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J. A. Curry
and
G. F. Herman

Abstract

The occurrence of cloudiness over the Beaufort Sea region of the Arctic Basin during June 1980, is related to the ambient large-scale meteorological conditions. Cloud data are obtained from a hand analysis of the visible and infrared Defense Meteorological Satellite Program (DMSP) images and from the U.S. Air Force Three-Dimensional Nephanalysis (3DNEPH). A comparison of the two cloud cover datasets showed good agreement for mid- and high-level cloudiness, but low-level cloudiness was significantly underestimated by the 3DNEPH. The study therefore uses a composite data set consisting of the 3DNEPH data at mid- and upper-levels, and the DMSP data at lower levels. Atmospheric data are obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) objective analysis, and large-scale heat and moisture budgets are constructed. The budgets are used to investigate the processes which contribute to relative humidity changes.

The budgets are related to the cloud cover for both the monthly cloudiness values averaged over the entire region, and for the twice-daily grid point values. Large amounts of low cloud cover during June are attributed primarily to the low level advection of moisture and a residual cooling due to radiation and boundary layer turbulence. The occurrence of midlevel cloudiness is associated with the large-scale transport of heat and moisture. Several relative humidity-based parameterizations currently used in GCMs were tested for their ability to diagnose June 1980 conditions in the Arctic using the initialized fields, but their performance was generally poor. The addition of other atmospheric parameters and budget terms in the empirical formulae provided some improvement, although the agreement with observations remained limited. While our results are dependent upon the quality of the atmospheric and cloud data in this region, they provide further examples of the deficiencies of simple diagnostic layered-cloud parameterizations.

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G. F. Herman
and
W. T. Johnson

Abstract

The performance of a recent version of the general circulation model used at the Goddard Laboratory for Atmospheric Science is evaluated with particular emphasis on its behavior at high latitudes of the Northern and Southern Hemispheres. A January–February climatology for the model was constructed by averaging eight 30-day means, each of which spanned the period from 15 January to 14 February. A mean July climatology was similarly defined on the basis of seven 30-day averages, each spanning the period 1–31 July.

Model-generated sea level pressure, 500 mb geopotential, and surface air temperature are compared with observed long-term climatologies. Sensible heat, evaporative, and radiative fluxes at the surface, and radiative fluxes at the top of the atmosphere also are compared with observed data.

In the Northern Hemisphere the major features that are satisfactorily simulated include the position and intensity of the Aleutian and Icelandic lows in winter; the central Arctic pressure distribution during winter and summer; and the summertime North Atlantic and North Pacific high pressure regimes. Sensible and evaporative heat fluxes and radiation budget parameters are not unreasonable, but a rigorous comparison is difficult because of data deficiencies. The most notable shortcomings of the model include its weak wintertime Asiatic high, and missing meridionality of the 500 mb flow over the North Pacific.

The GCM is less successful in simulating the observed climatology of the Southern Hemisphere. The 500 mb circumpolar flow is adequate, but the model does not successfully reproduce the stationary low pressure centers at the surface around the Antarctic continent. Simulated energy flux components do not disagree in any substantial way with the sparse observations that exist.

The performance of other major GCM's at high latitudes is summarized, and some implications for climatic experiments of GCM performance at high latitudes are discussed.

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J. A. Curry
and
G. F. Herman

Abstract

Aircraft measurements of infrared radiation and cloud microphysics that were collected during the June 1980 Arctic Stratus Experiment are presented and analyzed with the aid of an infrared radiative transfer model. The radiation measurements were obtained with the NCAR Electra's Eppley pyrgeometers and a Barnes PRT-6 radiometer, and the cloud particle observations were obtained with the Knollenberg FSSP and 200X probes.

The data were used to derive values of cloud emittances, mass and volume absorption coefficients, cloud reflectances, cooling rates and radiative extinction lengths. These parameters were found to be strongly dependent on the cloud drop size distribution, and a parameterization of the absorption coefficient in terms of liquid water content and droplet equivalent radius is presented. The window reflectance of the clouds was determined to be between 6.4 and 8.8%.

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G. F. Herman
and
J. A. Curry

Abstract

A series of clouds-radiation experiments was carried out in June 1980 in Arctic stratus clouds occurring over the Beaufort Sea using the NCAR Electra aircraft. This paper is an analysis of the hemispheric radiation fields obtained with Eppley pyranometers and silicon flux detectors, and the cloud microphysical data obtained with the Knollenberg FSSP and 200X probes. These data were collected in a series of 12 vertical profiles in a variety of boundary layer stratus and altostratus situations. The results are interpreted with the aid of a theoretical radiative transfer model developed by Slingo and Schrecker.

The Knollenberg measurements indicated a wide range of cloud liquid water contents which generally increased from cloud base to cloud top. Values of the equivalent radius of the droplet size distribution are presented. Based on the agreement between the radiation measurements and the model calculations, it is concluded that the Knollenberg FSSP probe produced realistic liquid water concentrations in the present study.

Cloud reflectivity, transmissivity, and absorptivity were obtained with the Eppley pyranometers over the total solar spectrum, and also in the visible and near infrared regions, while the silicon detector produced values over the total spectrum, and in the near-infrared. The reflectivity and transmissivity obtained with the two systems agreed well with each other, and also with the model calculations. The measured values of absorptivity were systematically greater than those predicted, particularly in the visible region.

The theoretical model was used to investigate in each cloud profile the roles of aerosols, gaseous and droplet absorptions, and surface reflectivity. In particular, it was found that Arctic summer aerosol did not significantly affect the bulk radiative properties of the clouds, but did have a large effect on the ratio of diffuse to total radiation above the cloud.

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R. E. Newell
,
G. F. Herman
,
G. J. Boer
, and
T. G. Dopplick

Abstract

Changes in radiative heating and cooling rates due to both the near Infrared and 15μbands of CO2 are computed for changes in CO2 concentration from 320 to 600 and 1000 ppmv. An increase in CO2 concentration leads to a smaller net change at the in the troposphere, little net change at the tropopause, and increased cooling in the stratosphere. The 15μ and near infrared effects act in the same sense in the tropopause but tend to compensate in the stratosphere, although the near infrared contribution is generally small when compared with that due to the 15μ band. Possible effects of the changes on the generation of zonal available potential energy are suggested.

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G. F. Herman
,
J. E. Walsh
,
W. H. Raymond
,
R. E. Schlesinger
, and
B. Ross

Abstract

Data sensitivity experiments are carried out with a regional objective analysis and forecast model to study the influence of selected components of the FGGE observing system in the high latitude North Atlantic and eastern Arctic. The objective analysts are obtained from an adaptation of the Australian Numerical Meteorology Research Center (ANMRC) analysis scheme, which utilizes a combination of the Successive Correction Method and a variational approach. Similarly, the forecasts are produced by a ten-level ANMRC regional model adapted to the Northern Hemisphere. First-guess fields are obtained from the 6 h FGGE and NOSAT forecasts produced with the fourth-order general circulation model at NASA's Goddard Laboratory for Atmospheres.

Four experiments are conducted, each consisting of three analyses and forecasts. The experiments investigate how forecast skill is affected by the inclusion of (i) satellite temperature retrievals and surface buoy and marine reports; (ii) retrievals, but without the buoy and marine data; (iii) buoy and marine reports in the presence of the satellite temperature data; and (iv) buoy and marine reports in the absence of the satellite temperature data. In general, both the forecast and the analysis models perform favorably when compared to the ECMWF analyses.

The forecast results are case-dependent. In one case the inclusion of the satellite data significantly reduces the 500 mb height and sea level pressure forecast error at 24 and 48 h, independently of whether or not the buoy/marine data are included. In the two other cases, the satellite data provide a very small reduction of error and a very slight increase in forecast error, respectively. The inclusion of the buoy and marine data always reduces the error in the sea level pressure forecast, but the effects on the 500 mb height forecasts are case-dependent. In some cases the inclusion of the buoy data allows additional secondary circulation features to be identified.

An experiment is also conducted to illustrate how the quality of the data used at the boundaries of the regional model can potentially degrade or improve the forecast in the interior of the region.

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Ivo G. S. van Hooijdonk
,
Herman J. H. Clercx
,
Cedrick Ansorge
,
Arnold F. Moene
, and
Bas J. H. van de Wiel

Abstract

We perform direct numerical simulation of the Couette flow as a model for the stable boundary layer. The flow evolution is investigated for combinations of the (bulk) Reynolds number and the imposed surface buoyancy flux. First, we establish what the similarities and differences are between applying a fixed buoyancy difference (Dirichlet) and a fixed buoyancy flux (Neumann) as boundary conditions. Moreover, two distinct parameters were recently proposed for the turbulent-to-laminar transition: the Reynolds number based on the Obukhov length and the “shear capacity,” a velocity-scale ratio based on the buoyancy flux maximum. We study how these parameters relate to each other and to the atmospheric boundary layer. The results show that in a weakly stratified equilibrium state, the flow statistics are virtually the same between the different types of boundary conditions. However, at stronger stratification and, more generally, in nonequilibrium conditions, the flow statistics do depend on the type of boundary condition imposed. In the case of Neumann boundary conditions, a clear sensitivity to the initial stratification strength is observed because of the existence of multiple equilibriums, while for Dirichlet boundary conditions, only one statistically steady turbulent equilibrium exists for a particular set of boundary conditions. As in previous studies, we find that when the imposed surface flux is larger than the maximum buoyancy flux, no turbulent steady state occurs. Analytical investigation and simulation data indicate that this maximum buoyancy flux converges for increasing Reynolds numbers, which suggests a possible extrapolation to the atmospheric case.

Open access
Ivo G. S. van Hooijdonk
,
Herman J. H. Clercx
,
Carsten Abraham
,
Amber M. Holdsworth
,
Adam H. Monahan
,
Etienne Vignon
,
Arnold F. Moene
,
Peter Baas
, and
Bas J. H. van de Wiel

Abstract

This study aims to find the typical growth rate of the temperature inversion during the onset of the stable boundary layer around sunset. The sunset transition is a very challenging period for numerical weather prediction, since neither accepted theories for the convective boundary layer nor those for the stable boundary layer appear to be applicable. To gain more insight in this period, a systematic investigation of the temperature inversion growth rate is conducted. A statistical procedure is used to analyze almost 16 years of observations from the Cabauw observational tower, supported by observations from two additional sites (Dome C and Karlsruhe). The results show that, on average, the growth rate of the temperature inversion (normalized by the maximum inversion during the night) weakly declines with increasing wind speed. The observed growth rate is quantitatively consistent among the sites, and it appears insensitive to various other parameters. The results were also insensitive to the afternoon decay rate of the net radiation except when this decay rate was very weak. These observations are compared to numerical solutions of three models with increasing complexity: a bulk model, an idealized single-column model (SCM), and an operational-level SCM. It appears only the latter could reproduce qualitative features of the observations using a first-order closure. Moreover, replacing this closure with a prognostic TKE scheme substantially improved the quantitative performance. This suggests that idealized models assuming instantaneous equilibrium flux-profile relations may not aid in understanding this period, since history effects may qualitatively affect the dynamics.

Open access
Atsumu Ohmura
,
Ellsworth G. Dutton
,
Bruce Forgan
,
Claus Fröhlich
,
Hans Gilgen
,
Herman Hegner
,
Alain Heimo
,
Gert König-Langlo
,
Bruce McArthur
,
Guido Müller
,
Rolf Philipona
,
Rachel Pinker
,
Charlie H. Whitlock
,
Klaus Dehne
, and
Martin Wild

To support climate research, the World Climate Research Programme (WCRP) initiated a new radiometric network, the Baseline Surface Radiation Network (BSRN). The network aims at providing validation material for satellite radiometry and climate models. It further aims at detecting long-term variations in irradiances at the earth's surface, which are believed to play an important role in climate change. The network and its instrumentation are designed 1) to cover major climate zones, 2) to provide the accuracy required to meet the objectives, and 3) to ensure homogenized standards for a long period in the future. The limits of the accuracy are defined to reach these goals. The suitable instruments and instrumentations have been determined and the methods for observations and data management have been agreed on at all stations. Measurements of irradiances are at 1 Hz, and the 1-min statistics (mean, standard deviation, and extreme values) with quality flags are stored at a centralized data archive at the WCRP's World Radiation Monitoring Center (WRMC) in Zurich, Switzerland. The data are quality controlled both at stations and at the WRMC. The original 1-min irradiance statistics will be stored at the WRMC for 10 years, while hourly mean values will be transferred to the World Radiation Data Center in St. Petersburg, Russia. The BSRN, consisting of 15 stations, covers the earth's surface from 80°N to 90°S, and will soon be joined by seven more stations. The data are available to scientific communities in various ways depending on the communication environment of the users. The present article discusses the scientific base, organizational and technical aspects of the network, and data retrieval methods; shows various application possibilities; and presents the future tasks to be accomplished.

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T. J. Garrett
,
B. C. Navarro
,
C. H. Twohy
,
E. J. Jensen
,
D. G. Baumgardner
,
P. T. Bui
,
H. Gerber
,
R. L. Herman
,
A. J. Heymsfield
,
P. Lawson
,
P. Minnis
,
L. Nguyen
,
M. Poellot
,
S. K. Pope
,
F. P. J. Valero
, and
E. M. Weinstock

Abstract

This paper presents a detailed study of a single thunderstorm anvil cirrus cloud measured on 21 July 2002 near southern Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). NASA WB-57F and University of North Dakota Citation aircraft tracked the microphysical and radiative development of the anvil for 3 h. Measurements showed that the cloud mass that was advected downwind from the thunderstorm was separated vertically into two layers: a cirrus anvil with cloud-top temperatures of −45°C lay below a second, thin tropopause cirrus (TTC) layer with the same horizontal dimensions as the anvil and temperatures near −70°C. In both cloud layers, ice crystals smaller than 50 μm across dominated the size distributions and cloud radiative properties. In the anvil, ice crystals larger than 50 μm aggregated and precipitated while small ice crystals increasingly dominated the size distributions; as a consequence, measured ice water contents and ice crystal effective radii decreased with time. Meanwhile, the anvil thinned vertically and maintained a stratification similar to its environment. Because effective radii were small, radiative heating and cooling were concentrated in layers approximately 100 m thick at the anvil top and base. A simple analysis suggests that the anvil cirrus spread laterally because mixing in these radiatively driven layers created horizontal pressure gradients between the cloud and its stratified environment. The TTC layer also spread but, unlike the anvil, did not dissipate—perhaps because the anvil shielded the TTC from terrestrial infrared heating. Calculations of top-of-troposphere radiative forcing above the anvil and TTC showed strong cooling that tapered as the anvil evolved.

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