Program Southern Great Plains Site Research Articles

Cloud Resolving Modeling of the ARM Summer 1997 IOP: Model Formulation, Results, Uncertainties, and Sensitivities

Abstract A new three-dimensional cloud resolving model (CRM) has been developed to study the statistical properties of cumulus convection. The model was applied to simulate a 28-day evolution of clouds over the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains site during the summer 1997 Intensive Observation Period. The model was forced by the large-scale advective tendencies and surface fluxes derived from the observations. The sensitivity of the results to the domain dimensionality and size, horizontal grid resolution, and parameterization of microphysics has been tested. In addition, the sensitivity to perturbed initial conditions has also been tested using a 20-member ensemble of runs. The model captures rather well the observed temporal evolution of the precipitable water and precipitation rate, although it severely underestimates the shaded cloud fraction possibly because of an inability to account for the lateral advection of clouds over the ARM site. The ensemble runs reveal t...

Comparison of Stratus Cloud Properties Deduced from Surface, GOES, and Aircraft Data during the March 2000 ARM Cloud IOP

Low-level stratus cloud microphysical properties derived from surface and Geostationary Operational Environmental Satellite (GOES) data during the March 2000 cloud intensive observational period (IOP) at the Atmospheric Radiation Measurement (ARM) program Southern Great Plains (SGP) site are compared with aircraft in situ measurements. For the surface retrievals, the cloud droplet effective radius and optical depth are retrieved from a δ2-stream radiative transfer model with the input of ground-based measurements, and the cloud liquid water path (LWP) is retrieved from ground-based microwave-radiometer-measured brightness temperature. The satellite results, retrieved from GOES visible, solar-infrared, and infrared radiances, are averaged in a 0.5° × 0.5° box centered on the ARM SGP site. The forward scattering spectrometer probe (FSSP) on the University of North Dakota Citation aircraft provided in situ measurements of the cloud microphysical properties. During the IOP, four low-level stratus cases were intensively observed by the ground- and satellite-based remote sensors and aircraft in situ instruments resulting in a total of 10 h of simultaneous data from the three platforms. In spite of the large differences in temporal and spatial resolution between surface, GOES, and aircraft, the surface retrievals have excellent agreement with the aircraft data overall for the entire 10-h period, and the GOES results agree reasonably well with the surface and aircraft data and have similar trends and magnitudes except for the GOES-derived effective radii, which are typically larger than the surface- and aircraft-derived values. The means and standard deviations of the differences between the surface and aircraft effective radius, LWP, and optical depth are −4% ± 20.1%, −1% ± 31.2%, and 8% ± 29.3%, respectively; while their correlation coefficients are 0.78, 0.92, and 0.89, respectively, during the 10-h period. The differences and correlations between the GOES-8 and aircraft results are of a similar magnitude, except for the droplet sizes. The averaged GOES-derived effective radius is 23% or 1.8 μm greater than the corresponding aircraft values, resulting in a much smaller correlation coefficient of 0.18. Additional surface–satellite datasets were analyzed for time periods when the aircraft was unavailable. When these additional results are combined with the retrievals from the four in situ cases, the means and standard deviations of the differences between the satellite-derived cloud droplet effective radius, LWP, and optical depth and their surface-based counterparts are 16% ± 31.2%, 4% ± 31.6%, and −6% ± 39.9%, respectively. The corresponding correlation coefficients are 0.24, 0.88, and 0.73. The frequency distributions of the two datasets are very similar indicating that the satellite retrieval method should be able to produce reliable statistics of boundary layer cloud properties for use in climate and cloud process models.

Assessment of the ECMWF Model Cloudiness and Surface Radiation Fields at the ARM SGP Site

The cloud and radiation fields produced by the operational ECMWF forecasts are assessed using observations from the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) site over the April–May 1999 period. Over the first 36 h of the forecasts, most of the model fields, taken over a 24-h time window (either 0–24, 6–30, or 12–36 h) are generally in good agreement with each other. Comparisons of model fields taken from any such 24-h time window with observations are therefore representative of the quality of the ECMWF model physical parameterizations. The surface radiation fluxes are assessed separately for clear-sky, overcast, and whole-sky situations. For clear-sky fluxes, differences between model and observations are linked to differences in humidity and temperature profiles, the characterization of aerosols, and potential problems in the radiation schemes. For clear-sky conditions, the downward longwave radiation is usually within the accuracy of the measurements. For overcast conditions, the agreement with observations is also usually good. On the other hand, the downward shortwave radiation is overestimated, whatever the conditions. Although this might be partly due to uncertainties in the aerosol content, the clear-sky overestimation of the downward shortwave radiation, when aerosols are specified from climatic values or observations, indicates an underestimation of the gaseous absorption. Model cloud occurrences and boundaries over the SGP Central Facility are compared with similar quantities derived from radar and micropulse lidar observations. Model cloud water is tentatively assessed through comparisons with the radar reflectivity measurements. Systematic deficiencies in the surface radiation fields in the presence of clouds are discussed with respect to differences between the model and observed cloud characteristics. Given the TL319 resolution of the ECMWF model at the time of the comparisons, both the day-to-day and within-the-day temporal variability are captured reasonably well by 24-h forecasts that include cloud–radiation interactions with 1-h time resolution. However, most of the differences with observations can be traced back either to deficiencies in the clear-sky shortwave radiation scheme or to problems in the cloud fraction and/or cloud water content.

Raman lidar measurements of the aerosol extinction‐to‐backscatter ratio over the Southern Great Plains

We derive profiles of the aerosol extinction‐to‐backscatter ratio, Sa, at 355 nm using aerosol extinction and backscatter profiles measured during 1998 and 1999 by the operational Raman lidar at the Department of Energy Atmospheric Radiation Measurement Program Southern Great Plains site in north central Oklahoma. Data from this Raman/Rayleigh‐Mie lidar, which measures Raman scattering from nitrogen as well as the combined molecular (Rayleigh) and aerosol (Mie) scattering at the laser wavelength, are used to derive aerosol extinction and backscattering independently as a function of altitude. Because this lidar operates at 355 nm, where molecular backscattering is comparable to aerosol backscattering, Sa retrievals are generally limited to conditions where aerosol extinction at 355 nm is >0.03 km−1. The mean value of Sa at 355 nm derived for this period was 68 sr with a standard deviation of 12 sr. Sa was generally about 5–10 sr higher during high aerosol optical thickness (AOT) (>0.3) conditions than during low AOT (<0.1). A similar increase in Sa was found when the relative humidity increased from 30 to 80%. Large (>15%) variations in the vertical profile of Sa occurred about 30% of the time, which implies that significant variability in the vertical distribution of the aerosol size distribution, shape, and/or composition often occurs. The Raman lidar measurements of Sa were compared with estimates of particle size and refractive index derived from an algorithm that uses ground‐based Sun photometer measurements of Sun and sky radiance. For 17 cases of coincident Raman lidar and Sun and sky radiance measurements, Sa was linearly correlated with the aerosol fine mode effective radius and the volume ratio of fine/coarse particles.

The Composite Characteristics of Cirrus Clouds: Bulk Properties Revealed by One Year of Continuous Cloud Radar Data

Abstract The properties of midlatitude cirrus clouds are examined using one year of continuous vertically pointing millimeter-wave cloud radar data collected at the Atmospheric Radiation Measurement Program Southern Great Plains site in Oklahoma. The goal of this analysis is to present the cloud characteristics in a manner that will aid in the evaluation and improvement of cirrus parameterizations in large-scale models. Using a temperature- and radar reflectivity–based definition of cirrus, the occurrence frequency of cirrus, the vertical location and thickness of cirrus layers, and other fundamental statistics are examined. Also the bulk microphysical properties of optically thin cirrus layers that occur in isolation from other cloud layers are examined. During 1997, it is found that cirrus were present 22% of the time, had a mean layer thickness of 2.0 km, and were most likely to occur in the 8.5–10-km height range. On average, the cirrus clouds tended to be found in layers in which the synoptic-scale v...

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 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.