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J. P. Hacker and W. M. Angevine

Abstract

Experiments with the single-column implementation of the Weather Research and Forecasting Model provide a basis for deducing land–atmosphere coupling errors in the model. Coupling occurs both through heat and moisture fluxes through the land–atmosphere interface and roughness sublayer, and turbulent heat, moisture, and momentum fluxes through the atmospheric surface layer. This work primarily addresses the turbulent fluxes, which are parameterized following the Monin–Obukhov similarity theory applied to the atmospheric surface layer. By combining ensemble data assimilation and parameter estimation, the model error can be characterized. Ensemble data assimilation of 2-m temperature and water vapor mixing ratio, and 10-m wind components, forces the model to follow observations during a month-long simulation for a column over the well-instrumented Atmospheric Radiation Measurement (ARM) Central Facility near Lamont, Oklahoma. One-hour errors in predicted observations are systematically small but nonzero, and the systematic errors measure bias as a function of local time of day. Analysis increments for state elements nearby (15 m AGL) can be too small or have the wrong sign, indicating systematically biased covariances and model error. Experiments using the ensemble filter to objectively estimate a parameter controlling the thermal land–atmosphere coupling show that the parameter adapts to offset the model errors, but that the errors cannot be eliminated. Results suggest either structural errors or further parametric errors that may be difficult to estimate. Experiments omitting atypical observations such as soil and flux measurements lead to qualitatively similar deductions, showing the potential for assimilating common in situ observations as an inexpensive framework for deducing and isolating model errors.

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A. G. Williams, J. M. Hacker, and H. Kraus

Abstract

The structure of the intertropical convergence zone ITCZ cloud-topped marine atmospheric boundary layer away from the most intense mesoscale convective systems during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) is investigated. Eight vertical profiles taken by the Australian Cessna research aircraft are analyzed, representing the successive influence of a growing small cluster of precipitating cumulus upon the subcloud layer. On the basis of conclusions from a spectral analysis in Part I of this study, results are partitioned into contributions from three distinct categories: (a) small-scale (<2 km) processes, corresponding to small eddies contained and forced mainly within the subcloud layer and weakly active cumulus motions; (b) cloud-scale (>2 km) processes, corresponding to meso-γ-scale motions associated mainly with the action of precipitating cumulus clouds and larger motions; and (c) extreme processes, corresponding to contributions from events at the tail of the small-scale statistical flux distribution. Such events are associated with downdrafts below precipitating cumulus, updrafts at gustfronts, and the effects of moisture contamination on thermodynamic data, and can act to significantly skew the flux distribution. In the presence of vigorous cumuli, cloud root circulations (including compensating downdrafts) force significant cloud-scale fluxes in the upper subcloud layer. When conditions become highly disturbed, these fluxes dominate and the vast majority of small-scale humidity transport is concentrated into the cloud root regions. Precipitation produces strong downdrafts and outflows of evaporatively cooled air in the lower subcloud layer, markedly increasing temperature and velocity variances. Neither cloud root circulations nor outflows are supported by cloud-scale buoyancy, with the former being fed by pressure and momentum forces while the latter are formed via small-scale (extreme) buoyancy effects. Small-scale (surface forced) processes moisten and slow the subcloud layer as a whole, while cloud processes cause drying and often acceleration due to enhanced cloud–subcloud-layer exchanges. Processes on all scales lead to net warming of the subcloud layer in the present dataset. Although in zero or low precipitation cases the mean structure of the mixed layer may still be represented to some degree by existing simple zero-order jump models, significant adjustments are required to such models in order to account for the effects of cloud-scale processes under disturbed conditions. In particular, the enhancement of cloud–subcloud-layer exchanges by cloud root processes and the effects of increased horizontal wind variances upon surface fluxes requires attention. A new velocity scale is suggested, based on large-scale vertical velocity at cloud base, which may be useful in the formulation of newparameterizations.

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A. G. Williams, H. Kraus, and J. M. Hacker

Abstract

Spectral analysis of high-resolution turbulence data from the South Australian Cessna research aircraft is performed in an investigation of the multiscale nature of vertical transport processes in the atmospheric boundary layer (ABL) during TOGA COARF. The flights were conducted in the vicinity of large cloud cluster systems in the intertropical convergence zone, but away from the most intense mesoscale (100s of km) convective systems within the clusters. A number of very long (up to 430 km) and low (20-70 m) continuous data runs, composing an excellent dataset for studying the spectral composition of near-surface fluxes, are complemented by eight “stack” patterns providing important information regarding vertical variations. The ABL in these regions is found to be highly horizontally heterogeneous, due to the intrusion of cool air masses associated with precipitating cumulus and cumulonimbus clouds, and the action of lines of convention on a range of scales. Not only does this lead to large variations in the surface turbulent flux field, but it can also generate significant direct fluxes in a submesoscale (20–50 km) range at low altitudes, which are not expected to be controlled by ABL parameters. That is, enhanced motions resulting from the action of precipitating cumulus clouds in the presence of wind shear can lead to strong entrainment of air into the subcloud layer, and, in addition, gravity waves generated above the ABL can also influence subcloud motion. Analysis of the form and consistency of the cospectra suggests that, despite the absence of a clear “gap” in the power spectra of the major variables, it is nevertheless possible to achieve a reasonable partitioning between “ABL turbulence” and the larger-scale processes via a simple spectral separation with a crossover wavelength at around 2 km. This useful characteristic appears to reflect an ability of the ABL turbulence to maintain a high degree of coherency in spite of the changing conditions imposed by the mesoscale disturbances.

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Jeffrey T. Kiehl, Julie M. Caron, and James J. Hack

Abstract

Climate model simulations of the latter part of the twentieth century indicate a warming of the troposphere that is equal to or larger than the warming at the surface, while satellite observations from the Microwave Sounding Unit (MSU) indicate little warming of the troposphere relative to surface observations. Recently, Fu et al. proposed a new approach to retrieving free tropospheric temperature trends from MSU data that better accounts for stratospheric cooling, which contaminates the tropospheric signal and leads to a smaller trend in tropospheric warming. In this study, climate model simulations are used as a self-consistent dataset to test these retrieval algorithms. The two methods of retrieving tropospheric temperature trends are applied to three climate model simulations of the twentieth century. The Fu et al. algorithm is found to be in very good agreement with the model-simulated tropospheric warming, indicating that it accurately accounts for cooling from the lower stratosphere.

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J. T. Kiehl, J. J. Hack, M. H. Zhang, and R. D. Cess

Abstract

Recent studies by Cess et al. and Ramanathan et al. find that clouds absorb significantly more shortwave radiation than currently modeled by general circulation models. Initial calculations for the global annual shortwave energy budget imply that including the additional shortwave cloud absorption leads to an additional 22 W m−2 absorption in the atmosphere, with an equivalent reduction of shortwave flux at the surface. The present study investigates the climate implications of enhanced cloud absorption with the use of the National Center for Atmospheric Research Community Climate Model. The GCM response to this forcing is to warm the upper troposphere by as much as 4 K. The additional shortwave heating in the upper troposphere reduces the strength of the Hadley circulation by 12% and leads to lower surface wind speeds in the Tropics. In turn, these lower wind speeds lead to a 25 W m−2 reduction in surface latent heat flux.

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R. E. Carbone, J. W. Wilson, T. D. Keenan, and J. M. Hacker

Abstract

Diurnally forced convection was observed over the Tiwi Islands, north of the Australian continent, as part of the Maritime Continent Thunderstorm Experiment. Immature peninsula-scale (5–15 km) sea breezes were observed to initiate moist convection early each day, principally through convergence that results from the confluence or collision of peninsula breeze fronts. Convection initiated by peninsula-scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event. Mature island-scale (∼100 km) breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection.

The initiation of mesoscale convective systems (MCSs) is observed to be a direct consequence of breeze front collisions for only ∼20% of the days on which organized convection develops. This is referred to as “type A” forcing and it occurs when normal convective development is delayed or otherwise suppressed. Type A forcing is nature’s backup mechanism and it is less likely to produce large or strong mesoscale convective systems when compared to the general population of events.

On approximately 80% of days during which organized convection develops, a multiple-stage forcing process evolves through complex interactions between preferred sea breezes and convectively generated cold pools. So-called type B forcing emerges 1–3 h before penetration of the sea-breeze fronts to the interior island. Type B evolution has at least four stages: 1) leeward- or other preferred-coast sea-breeze showers that develop small cold pools, 2) showers that travel inland when their cold pools become denser than the marine boundary layer, 3) westward propagation of squalls that result from a merge or maturation of small cold pools, and 4) interaction between a gust front and a zonally oriented sea-breeze front of island scale (∼100 km). A collision of gust fronts, emanating from separate convective areas over Bathurst and Melville Islands, can excite a fifth stage of development associated with many of the strongest systems.

A principal finding of this study is that all MCSs over the Tiwi Islands can be traced backward in time to the initiation of convection by island-scale sea breezes, usually of type B near leeward coasts. Subsequent convective evolution is characteristic of traveling free convection elsewhere in that it organizes according to cold pool, shear balance, and mean flow factors. The presence of a critical level in the lower troposphere is a unique aspect of the theoretical “optimal condition” associated with island convection in a low-level jet regime; however, the data presented here suggest that the effects of surface layer stagnation may be of greater practical importance.

Since the aforestated conclusions are based on time series of rather limited duration, the reader is cautioned as to uncertainty associated with the climatological frequency of events as described herein. Furthermore, the authors have not examined external forcings, which may be associated with large-scale circulations.

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Huang Xinmei, T. J. Lyons, R. C. G. Smith, J. M. Hacker, and P. Schwerdtfeger

Abstract

A model is developed to evaluate surface heat flux densities using the radiant surface temperature and red and near-infrared reflectances from the NOAA Advanced Very High Resolution Radiometer sensor. Net radiation is calculated from an empirical formulation and albedo estimated from satellite observations. Infrared surface temperature is corrected to aerodynamic surface temperature in estimating the sensible heat flux and the latent flux is evaluated as the residual of the surface energy balance. When applied to relatively homogeneous agricultural and native vegetation, the model yields realistic estimates of sensible and latent heat flux density in the surface layer for cases where either the sensible or latent flux dominates.

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James W. Hurrell, James J. Hack, Dennis Shea, Julie M. Caron, and James Rosinski

Abstract

A new surface boundary forcing dataset for uncoupled simulations with the Community Atmosphere Model is described. It is a merged product based on the monthly mean Hadley Centre sea ice and SST dataset version 1 (HadISST1) and version 2 of the National Oceanic and Atmospheric Administration (NOAA) weekly optimum interpolation (OI) SST analysis. These two source datasets were also used to supply ocean surface information to the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40). The merged product provides monthly mean sea surface temperature and sea ice concentration data from 1870 to the present: it is updated monthly, and it is freely available for community use. The merging procedure was designed to take full advantage of the higher-resolution SST information inherent in the NOAA OI.v2 analysis.

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James J. Hack, Julie M. Caron, G. Danabasoglu, Keith W. Oleson, Cecilia Bitz, and John E. Truesdale

Abstract

The latest version of the Community Climate System Model (CCSM) Community Atmosphere Model version 3 (CAM3) has been released to allow for numerical integration at a variety of horizontal resolutions. One goal of the CAM3 design was to provide comparable large-scale simulation fidelity over a range of horizontal resolutions through modifications to adjustable coefficients in the parameterized treatment of clouds and precipitation. Coefficients are modified to provide similar cloud radiative forcing characteristics for each resolution. Simulations with the CAM3 show robust systematic improvements with higher horizontal resolution for a variety of features, most notably associated with the large-scale dynamical circulation. This paper will focus on simulation differences between the two principal configurations of the CAM3, which differ by a factor of 2 in their horizontal resolution.

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Donald E. Wroblewski, Owen R. Coté, Jorg M. Hacker, and Ronald J. Dobosy

Abstract

High-resolution measurements obtained from NOAA “best” atmospheric turbulence (BAT) probes mounted on an EGRETT high-altitude research aircraft were used to characterize turbulence in the upper troposphere and lower stratosphere at scales from 2 m to 20 km, focusing on three-dimensional behavior in the sub-kilometer-scale range. Data were analyzed for 129 separate level flight segments representing 41 h of flight time and 12 600 km of wind-relative flight distances. The majority of flights occurred near the tropopause layer of the winter subtropical jet stream in the Southern Hemisphere. Second-order structure functions for velocity and temperature were analyzed for the separate level-flight segments, individually and in various ensembles. A 3D scaling range was observed at scales less than about 100 m, with power-law exponents for the structure functions of the velocity component in the flight direction varying mostly between 0.4 and 0.75 for the separate flight segments, but close to ⅔ for the ensemble-averaged curves for all levels and for various subensembles. Structure functions in the 3D scaling range were decoupled from those at scales greater than 10 km, with the large-scale structure functions showing less variation than those at smaller scales. Weakly anisotropic behavior was observed in the 3D range, with structure parameters for the lateral and vertical velocities on the same order as those in the flight direction but deviating from the expected isotropic value. Anisotropy was correlated with turbulence intensity, with greater anisotropy associated with weaker turbulence.

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