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John W. Bergman

Abstract

The large-scale diurnal variation of cloud cover is derived from diurnal variations of temperature, density, water content, and static stability in a linearized calculation. Forced by the diurnal cycle of solar heating, the calculated cloud distribution is broadly consistent with observed diurnal variations under maritime nonconvective, maritime convective, and continental convective conditions.

The calculated diurnal variation of low-cloud fraction follows primarily from the diurnal variation of temperature, which creates a diurnal variation of saturation vapor pressure. The calculated diurnal amplitude of low-cloud fraction is large under maritime nonconvective conditions, in which a well-mixed boundary layer promotes the transition between cloudy and clear conditions. The amplitude is further enhanced under continental conditions by the diurnal variation of vertical heat transport from the surface. The diurnal variation of high-cloud fraction under continental conditions follows primarily from the diurnal variation of low-level stability, which is large if the diurnal amplitude of surface temperature is large. The diurnal variation of high-cloud fraction under maritime convective conditions follows primarily from the diurnal variation of stability at cloud top, which controls the probability that convective cloud top exists in a height interval Δz. The role of clouds in radiative heating is then important because high clouds concentrate shortwave heating in the upper troposphere, which enhances the diurnal variation of stability there.

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John W. Bergman and Prashant D. Sardeshmukh

Abstract

Single column models (SCMs) provide an economical framework for assessing the sensitivity of atmospheric temperature and humidity to natural and imposed perturbations, and also for developing improved representations of diabatic processes in weather and climate models. Their economy is achieved at the expense of ignoring interactions with the circulation dynamics; thus, advection by the large-scale flow is either prescribed or neglected. This artificial decoupling of the diabatic and adiabatic tendencies can often cause rapid error growth in SCM integrations, especially in the Tropics where large-scale vertical advection is important. As a result, SCMs can quickly develop highly unrealistic thermodynamic structures, making it pointless to study their subsequent evolution.

This paper suggests one way around this fundamental difficulty through a simple coupling of the diabatic and adiabatic tendencies. In essence, the local vertical velocity at any instant is specified by a formula that links the local vertical temperature advection to the evolution of SCM-generated diabatic heating rates up to that instant. This vertical velocity is then used to determine vertical humidity advection, and also horizontal temperature and humidity advection under an additional assumption that the column is embedded in a uniform environment. The parameters in the formula are estimated in a separate set of calculations, from the approach to equilibrium of a linearized global primitive equation model forced by steady heat sources. As a test, the parameterized dynamics are used to predict the linear model's local response to oscillating heat sources, and found to perform remarkably well over a wide range of space and time scales. In a second test, the parameterization is found to capture important aspects of a general circulation model's vertical advection and temperature tendencies and their lead–lag relationships with diabatic heating fluctuations at convectively active locations in the Tropics.

When implemented in the NCAR SCM, the dynamically coupled SCM shows a clear improvement over its uncoupled counterpart for tropical conditions observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Coupling effectively stabilizes the SCM. As a result, short-term prediction errors are substantially reduced, the ensemble spread is reduced in ensemble runs, and the SCM is able to maintain realistic thermodynamic structures in extended runs. Such a dynamically coupled SCM should therefore be more useful not only for isolating physical parameterization errors in weather and climate models, but also for economical simulations of regional climate variability.

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John W. Bergman and Murry L. Salby

Abstract

The ISCCP-C2 cloud climatology is used to describe the three-dimensional structure of cloud diurnal variations and to investigate their relationship to local climatological conditions. The latter follows from the regression of diurnal components onto climatological state variables.

Four important diurnal cloud categories are identified. The diurnal variation of maritime high-cloud fraction Chi maximizes at 1700 local solar time (LST) and is strongest over maritime convective locations where the mean high-cloud fraction is C hi > 0.1. The diurnal variation of maritime low-cloud fraction maximizes at 0400 LST and is strong over maritime nonconvective locations where C hi < 0.1. Diurnal variations of high-cloud fraction (persistent during the night, minimum at 1100 LST) and low-cloud fraction (1300 LST maximum) are strong over all continental locations in the latitude band 40°S–40°N.

In each cloud category, most of the diurnal amplitude and phase at individual locations is explained by the regression of diurnal amplitude onto only three climatological state variables. For most categories, the diurnal amplitude has its strongest relationship with mean cloud fraction. The relationship between relative diurnal amplitude (amplitude divided by the mean) and other climatological properties is then particularly meaningful. The relative amplitude of maritime high-cloud fraction is related to the mean total-cloud fraction and the noon-time solar zenith angle, which measures the solar diurnal amplitude. The diurnal amplitude of maritime low-cloud fraction does not have its strongest relationship with the mean low-cloud fraction, but has strong relationships to the upper-level cloud fraction, cloud-top height, and the solar diurnal amplitude. The relative amplitude of continental high-cloud fraction is related most strongly to the time-mean surface temperature, the diurnal amplitude of surface temperature, and the solar diurnal amplitude. The relative amplitude of continental low-cloud fraction has strong relationships with atmospheric moisture content and the diurnal amplitude of surface temperature.

In contrast to amplitude, diurnal phase does not exhibit a strong relationship with any climatological variable. Instead, it is uniform within individual categories, which makes cloud diurnal variations independent of geographical location and, therefore, highly spatially coherent. The spatial coherence of cloud diurnal variations makes them an important ingredient of climate, one that affords some predictability in terms of local climatological conditions.

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John W. Bergman and Harry H. Hendon

Abstract

Seasonal variations of cloud radiative forcing (CRF) are calculated from observed cloud properties in the International Satellite Cloud Climatology Project over the Pacific between 30°S and 30°N. Using 7 yr of data, the first annual harmonic of CRF is statistically significant with respect to the background red noise spectrum at better than a 0.99 confidence level at most locations. It is significant with respect to calculation error at better than a 0.90 confidence level at those same locations. Calculated annual variations are strongest in the subtropics and equatorial east Pacific.

In a linear analysis, annual variations of CRF are attributed to individual annual variations of cloud properties, insolation, or surface temperature. At higher latitudes, the seasonal cycle of CRF at the top of the atmosphere (TOA) and at the surface is dominated by the shortwave (SW) component and results primarily from the seasonal cycle of insolation interacting with the time mean cloud field. Annual variations of cloud fraction and of cloud optical depth are both important in the Tropics, particularly in the east Pacific. Longwave (LW) CRF at TOA is strongest at locations where the seasonal cycle of convection is strong. At those locations, annual variations of CRF result primarily from annual variations of cloud height and not from annual variations of cloud fraction. At the surface, annual variations of LW CRF are small throughout. The annual variations of atmospheric CRF are dominated by the LW component, with the SW component contributing about 20%. As with LW CRF at TOA, annual variations of atmospheric CRF are strongest over convective locations and result from annual variations of cloud height.

The impact of cloud radiative forcing on zonal circulations in the equatorial Pacific and on SST in the east Pacific was analyzed. CRF represents approximately 20% of the annual variations of diabatic heating rates over convective locations and 50% or better at nonconvective locations. Annual variations of atmospheric CRF, when strong, tend to be in phase with those of total diabatic heating rates, indicating that clouds reinforce tropical circulations driven by latent heating.

The role of clouds is particularly important in the east Pacific between 85° and 105°W. Atmospheric CRF is a major component of total diabatic heating over the cold tongue, where seasonal variations of SST are strongest. If seasonal variations of SST in the cold tongue result from seasonal variations of upwelling driven by meridional wind variability, then CRF may play an important role. In contrast, CRF at the surface has only a weak seasonal cycle, with a phase that is not consistent as a forcing for seasonal variations of SST.

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John W. Bergman and Murry L. Salby

Abstract

The contribution to time-mean energetics from cloud diurnal variations is investigated. Cloud diurnal contributions to radiative fluxes follow as the differences between time-mean radiative fluxes based on diurnally varying cloud properties and those based on fixed cloud properties. Time-mean energetics under both conditions are derived from an observationally driven radiative transfer calculation in which cloud cover, temperature, and moisture are prescribed from satellite observations.

Cloud diurnal contributions to time-mean energetics arise from the nonlinear dependence of radiative fluxes on diurnally varying properties. Diurnal variations of cloud fractional coverage and solar flux are the main factors of the cloud diurnal contributions to shortwave (SW) flux, although the diurnal variation of cloud type is also important. The cloud diurnal contribution to longwave (LW) flux at the top of the atmosphere (TOA) is produced by diurnal variations of cloud fractional coverage, cloud-top height, and surface temperature. The cloud diurnal contribution to LW flux at the surface is produced by diurnal variations of cloud fractional coverage and cloud-base height. Cloud diurnal contributions to SW fluxes at the surface and TOA are much larger than the contribution to SW atmospheric absorption. The contribution to radiative heating in the atmosphere is concentrated inside the cloud layer. Its vertical profile changes sign, so the cloud diurnal contribution to atmospheric energetics is significantly larger than is implied by the column average.

Cloud diurnal contributions to SW flux at the surface and TOA are 5–15 W m−2 over continental and maritime subsidence regions, where the diurnal variation of cloud fractional coverage is large. The contributions to LW fluxes are 1–5 W m−2 over continental regions, where diurnal variations of cloud fractional coverage and surface temperature are large. A cancellation between contributions of opposite sign makes the cloud diurnal contributions to globally averaged energetics much smaller than regional contributions. However, a shift in regional climate from one dominated by high clouds to one dominated by low clouds can alter time-mean surface energetics by as much as 20 W m−2.

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John W. Bergman and Harry H. Hendon

Abstract

The role of clouds for low-latitude atmospheric circulations is examined in a linearized calculation forced by diabatic heating rates. A comparison of the circulation calculated from total diabatic heating, obtained from reanalysis data, with observed fields determines which aspects of the calculation are realistic and which are not. The role of clouds is quantified by the circulation calculated from atmospheric cloud radiative forcing, which, in turn, has been calculated with the National Center for Atmospheric Research radiative transfer model using cloud properties observed in the International Satellite Cloud Climatology Project.

In general, cloud radiative forcing contributes about 20% to the magnitude of low-latitude circulations. It typically reinforces the circulation that is driven by convective latent heating. Cloud radiative forcing tends to have a stronger influence in the lower troposphere than at upper levels. It influences local circulations more than remote ones. In particular, cloud radiative forcing from local low cloud cover is the dominant source of diabatic heating influencing subtropical circulations over the eastern oceans. Cloud radiative forcing from low clouds is also found to be important for seasonal variations of meridional winds over the cold tongue in the eastern Pacific. This indicates that atmospheric cloud radiative forcing, and not just surface forcing, is important for ocean–atmospheric coupling there.

Additional calculations are performed that test the sensitivity of the atmospheric circulation to different sources of diabatic heating rates. These sources include radiative heating rates that have been calculated from different cloud data, different cloud overlap assumptions, and enhanced cloud short-wave absorptivity. The principal conclusions of this investigation are unchanged by these calculations. However, enhanced short-wave absorption by clouds systematically reduces the impact of clouds on atmospheric circulations.

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John W. Bergman and Prashant D. Sardeshmukh

Abstract

Single column models (SCMs) provide an economical framework for developing and diagnosing representations of diabatic processes in weather and climate models. Their economy is achieved at the price of ignoring interactions with the circulation dynamics and with neighboring columns. It has recently been emphasized that this decoupling can lead to spurious error growth in SCM integrations that can totally obscure the error growth due to errors in the column physics that one hopes to isolate through such integrations. This paper suggests one way around this “existential crisis” of single column modeling. The basic idea is to focus on short-term SCM forecast errors, at ranges of 6 h or less, before a grossly unrealistic model state develops and before complex diabatic interactions render a clear diagnosis impossible.

To illustrate, a short-term forecast error diagnosis of the NCAR SCM is presented for tropical conditions observed during the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). The 21-day observing period is divided into 84 6-h segments for this purpose. The SCM error evolution is shown to be nearly linear over these 6-h segments and, indeed, apart from a vertical mean bias, to be mainly an extrapolation of initial tendencies. The latter are then decomposed into contributions by various components of the column physics, and additional 6-h integrations are performed with each component separately and in combination with others to assess its contribution to the 6-h errors. Initial tendency and 6-h error diagnostics thus complement each other in diagnosing column physics errors by this approach.

Although the SCM evolution from one time step to the next is nearly linear, the finite-amplitude adjustments made multiple times within each time step to the temperature and humidity to remove supersaturation and convective instabilities make it necessary to consider nonlinear interactions between the column physics components. One such particularly strong interaction is identified between vertical diffusion and deep convection. The former, though nominally small, is shown to have a profound impact on both the amplitude and timing of the latter, and thence on the small imbalance between the total diabatic heating and adiabatic cooling of ascent in the column. The SCM diagnosis thus suggests that misrepresentation of this interaction, in addition to that of the interacting components themselves, might be a major contributor to the NCAR GCM's tropical simulation errors.

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John W. Bergman and Philip J. Rasch

Abstract

A parameterization for specifying subgrid-scale cloud distributions in atmospheric models is developed. The fractional area of a grid-scale column in which clouds from two levels overlap (i.e., the cloud overlap probability) is described in terms of the correlation between horizontal cloudiness functions in the two levels. Cloud distributions that are useful for radiative transfer and cloud microphysical calculations are then determined from cloud fraction at individual model levels and a decorrelation depth. All pair-wise overlap probabilities among cloudy levels are obtained from the cloudiness correlations. However, those probabilities can overconstrain the determination of the cloud distribution. It is found that cloud fraction in each level along with the overlap probabilities among nearest neighbor cloudy levels is sufficient to specify the full cloud distribution.

The parameterization has both practical and interpretative advantages over existing parameterizations. The parameterized cloud fields are consistent with physically meaningful distributions at arbitrary vertical resolution. In particular, bulk properties of the distribution, such as total cloud fraction and radiative fluxes calculated from it, approach asymptotic values as the vertical resolution increases. Those values are nearly obtained once the cloud distribution is resolved; that is, if the thickness of cloudy levels is less than one half of the decorrelation depth. Furthermore, the decorrelation depth can, in principle, be specified as a function of space and time, which allows one to construct a wide range of cloud distributions from any given vertical profile of cloud fraction.

The parameterization is combined with radiative transfer calculations to examine the sensitivity of radiative fluxes to changes of the decorrelation depth. Calculations using idealized cloud distributions display strong sensitivities (∼50 W m−2) to changes of decorrelation depth. Those sensitivities arise primarily from the sensitivity of total cloud fraction to that parameter. Radiative fluxes calculated from a version of the National Center for Atmospheric Research Community Climate Model (CCM) show only a small sensitivity. The reason for this small sensitivity is traced to the propensity of CCM to produce overcast conditions within individual model levels. Thus, in order for the parameterization to be fully useful, it is necessary that other cloud parameterizations in the atmospheric model attain a threshold of realism.

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John W. Bergman and Murry L. Salby

Abstract

The spectrum of equatorial wave activity propagating vertically into the stratosphere is calculated from high-resolution imagery of the global convective pattern. Synoptic Global Cloud Imagery (GCI), constructed from six satellites simultaneously observing the earth, is used to diabatically force the linearized primitive equations. Having resolution of 0.5 deg and 3 h, that imagery captures the dominant scales of organized convection, including several harmonics of the diurnal cycle. Its global coverage with high space–time resolution allows the GCI to represent heating variability and dynamical behavior excited by it over a wide range of scales.

The dynamical response above the heating is evaluated globally in terms of a space–time spectrum of Hough modes, one which includes planetary-scale Kelvin waves, Rossby waves, and gravity waves down to the resolution of the GCI. The geopotential response, which is indicative of temperature fluctuations observed by satellite, is very red in frequency. Therefore, planetary-scale waves with periods longer than two days dominate the spectrum of geopotential, while high-frequency gravity waves make a comparatively small contribution. Some 80% of the geopotential variance is accounted for by the Kelvin and gravest-symmetric Rossby modes, while the Rossby–gravity mode is comparatively weak. In horizontal eddy motion, the excited wave spectrum is still dominated by planetary-scale components. However, meridional wind fluctuations associated with the Rossby–gravity mode have variance comparable to that of zonal wind fluctuations associated with the Kelvin mode, even though the Rossby–gravity mode is nearly invisible in the geopotential response. Estimates of tropospheric heating lead to amplitudes and propagation characteristics that are broadly consistent with satellite and radiosonde observations of wave activity in the lower stratosphere.

The space–time spectrum of EP flux is significantly whiter than the response in either geopotential or motion. Gravity waves of small scale and high frequency carry a large fraction of the upward flux. Although it dominates eastward variance of geopotential and motion, the Kelvin mode carries only about 50% of the eastward EP flux at phase speeds of 20–40 m s−1 and only 35% of the total eastward flux transmitted to the stratosphere. The remainder is carried by the gravity wave spectrum, which carries nearly all of the westward flux at phase speeds greater than 20 m s−1. The gravity wave spectrum also contributes significantly at phase speeds of 10–20 m s−1, where only 25% of the flux is accounted for by zonal wavenumbers less than 20. The broad nature of the gravity wave spectrum suggests its absorption at critical levels will be distributed over a deep layer of the middle atmosphere.

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John W. Bergman and Harry H. Hendon

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The radiative transfer model from NCAR’s general circulation model CCM3 is modified to calculate monthly radiative fluxes and heating rates from monthly observations of cloud properties from the International Satellite Cloud Climatology Project and temperature and humidity from ECMWF analysis. The calculation resolves the three-dimensional structure of monthly to interannual variations of radiative heating and is efficient enough to allow a wide range of sensitivity tests.

Two modifications to the radiative transfer model improve the calculation of shortwave (SW) fluxes in a cloudy atmosphere. The first replaces an existing nonphysical parameterization of partially cloudy skies with a physically motivated one that increases substantially the accuracy of calculated SW fluxes while increasing the computational time of the calculation by only 10%. The second modification allows the specification of generalized cloud overlap properties. With these modifications, radiative fluxes are calculated from observed atmospheric properties without any tuning to observed fluxes.

Based on a comparison with top-of-the-atmosphere (TOA) fluxes observed in the Earth Radiation Budget Experiment, calculated SW and longwave (LW) fluxes at TOA have errors of less than 10 W m−2 at 2.5° horizontal resolution, with smaller errors over ocean than over land. Errors in calculated surface fluxes are 10–20 W m−2 based on sensitivity tests and comparisons to surface fluxes from the GEWEX Surface Radiation Budget. In contrast, TOA and surface fluxes from the NCEP/NCAR reanalysis data, which rely on cloud properties from a general circulation model, have errors larger than 30 W m−2. Errors in the calculated fluxes result primarily from uncertainties in the observed cloud properties and specified surface albedo, with somewhat smaller errors resulting from unobserved aspects of the vertical distribution of clouds. Errors introduced into the calculation by using monthly observations and neglecting high-frequency variations are small relative to other sources of error.

Substantial uncertainty is found in many details of the vertical structure of cloud radiative forcing, which underscores the importance of performing a wide variety of sensitivity calculations in order to understand the impact of clouds on radiative heating. However, certain general features of the calculated vertical structure of cloud radiative forcing in the atmosphere are robust. Deep vertical cloud distributions at locations of active tropical convection result in deep cloud radiative heating, whereas shallow cloud distributions in the subtropics result in low-level cloud radiative cooling there. Under all conditions, SW cloud radiative forcing is systematically of opposite sign to LW cloud radiative forcing, which reduces the impact of LW cloud radiative forcing.

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