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  • Author or Editor: G. G. Mace x
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Xiquan Dong
and
Gerald G. Mace

Abstract

A record of single-layer and overcast low-level Arctic stratus cloud properties has been generated using data collected from May to September 2000 at the Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) (71.3°N, 156.6°W) site near Barrow, Alaska. The record includes liquid-phase and liquid dominant mixed-phase Arctic stratus macrophysical, microphysical, and radiative properties, as well as surface radiation budget and cloud radiative forcing. The macrophysical properties consist of cloud fractions, cloud-base/top heights and temperatures, and cloud thickness derived from a ground-based radar and lidar pair, and rawinsonde sounding. The microphysical properties include cloud liquid water path and content, and cloud-droplet effective radius and number concentration obtained from microwave radiometer brightness temperature measurements, and the new cloud parameterization. The radiative properties contain cloud optical depth, effective solar transmission, and surface/cloud/top-of-atmosphere albedos derived from the new cloud parameterization and standard Epply precision spectral pyranometers. The shortwave, longwave, and net cloud radiative forcings at the surface are inferred from measurements by standard Epply precision spectral pyranometers and pyrgeometers. There are approximately 300 h and more than 3600 samples (5-min resolution) of single-layer and overcast low-level stratus during the study period. The 10-day averaged total and low-level cloud (Z top < 3 km) fractions are 0.87 and 0.55, and low-level cloud-base and -top heights are around 0.4 and 0.8 km. The cloud-droplet effective radii and number concentrations in the spring are similar to midlatitude continental stratus cloud microphysical properties, and in the summer they are similar to midlatitude marine stratus clouds. The total cloud fractions in this study show good agreement with the satellite and surface results compiled from data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) Arctic Cloud Experiment (ACE) and the Surface Heat Budget of the Arctic Ocean (SHEBA) (∼77°N, 165°W) field experiments in 1998. The cloud microphysics derived from this study are similar, in general, to those collected in past field programs, although these comparisons are based on data collected at different locations and years. At the ARM NSA site, the summer cooling period is much longer (2–3 months vs 1–2 weeks), and the summer cooling magnitude is much larger (−100 W m−2 vs −5 W m−2) than at the SHEBA ship under the conditions of all skies at the SHEBA and overcast low-level stratus clouds at the NSA site.

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Gerald G. Mace
and
Sally Benson-Troth

Abstract

Assumptions regarding the vertical overlap characteristics of horizontal cloudy layers have been shown to be important to both the radiation transfer and the cloud microphysics that are predicted in general circulation models. Certain reasonable assumptions regarding cloud-layer overlap have been applied in models up to now where vertically continuous cloudy layers were assumed to overlap maximally leading to a minimum in cloud cover, and layers separated by noncloudy layers were assumed to overlap randomly. However, these assumptions have not been systematically evaluated with a comprehensive dataset that can resolve simultaneously occurring cloud layers. Presented here is an analysis of cloud-layer overlap characteristics derived from 103 months of cloud radar data collected by continuously operating millimeter-wavelength instruments deployed at the Atmospheric Radiation Measurement (ARM) sites in the Tropics, middle latitudes, and the Arctic. Using an approach recently proposed by Hogan and Illingworth, it is shown that an assumption of random overlap for layers separated by noncloudy layers is supported by observations. However, that the overlap characteristics of vertically continuous layers cannot be considered maximal is also shown. Indeed, vertically continuous cloudy layers do not appear to lend themselves to a simple overlap assumption. Therefore, to avoid significant biases in diagnosed cloud cover, the overlap properties of these layers in models will need to be parameterized. It is shown that the cloud-layer overlap characteristics in the middle latitudes do appear to be a strong function of season, suggesting that an overlap parameterization in terms of cloud system type may be possible.

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Gerald G. Mace
and
Sally Benson

Abstract

Data collected at the Atmospheric Radiation Measurement (ARM) Program ground sites allow for the description of the atmospheric thermodynamic state, cloud occurrence, and cloud properties. This information allows for the derivation of estimates of the effects of clouds on the radiation budget of the surface and atmosphere. Herein 8 yr of continuous data collected at the ARM Southern Great Plains (SGP) Climate Research Facility (ACRF) are analyzed, and the influence of clouds on the radiative flux divergence of solar and infrared energy on annual, seasonal, and monthly time scales is documented. Given the uncertainties in derived cloud microphysical properties that result in calculated radiant flux errors, it is demonstrated that the ability to quantitatively resolve all but the largest heating and cooling influences by clouds is marginal for averaging periods less than 1 month. Concentrating on seasonal and monthly averages, it is found that the net column-integrated radiative effect of clouds on the atmosphere is nearly neutral at this middle-latitude location. However, a net heating of the upper troposphere by upper-tropospheric clouds and a cooling of the lower troposphere by boundary layer clouds is documented. The balance evolves over the course of an annual cycle as the troposphere deepens in summer and boundary layer clouds become less frequent relative to upper-tropospheric clouds. Although the top-of-atmosphere IR radiative effect is nearly invariant through the annual cycle, the seasonally varying heating profile is determined largely by the convergence of IR flux because solar heating is offset by IR cooling within the column.

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Gerald G. Mace
and
Forrest J. Wrenn

Abstract

The International Satellite Cloud Climatology Project (ISCCP) provides a multidecadal and global description of cloud properties that are often grouped into joint histograms of column visible optical depth τ and effective cloud-top pressure P top. It has not been possible until recently to know the actual distributions of hydrometeor layers within the ISCCP P topτ bins. Distributions of hydrometeor layers within the ISCCP P topτ conditional probability space using measurements from the CloudSat Cloud Profiling Radar and the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar within two 40° × 40° regions in the eastern and western equatorial Pacific over a 2-yr period are examined. With the exception of thin cirrus and stratocumulus, the authors show that of the P topτ types that are commonly analyzed, none of the types contain unique distributions of geometrically defined layer types but tend to be populated by diverse sets of hydrometeor layers whose bulk profile properties conspire to render specific radiative signatures when interpreted by two-channel visible and IR sensors from space. In comparing the geometric distribution of cloud layers for common P topτ types, it is found that the ISCCP Cirrostratus, Deep Convection, and Stratocumulus types appear to have been drawn from a common geometric distribution of hydrometeor layers. The other six common ISCCP P topτ types do not share this feature. The authors can confidently reject an assumption that even though they have common top-of-atmosphere radiative signatures, they do not appear to share a common distribution of cloud layers and therefore are likely to have significantly different radiative heating profiles and different surface radiative forcing even though their top-of-atmosphere radiative signatures are similar.

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Elizabeth Berry
,
Gerald G. Mace
, and
Andrew Gettelman

Abstract

The distribution of clouds and their radiative effects in the Community Atmosphere Model, version 5 (CAM5), are compared to A-Train satellite data in Southeast Asia during the summer monsoon. Cloud radiative kernels are created based on populations of observed and modeled clouds separately in order to compare the sensitivity of the TOA radiation to changes in cloud fraction. There is generally good agreement between the observation- and model-derived cloud radiative kernels for most cloud types, meaning that the clouds in the model are heating and cooling like clouds in nature. Cloud radiative effects are assessed by multiplying the cloud radiative kernel by the cloud fraction histogram. For ice clouds in particular, there is good agreement between the model and observations, with optically thin cirrus producing a moderate warming effect and cirrostratus producing a slight cooling effect, on average. Consistent with observations, the model also shows that the median value of the ice water path (IWP) distribution, rather than the mean, is a more representative measure of the ice clouds that are responsible for heating. In addition, in both observations and the model, it is cirrus clouds with an IWP of 20 g m−2 that have the largest warming effect in this region, given their radiative heating and frequency of occurrence.

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Elizabeth Berry
,
Gerald G. Mace
, and
Andrew Gettelman

Abstract

Using information from the A-Train satellites, the properties and radiative effects of eastern Pacific Ocean boundary layer clouds are evaluated in the Community Atmosphere Model, version 5 (CAM5), from the summer of 2007 and 2008. The cloud microphysical properties are inferred using measurements from CloudSat and CALIPSO (CC) that are then used to calculate the broadband radiative flux profiles. Accounting appropriately for sampling differences between the measurements and the simulation, evidence of the “too few, too bright” low cloud bias is found in CAM5. Single-layer low clouds have a frequency of occurrence of 42% from CC, as compared with just 29% in CAM5, and the averaged cloud radiative kernel (CRK) for the model shows stronger cooling. For stratocumulus in particular, the cooling in the model CRK is larger by a factor of 2 relative to the observations, implying an overly sensitive tropical low cloud feedback. Differences in the day/night occurrence of stratocumulus help to explain some of the difference in the CRK. The cloud-type microphysics for liquid clouds is represented reasonably well by the model, with a tendency for smaller water paths and smaller effective radii. Overall, the occurrence and CRK have partially compensating errors such that the net cooling at the top of the atmosphere for eastern Pacific low clouds is −43 W m−2 in CAM5, as compared with −32 W m−2 from CC. The cooling effect in the model is accomplished by fewer low clouds with a narrower range of properties, as compared with more clouds with a broader range of properties in the observation-based dataset.

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Gerald G. Mace
,
Sally Benson
, and
Erik Vernon

Abstract

The properties of cirrus clouds observed at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) in Oklahoma are documented from a nearly continuous 6-yr record of 35-GHz cloud radar data. Cirrus frequency over the ACRF is 23% and 28% of the time in the warm (May–September) and cold seasons (November–March), respectively, with maxima and minima during the period studied of 30% and 16% in the warm season and 34% and 24% in the cold seasons. Cirrus, as defined here, reveal a seasonal oscillation in their macroscale properties that can be traced to the seasonal deepening of the troposphere in the Southern Plains region. While the average bulk microphysical properties do not change significantly from season to season, the variability of certain parameters demonstrates seasonal change. It is shown that the properties of cirrus clouds vary perceptively with the large-scale vertical motion. Using NCEP–NCAR reanalysis data to define the large-scale meteorological state when cirrus are observed at the ACRF, the authors find that cirrus tend to exist within a maximum in upper-tropospheric humidity and downstream of the peak upper-tropospheric vertical motion. Cirrus that exist in large-scale ascent upstream of the synoptic-scale middle-tropospheric ridge axis are shown to have higher water contents than cirrus that exist in large-scale subsidence downstream of the ridge axis, although the overall nature of the statistical distributions of water contents do not change greatly, suggesting that it may be difficult to parameterize the properties of cirrus based solely on large-scale vertical motion. The layer-mean particle size, on the other hand, shows no such sensitivity to the large-scale vertical motion.

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Steven M. Lazarus
,
Steven K. Krueger
, and
Gerald G. Mace

Abstract

Cloud amount statistics from three different sources were processed and compared. Surface observations from a National Centers for Environmental Prediction dataset were used. The data (Edited Cloud Report; ECR) consist of synoptic weather reports that have been edited to facilitate cloud analysis. Two stations near the Southern Great Plains (SGP) Cloud and Radiation Test Bed (CART) in north-central Oklahoma (Oklahoma City, Oklahoma and Wichita, Kansas) were selected. The ECR data span a 10-yr period from December 1981 to November 1991. The International Satellite Cloud Climatology Project (ISCCP) provided cloud amounts over the SGP CART for an 8-yr period (1983–91). Cloud amounts were also obtained from Micro Pulse Lidar (MPL) and Belfort Ceilometer (BLC) cloud-base height measurements made at the SGP CART over a 1-yr period. The annual and diurnal cycles of cloud amount as a function of cloud height and type were analyzed. The three datasets closely agree for total cloud amount. Good agreement was found in the ECR and MPL–BLC monthly low cloud amounts. With the exception of summer and midday in other seasons, the ISCCP low cloud amount estimates are generally 5%–10% less than the others. The ECR high cloud amount estimates are typically 10%–15% greater than those obtained from either the ISCCP or MPL–BLC datasets. The observed diurnal variations of altocumulus support the authors’ model results of radiatively induced circulations.

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Gerald G. Mace
,
Eugene E. Clothiaux
, and
Thomas P. Ackerman

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 vertical velocity was weakly ascending. The mean synoptic-scale vertical motion in the upper troposphere as derived from Rapid Update Cycle model output was +0.2 cm s−1. However, a significant fraction of the layers (33%) were found where the upper-tropospheric large-scale vertical velocity was clearly descending (w < −1.5 cm s−1). Microphysical properties were computed for that subset of cirrus events that were optically thin (infrared emissivity < 0.85) and occurred with no lower cloud layers. This subset of cirrus had mean values of ice water path, effective radius, and ice crystal concentration of 8 g m−2, 35 μm, and 100 L−1, respectively. Although all the cloud properties demonstrated a high degree of variability during the period considered, the statistics of these properties were fairly steady throughout the annual cycle. Consistent with previous studies, it is found that the cloud microphysical properties appear to be strongly correlated to the cloud layer thickness and mean temperature. Use of these results for parameterization of cirrus properties in large-scale models is discussed.

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Betty Carlin
,
Qiang Fu
,
Ulrike Lohmann
,
Gerald G. Mace
,
Kenneth Sassen
, and
Jennifer M. Comstock

Abstract

High ice cloud horizontal inhomogeneity is examined using optical depth retrievals from four midlatitude datasets. Three datasets include ice cloud microphysical profiles derived from millimeter cloud radar at the Southern Great Plains Atmospheric Radiation Measurement site in Oklahoma. A fourth dataset combines lidar and midinfrared radiometry (LIRAD), and is from the Facility for Atmospheric Remote Sensing at the University of Utah, Salt Lake City, Utah. Plane-parallel homogeneous (PPH) calculations of domain-averaged solar albedo for these four datasets are compared to independent column approximation (ICA) results. A solar albedo bias up to 25% is found over a low reflective surface at a high solar zenith angle. A spherical solar albedo bias as high as 11% is shown. The gamma-weighted radiative transfer (GWRT) scheme is shown to be an effective correction for the solar albedo bias suitable for GCM applications. The GWRT result was, in all cases, within 1–2 W m−2 of the ICA outgoing solar flux. The GWRT requires a parameterization of the standard deviation of cloud optical depth. It is suggested that the domain-averaged cloud optical depth and ice water path together can be used in a parameterization to account for 80% of the standard deviation in optical depth.

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