

A 25‐month database of the macrophysical, microphysical, and radiative properties of isolated and overcast low‐level stratus clouds has been generated using a newly developed parameterization and surface measurements from the Atmospheric Radiation Measurement central facility in Oklahoma. The database (5‐min resolution) includes two parts: measurements and retrievals. The former consist of cloud base and top heights, layer‐mean temperature, cloud liquid water path, and solar transmission ratio measured by a ground‐based lidar/ceilometer and radar pair, radiosondes, a microwave radiometer, and a standard Eppley precision spectral pyranometer, respectively. The retrievals include the cloud‐droplet effective radius and number concentration and broadband shortwave optical depth and cloud and top‐of‐atmosphere albedos. Stratus without any overlying mid or high‐level clouds occurred most frequently during winter and least often during summer. Mean cloud‐layer altitudes and geometric thicknesses were higher and greater, respectively, in summer than in winter. Both quantities are positively correlated with the cloud‐layer mean temperature. Mean cloud‐droplet effective radii range from 8.1 μm in winter to 9.7 μm during summer, while cloud‐droplet number concentrations during winter are nearly twice those in summer. Since cloud liquid water paths are almost the same in both seasons, cloud optical depth is higher during the winter, leading to greater cloud albedos and lower cloud transmittances.

10.2172/1245980

A Surface Energy Balance System (SEBS) has been installed collocated with each deployed Eddy Correlation Flux Measurement System (ECOR) at the Atmospheric Radiation Measurement (ARM) Climate Research Facility’s Southern Great Plains (SGP) site, North Slope of Alaska (NSA) site, first ARM Mobile Facility (AMF1), second ARM Mobile Facility (AMF2), and third ARM Mobile Facility (AMF3) at Oliktok Point (OLI). A SEBS was also deployed with the Tropical Western Pacific (TWP) site, before it was decommissioned. Data from these sites, including the retired TWP, are available in the ARM Data Archive. The SEBS consists of upwelling and downwelling solar and infrared radiometers within one net radiometer, a wetness sensor, and soil measurements. The SEBS measurements allow the comparison of ECOR sensible and latent heat fluxes with the energy balance determined from the SEBS and provide information on wetting of the sensors for data quality purposes.

A comparison of single column model simulations of summertime midlatitude continental convection

Eleven different single‐column models (SCMs) and one cloud ensemble model (CEM) are driven by boundary conditions observed at the Atmospheric Radiation Measurement (ARM) program southern Great Plains site for a 17 day period during the summer of 1995. Comparison of the model simulations reveals common signatures identifiable as products of errors in the boundary conditions. Intermodel differences in the simulated temperature, humidity, cloud, precipitation, and radiative fluxes reflect differences in model resolution or physical parameterizations, although sensitive dependence on initial conditions can also contribute to intermodel differences. All models perform well at times but poorly at others. Although none of the SCM simulations stands out as superior to the others, the simulation by the CEM is in several respects in better agreement with the observations than the simulations by the SCMs. Nudging of the simulated temperature and humidity toward observations generally improves the simulated cloud and radiation fields as well as the simulated temperature and humidity but degrades the precipitation simulation for models with large temperature and humidity biases without nudging. Although some of the intermodel differences have not been explained, others have been identified as model problems that can be or have been corrected as a result of the comparison.

Path radiance technique for retrieving aerosol optical thickness over land

The key issue in retrieving aerosol optical thickness over land from shortwave satellite radiances is to identify and separate the signal due to scattering by a largely transparent aerosol layer from the noise due to reflection by the background surface, where the signal is relatively uniform compared to the highly inhomogeneous surface contribution. Sensitivity studies in aerosol optical thickness retrievals reveal that the apparent reflectance at the top of the atmosphere is very susceptible to the surface reflectance, especially when aerosol optical thickness is small. Uncertainties associated with surface reflectance estimation can greatly amplify the error of the aerosol optical thickness retrieval. To reduce these uncertainties, we have developed a “path radiance” method to retrieve aerosol optical thickness over land by extending the traditional technique that uses the “dark object” approach to extract the aerosol signal. This method uses the signature of the correlation of visible and middle‐IR reflectance at the surface and couples the correlation with the atmospheric effect. We have applied this method to a Landsat TM (Thematic Mapper) image acquired over the Oklahoma southern Great Plains site of the Department of Energy Atmospheric Radiation Measurement (ARM) program on September 27, 1997, a very clear day (aerosol optical thickness of 0.07 at 0.5 μm) during the first Landsat Intensive Observation Period. The retrieved mean aerosol optical thickness for TM band 1 at 0.49 μm and band 3 at 0.66 μm agree very well with the ground‐based Sun photometer measurements at the ARM site. The ability to retrieve small aerosol optical thickness makes this path radiance technique promising. More importantly, the path radiance is relatively insensitive to surface inhomogeneity. The retrieved mean path radiances in reflectance units have very small standard deviations for both TM blue and red bands. This small variability of path radiance further supports the current aerosol retrieval method.

An evaluation of NCEP Eta model predictions of surface energy budget and cloud properties by comparison with measured ARM data

Time‐series output from the Eta forecast model of the National Weather Service's National Centers for Environmental Prediction was evaluated by comparison with measured values for a location in Oklahoma. The measured data were drawn from the archives of the Department of Energy's Atmospheric Radiation Measurement program for the southern Great Plains site. Surface energy budget components and cloud indicators were examined for the first half of 1997. Overall, the Eta surface energy budget was found to be nearly balanced, as intended by the model physics, except for one instance when light snowfall occurred in the spring. Despite this balance, an average 50 W m−2 excess in incoming solar radiation was found. Approximately half of this excess was attributed to deficient extinction of shortwave radiation by water vapor and aerosols in the model, while the remainder appeared to be due to cloud treatment errors. The excess shortwave flux was offset by a smaller negative bias in the downwelling infrared flux and the use of slightly high albedos, in addition to errors of lesser magnitude in the latent and sensible heat fluxes. The upwelling infrared flux and ground heat flux were closer to measured values. Ambiguity in the definition of a cloud as well as measurement limitations hindered the analysis of cloud prediction. However, a rough cloud comparison indicated that the Eta model has more difficulty predicting convection than the movement of large storm systems. In addition, cirrus clouds were predicted too frequently in the winter, while low and midlevel clouds were often missed in the spring. This study demonstrates that single‐point time‐series data can be used effectively both to ascertain the quality of model output and to investigate the treatment of individual physical processes within models.

The rotating shadowband spectroradiometer (RSS) at SGP

The RSS provides continuous spectral measurements of total‐horizontal, diffuse‐horizontal, and direct‐normal irradiance from 360 to 1100 nm using the automated shadowband technique. We show instrument performance, calibration method and accuracies, case data from the first‐generation instrument operated at the Southern Great Plains (SGP) site, and comparisons with the improved design now starting operation.

Joint statistics of photon pathlength and cloud optical depth

A mean pressure‐ and temperature‐weighted photon pathlength in the atmosphere can be inferred from moderate resolution measurements in the O2 A‐band. We show a pathlength retrieval method and calibration results for measurements from a Rotating Shadowband Spectroradiometer (RSS), and present the joint statistics of pathlength and cloud optical depth for cloudy skies observed at the Southern Great Plains (SGP) site from Sept. 30 to Dec. 22, 1997. Two different population branches are apparent in the scattergram of the pathlength vs. cloud optical depth; we attribute these to 1) single layer cases exhibiting small variations of pathlength enhancement over large optical depth ranges; 2) multiple layer cases with large variances of enhanced photon pathlengths.

Examining Horizontal Variability in Ground-Based Microwave Retrievals ofLiquid Water Path

Abstract Estimating horizontal length scales associated with variability in cloud liquid water path (LWP) is important in improving both satellite retrievals of cloud properties and in improving cloud models. A microwave radiometer (MWR) at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site provides measurements of the horizontal variability in cloud LWP. Estimating the length scales in time series is typically done using Fourier analysis, but Fourier methods can be quite badly biased by the presence of intermittency in the data. Cloud LWP can contain very localized structures due to convective plumes, so there is some question as to whether decomposing the measurements into a series of Fourier modes will provide physically meaningful results. To explore this problem, LWP measurements are simultaneously decomposed into a series of Fourier modes along with a series of discrete pulses. The pulses in the fitted basis set tend to pick up intermittency, so that the resulting magnitudes of F...

Raman lidar measurements of aerosol extinction and backscattering: 2. Derivation of aerosol real refractive index, single‐scattering albedo, and humidification factor using Raman lidar and aircraft size distribution measurements

Aerosol backscattering and extinction profiles measured by the NASA Goddard Space Flight Center Scanning Raman Lidar (SRL) during the remote cloud sensing (RCS) intensive operations period (IOP) at the Department of Energy Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) site during two nights in April 1994 are discussed. These profiles are shown to be consistent with the simultaneous aerosol size distribution measurements made by a PCASP (Passive Cavity Aerosol Spectrometer Probe) optical particle counter flown on the University of North Dakota Citation aircraft. We describe a technique which uses both lidar and PCASP measurements to derive the dependence of particle size on relative humidity, the aerosol real refractive index n, and estimate the effective single‐scattering albedo ω0. Values of n ranged between 1.4–1.5 (dry) and 1.37–1.47 (wet); ω0 varied between 0.7 and 1.0. The single‐scattering albedo derived from this technique is sensitive to the manner in which absorbing particles are represented in the aerosol mixture; representing the absorbing particles as an internal mixture rather than the external mixture assumed here results in generally higher values of ω0. The lidar measurements indicate that the change in particle size with relative humidity as measured by the PCASP can be represented in the form discussed by Hanel [1976] with the exponent γ = 0.3 ± 0.05. The variations in aerosol optical and physical characteristics captured in the lidar and aircraft size distribution measurements are discussed in the context of the meteorological conditions observed during the experiment.

Raman lidar measurements of aerosol extinction and backscattering: 1. Methods and comparisons

This paper examines the aerosol backscattering and extinction profiles measured at night by the NASA Goddard Space Flight Center Scanning Raman Lidar (SRL) during the remote cloud sensing (RCS) intensive operations period (IOP) at the Department of Energy Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) site in April 1994. These lidar data are used to derive aerosol profiles for altitudes between 0.015 and 5 km. Since this lidar detects Raman scattering from nitrogen and oxygen molecules as well as the elastic scattering from molecules and aerosols, it measures both aerosol backscattering and extinction simultaneously. The aerosol extinction/backscattering ratio varied between approximately 30 sr and 75 sr at 351 nm. Aerosol optical thicknesses derived by integrating the lidar profiles of aerosol extinction measured at night between 0.1 and 5 km are found to be about 10–40% lower than those measured by a Sun photometer during the day. This difference is attributed to the contribution by stratospheric aerosols not included in the lidar estimates as well as to diurnal differences in aerosol properties and concentrations. Aerosol profiles close to the surface were acquired by pointing the lidar nearly horizontally. Measurements of aerosol scattering from a tower‐mounted nephelometer are found to be 40% lower than lidar measurements of aerosol extinction over a wide range of relative humidities even after accounting for the difference in wavelengths. The reasons for this difference are not clear but may be due to the inability of the nephelometer to accurately measure scattering by large particles.