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R. L. Miller

Abstract

The ability of subtropical stratus low cloud cover to moderate or amplify the tropical response to climate forcing such as increased CO2 is considered. Cloud radiative forcing over the subtropics is parameterized using an empirical relation between stratus cloud cover and the difference in potential temperature between 700 mb (a level that is above the trade inversion) and the surface. This relation includes the empirical negative correlation between SST and low cloud cover and is potentially a positive feedback to climate forcing.

Since potential temperature above the trade inversion varies in unison across the Tropics as a result of the large-scale circulation and because moist convection relates tropospheric temperature within the convecting region to variations in surface temperature and moisture, the subtropical potential temperature at 700 mb depends upon surface conditions within the convecting region. As a result, subtropical stratus cloud cover and the associated feedback depend upon the entire tropical climate and not just the underlying SST.

A simple tropical model is constructed, consisting of separate budgets of dry static energy and moisture for the convecting region (referred to as the “warm” pool) and the subtropical descending region (the “cold” pool). The cold pool is the location of stratus low clouds in the model. Dynamics is implicitly included through the assumption that temperature above the boundary layer is horizontally uniform as a result of the large-scale circulation. The tropopause and warm pool surface are shown to be connected by a moist adiabat in the limit of vanishingly narrow convective updrafts.

Stratus low cloud cover is found to be a negative feedback, increasing in response to doubled CO2 and reducing the tropically averaged warming in comparison to the warming with low cloud cover held fixed. Increased low cloud cover is shown to result from the increased difference in surface temperature between the warm and cold pools, and the increased low-level static stability over the warm pool, equal to the increase in potential temperature along the moist adiabat originating in the warm pool mixed layer.

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R. L. Miller

Abstract

This study calculates the adjustment to radiative forcing in a simple model of a mixed layer ocean coupled to the overlying atmosphere. One application of the model is to calculate how dust aerosols perturb the temperature of the atmosphere and ocean, which in turn influence tropical cyclone development. Forcing at the top of the atmosphere (TOA) is the primary control upon both the atmospheric and ocean temperature anomalies, both at equilibrium and during most of the adjustment to the forcing. Ocean temperature is directly influenced by forcing only at the surface, but is indirectly related to forcing at TOA due to heat exchange with the atmosphere. Within a few days of the forcing onset, the atmospheric temperature adjusts to heating within the aerosol layer, reducing the net transfer of heat from the ocean to the atmosphere. For realistic levels of aerosol radiative forcing, the perturbed net surface heating strongly opposes forcing at the surface. This means that surface forcing dominates the ocean response only within the first few days following a dust outbreak, before the atmosphere has responded. This suggests that, to calculate the effect of dust upon the ocean temperature, the atmospheric adjustment must be taken into account explicitly and forcing at TOA must be considered in addition to the surface forcing. The importance of TOA forcing should be investigated in a model where vertical and lateral mixing of heat are calculated with fewer assumptions than in the simple model presented here. Nonetheless, the fundamental influence of TOA forcing appears to be only weakly sensitive to the model assumptions.

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R. L. Miller and X. Jiang

Abstract

The effect of wind-evaporative feedbacks upon ENSO, and the coupling of Pacific and Indian Ocean variability, is considered based upon a 110-yr simulation from a coupled ocean and atmosphere general circulation model.

ENSO-like modes, which propagate westward, are found in the model Pacific Ocean. Examination of the SST budget shows that the modes amplify and propagate as a result of changes in the surface energy flux and upwelling rates. Surface flux variability is dominated by the solar and evaporative components, and wind-evaporative feedbacks are shown to lead to growth and westward propagation of coupled anomalies in the model Pacific, a region of mean easterly winds. Eastward propagating coupled modes in the model Indian Ocean, a region of mean equatorial westerlies, are also found and are attributed to the same feedback.

Interaction of the Pacific and Indian Ocean modes through the evaporation field is demonstrated, and their relevance to observed coupled ocean-atmosphere variability is considered.

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R. L. Miller and I. Tegen

Abstract

The effect of soil dust aerosols upon the tropical climate is estimated by forcing a simple model of a tropical direct circulation. The model consists of a region vertically mixed by deep convection and a nonconvecting region, for which budgets of dry static energy and moisture are constructed. Dynamical effects are included implicitly, by prohibiting horizontal temperature contrasts above the boundary layer.

Dust aerosols absorb sunlight to a greater extent than industrial sulfate and sea-salt aerosols. In a companion study, where the climate response to dust is calculated using an atmospheric general circulation model, the global-average dust radiative forcing is negligible at the top of the dust layer, in comparison to the large reduction of the net flux at the surface. Thus, dust aerosols redistribute radiative heating from the surface into the dust layer, unlike industrial sulfates and sea salt, which through reflection reduce the total radiative energy gained by the column.

The simple model is perturbed by a reduction in the net radiative flux at the surface. Forcing at the top of the dust layer is idealized to be zero. Cooling occurs at the surface of the nonconvecting region, but surface temperature within the convecting region is only slightly perturbed. It is shown that the disproportionately small response within the convecting region is a consequence of the trivial radiative forcing at the top of the dust layer, and the occurrence of deep convection, which prevents the surface temperature from changing without a corresponding change of the emitting temperature in the upper troposphere.

Additional experiments, where the absorptivity of the dust particles is varied, indicate that the anomalous surface temperature is most sensitive to the radiative forcing at the top of the dust layer. The reduction of the surface net radiation is less important per se but introduces an asymmetry in the response between the convecting and nonconvecting regions through the radiative forcing within the dust layer, which is the difference between the forcing at the surface and the layer top. This heating can offset radiative cooling above the boundary layer, reducing the strength of the circulation that links the nonconvecting and convecting regions. The weakened circulation requires cooling of the nonconvecting region relative to the convecting region in order to maintain the export of energy from the latter to the former.

It is suggested that the “semi-indirect” effect of aerosols, wherein cloud cover is changed in response to aerosol heating, is sensitive to the vertical extent and magnitude of the aerosol forcing.

The experiments suggest that dust optical properties (to which the top of the atmosphere forcing is sensitive) should be allowed to vary with the mineral composition of the source region in a computation of the climate response. More extensive measurements of the dust optical properties, along with the vertical distribution of the dust layer, are needed to reduce the uncertainty of the climate response to dust aerosols.

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R. L. Miller and I. Tegen

Abstract

The effect of radiative forcing by soil dust aerosols upon climate is calculated. Two atmospheric GCM (AGCM) simulations are compared, one containing a prescribed seasonally varying concentration of dust aerosols, and the other omitting dust. Each simulation includes a mixed layer ocean model, which allows SST to change in response to the reduction in surface net radiation by dust. Dust aerosols reduce the surface net radiation both by absorbing and reflecting sunlight. For the optical properties of the dust particles assumed here, the reflection of sunlight is largely offset by the trapping of upwelling longwave radiation, so that the perturbation by dust to the net radiation gain at the top of the atmosphere is small in comparison to the surface reduction. Consequently, the radiative effect of soil dust aerosols is to redistribute heating from the surface to within the dust layer.

Beneath the dust layer, surface temperature is reduced on the order of 1 K, typically in regions where deep convection is absent. In contrast, surface temperature remains unperturbed over the Arabian Sea during Northern Hemisphere (NH) summer, even though the dust concentration is highest in this region. It is suggested that the absence of cooling results from the negligible radiative forcing by dust at the top of the atmosphere, along with the frequent occurrence of deep convection, which ties the surface temperature to the unperturbed value at the emitting level.

Where convection is absent, cooling at the surface occurs because radiative heating by dust reduces the rate of subsidence (and the corresponding mass exchange with the convecting region). Thus, the temperature contrast between these two regions must increase to maintain the original transport of energy, which is unperturbed by dust. It is suggested that cooling over the Arabian Sea during NH winter, despite the much smaller dust loading, is permitted by the absence of convection during this season. Thus, the change in surface temperature forced by dust depends upon the extent of overlap between the dust layer and regions of deep convection, in addition to the magnitude of the radiative forcing.

Surface temperature is also reduced outside of the dust cloud, which is unlikely to result solely from natural variability of the AGCM.

It is suggested that the perturbation by dust to Indian and African monsoon rainfall may depend upon the extent to which ocean dynamical heat transports are altered by dust.

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R. L. Miller and R. S. Lindzen

Abstract

African waves are believed to originate as shear instabilities, although in certain cases rainfall is organized so that latent heating contributes to wave growth. What determines whether the shear instability can organize rainfall is considered here; in particular, why African waves organize rainfall mainly during the late summer, despite the regular occurrence of shear instability and rainfall throughout the season.During GATE, moisture convergence by the waves was also largest toward the late summer. It is assumed that an African wave will organize rainfall if it converges moisture—as measured by the ascent at the top of the moist layer—with sufficient amplitude. The wave amplitude is specified at some level beneath the 600-mb African jet, whose instability is a plausible source of the wave. The ascent is calculated using the quasigeostrophic potential vorticity and thermodynamic equations, and depends on the zonal wind separating the unstable jet from the top of the moist layer.Before turning to the example of the African jet, the more general behavior of the model is considered. In the absence of shear, a wave can arrive at the moist layer with undiminished amplitude. However, the ascent corresponding to this wave is small—less than the estimated ascent for Phase I of GATE when rainfall remained unorganized. For larger values of the shear, this threshold can be exceeded, although the ascent decays beneath the jet. Thus, the question arises whether a wave source can organize rainfall from an arbitrarily large distance above the moist layer. It is suggested that organization can only occur if the unstable jet is within a few kilometers of the moist layer and separated by large shear, although exceptions are noted.The calculation is applied to a wind profile resembling the observed 600-mb African jet. The wave amplitude decays beneath the jet so that the ascent at the top of the moist layer increases as the separation of the jet and moist layer decreases. Evidence is presented that the waves are closer to the moist layer during the late summer, resulting in larger ascent at this time.Large variations in the ascent can also occur even if the separation of the jet and moist layer remains constant. It is shown that the ascent can vary greatly as a result of small changes in the jet that are within its observed summer variability.

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R. A. Kropfli and L. J. Miller

Abstract

No abstract available.

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Paul L. Smith and James R. Miller Jr.

Abstract

No abstract available.

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R. A. Kropfli and L. S. Miller

Abstract

The NOAA/WPL dual-Doppler radar System has been used to determine the three-dimensional kiematic, structure of a convective storm during its decaying Stage which grew in a weakly sheared environment. The internal flow appears Similar in many respects to the Wokingham storm described by Browning and Ludlam even though the latter existed in a strongly sheared environment. Among the similar features are an upshear tilted updraft, a surface gust front, the intrusion of middle-level cool dry air, a precipitation-filled down-draft, and a vortex pattern suggestive of obstacle flow.

Quantitative flux results are presented. Profiles of mass, water vapor, energy, momentum and vorticity fluxes were computed using the Doppler data and other supporting data from the National Hail Research Experiment surface network and upper air soundings.

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R. L. Miller and A. D. Del Genio

Abstract

Simulations of natural variability by two GCMs are examined. One GCM is a sector model, allowing relatively rapid integration without simplification of the model physics, which would potentially exclude mechanisms of variability. Two mechanisms are found in which tropical surface temperature and SST vary on interannual and longer times. Both are related to changes in cloud cover that modulate SST through the surface radiative flux.

Over the equatorial ocean, SST and surface temperature vary on an interannual timescale, which is determined by the magnitude of the associated cloud cover anomalies. Over the subtropical ocean, variations in low cloud cover drive SST variations. In the sector model, the variability has no preferred timescale, but instead is characterized by a “red” spectrum with increasing power at longer periods. In the terrestrial GCM, SST variability associated with low cloud anomalies has a decadal timescale and is the dominant form of global temperature variability.

Both GCMs are coupled to a mixed layer ocean model, where dynamical heat transports are prescribed, thus filtering out ENSO and thermohaline circulation variability. The occurrence of variability in the absence of dynamical ocean feedbacks suggests that climatic variability on long times can arise from atmospheric processes alone.

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