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Eric A. Smith
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Eric A. Smith
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Eric A. Smith
and
Matthew R. Smith

Abstract

The tropical radiation balance is investigated on an interannual time scale using a five-year(1979–83) dataset obtained from the Nimbus-7 Earth Radiation Budget (ERB) experiment. The study emphasizes the separate contributions to interannual fluctuations in the global radiation balance by the tropics and extratropics. An attempt is made to Identify source regions within the tropics that give rise to the fluctuations and to quantify the effect of the fluctuations on zonal heat transport.

Superimposed on the five-year global trend pattern of net radiation are large amplitude nonseasonal variations largely confined to tropical latitudes. The significant regions are the Southwest–East Asian (SW–EA) monsoon and two regions associated with the ascent and descent branches of the Pacific Walker Cell. A “cloud reciprocity index” is formulated in order to examine the degree to which extended cloud systems over the oceanic tropics can induce these interannual fluctuations in the radiation balance. The SW–EA monsoon and the eastern Pacific exhibit low-index patterns, suggesting that these are the two dominant sources of the anomalies.

The impact of the fluctuations is examined in terms of external entropy exchange (EEE). Paltridge's theory that climate fluctuations are controlled by a minimum EEE constraint is partially supported. The impact of tropical fluctuations on zonal heat transport is examined. The amplitudes in the year-to-year tropical transport residuals are found to be as high as 50% of, and generally out of phase with, the total global residual. The SW–EA monsoon and the eastern Pacific can explain a large portion of the total tropical residual during specific years.

Simultaneous and lagged spatial correlation analyses are used to determine the degree to which the radiative anomalies associated with the SW–EA monsoon region are coupled to other centers of variability. The simultaneous correlations with net radiation are dissimilar to those found with the albedo and outgoing longwave radiation, particularly in terms of seasonal forcing. The organization of lagged albedo anomaly correlation patterns suggest that predictive indicators of the SW–EA monsoon behavior may be found in the tropical ocean basins.

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Amita V. Mehta
and
Eric A. Smith

Abstract

Infrared radiative cooling rates are calculated over the Asian summer monsoon between 5°S–20°N and 40°–135°E at a spatial resolution of 5° × 5° for the summer seasons of 1984 and 1987. A medium spectral resolution infrared radiative transfer model with specified temperature, moisture, clouds, and trace gas distributions is used to obtain the cooling rate profiles. Cloud distributions for the two summers are obtained from Indian National Satellite measurements. Seasonal mean and intraseasonal variations of clouds and radiative cooling rates over a 21–76-day range of periods are examined.

The analysis identifies centers over the central and eastern Indian Ocean, and western Pacific Ocean, along the equator, and along 15°N, where seasonal mean cloud amounts range from 40% to 80% with cloud tops mostly in the middle and upper troposphere. Intraseasonal variability of clouds is also large over these centers (% variances >25%). Consistently, seasonal mean cooling rates are at a maximum (3°–5°C day−1) in the upper troposphere between 300 and 400 mb, related to cloud-top cooling. The cooling rates below 400 mb are between 1° and 3°C day−1. The cooling rates exhibit intraseasonal amplitudes of 1.0°–1.5°C day−1. The largest amplitudes are found between 300 and 500 mb, indicating that cooling rate variability is directly related to intraseasonal variability of convective clouds. Spatial distributions of clouds and cooling rates remain similar during the 1984 and 1987 summer seasons. However, during 1987, intraseasonal amplitudes of deep convective cloud amount and cooling rate over the Indian Ocean are 10%–15% larger than in 1984.

It is shown that intraseasonal variability of cooling rates over the Indian Ocean can perturb convective heating by 10%–30% in the upper and lower troposphere. Based on a one-dimensional radiative–convective equilibrium model, it is estimated that the radiative damping timescale over the Indian Ocean region is ∼3 days. Based on this damping timescale and in conjunction with a model of equatorial Kelvin waves with first baroclinic mode, it is hypothesized that the variable cloud-radiative cooling rates can alter phase speeds of Kelvin waves by up to 60%. This helps explain why the frequency range of intraseasonal oscillations is so broad.

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Eric A. Smith
and
Lei Shi

Abstract

The role of the Tibetan Plateau on the behavior of the surface longwave radiation budget is examined, and the behavior of the vertical profile of longwave cooling over the plateau, including its diurnal variation, is quantified. The investigation has been conducted with the aid of datasets obtained during the 1986 Tibetan Plateau Meteorological Experiment (TIPMEX-86). A medium spectral-resolution infrared radiative transfer model using a simple modification for applications in idealized complex (valley) terrain is developed for the study. This study focuses on the clear-sky case where the surface effects are most significant.

The TIPMEX-86 data, obtained during the spring-summer transition into the East Asian monsoon season, are used to help validate the surface longwave radiation budget at two sites of varying elevation: Lasa (3650 m) and Naqu (4500 m). Based on the degree to which skin-temperature boundary conditions control the magnitude of infrared cooling, we define the concept of relative longwave heating and explain its influence on the vertical infrared cooling-rate profile. Relative longwave radiative heating at the higher-elevation Naqu site is found to be twice as large as that corresponding to the lower-elevation Lasa site located within a valley. Besides reducing the infrared cooling rates, it is shown that relative longwave heating extends the period of the day over which the plateau acts as a direct heat source to the atmosphere. Computational results from the infrared model help substantiate observational analyses that indicate surface longwave net radiation at the high-elevation site, on clear days, exceeds 300 W m−2; this is an order of magnitude greater than typical of sea-level oceanic conditions. As a result of the unique meteorological and surface conditions, total infrared flux convergence occurs within the deep planetary boundary layer (i.e., infrared heating of the cloud-free lower atmosphere) at the high-elevation site during the afternoon. An important characteristic of the daytime longwave heating process of the lower layers is how it turns off like a switch at approximately 1800 MST, transforming almost immediately to maximum cooling of the lower layers.

Atmospheric longwave cooling is significantly influenced by variations in the biophysical composition of the surface and the associated thermal diurnal cycle. It is estimated that natural variations of surface emissivity could modulate longwave cooling by up to 40%. The largest impact would occur at a time when the surface temperature is high and the relative longwave radiative heating of the lower atmosphere by the surface reaches its maximum value.

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Lei Shi
and
Eric A. Smith

Abstract

During the summer east Asian monsoon transition period in 1979, a meteorological field experiment entitled the Qinghai-Xizang Plateau Meteorological Experiment (QXPMEX-79) was conducted over the entire Tibetan Plateau. Data collected on and around the plateau during this period, in conjunction with a medium spectral-resolution infrared radiative transfer model, are used to gain an understanding of how elevation and surface biophysical factors, which are highly variable over the large-scale plateau domain, regulate the spatial distribution of clear-sky infrared cooling during the transition phase of the summer monsoon.

The spatial distribution of longwave cooling over the plateau is significantly influenced by variations in biophysical composition, topography, and elevation, the surface thermal diurnal cycle, and various climatological factors. An important factor is soil moisture. Bulk clear-sky longwave cooling rates are larger in the southeast sector of the plateau than in the north. This is because rainfall is greatest in the southeast, whereas the north is highly desertified and relative longwave radiative heating by the surface is greatest. Another important phenomenon is that the locale of a large-scale east-west-aligned spatial gradient in radiative cooling propagates northward with time. During the premonsoon period (May–June), the location of the strong spatial gradient is found in the southeastern margin of the plateau. Due to changes in surface and atmospheric conditions after the summer monsoon commences, the high gradient sector is shifted to the central Qinghai region. Furthermore, an overall decrease in longwave cooling takes place in the lower atmosphere immediately prior to the arrival of the active monsoon.

The magnitude of longwave cooling is significantly affected by skin-temperature boundary conditions at plateau altitudes. If skin-temperature discontinuities across the surface-atmosphere interface are neglected, bulk cooling rates will be in error up to 1°C day−1. The high surface skin temperatures, particularly in the afternoon, lead to significant relative longwave radiative heating in the lower atmosphere for which the impact in terms of vertical depth is shown to increase rather dramatically as a function of the elevation of the terrain. The significance of these results in the context of previous heat budget studies of the plateau suggest that the radiative heating term (QR ) used by previous investigators contains far too much longwave cooling, and thus in a classic formulation of the Yanai Q 1 balance equation, would lead to underestimation of sensible heating into the atmospheric column.

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Byung-Ju Sohn
and
Eric A. Smith

Abstract

Cloud–radiative forcing calculations based on Nimbus-7 radiation budget and cloudiness measurements reveal that cloud-induced longwave (LW) warming (cloud greenhouse influence) is dominant over the tropics, whereas cloud-induced shortwave (SW) cooling (cloud albedo influence) is dominant over mid- and high latitudes. The average SW cloud cooling taken over the area of the globe from 65°N to 65°S is −27.8 W m−2. This magnitude slightly overcomes LW cloud warming (−25.7 W m−2), resulting in a small net cooling effect of −2.1 W m−2 over 93% of the earth.

A 6-year zonally averaged mean cloudy- and clear-sky net radiation flux analysis shows that there are three distinct regimes in terms of net cloud warming or cooling, that is, warming in the tropics (between 20°N and 20°S) and in the high latitudes (poleward of 55°) and cooling in the extratropical latitudes between 20° and 55° in both hemispheres. These distributions reinforce the intensities of the Hadley and Ferrel meridional circulation cells. This stems from strong warming due to high-level clouds in the tropics and strong cooling due to mid- and low-level clouds at extratropical latitudes. The magnitude of the contribution by cloud forcing is found to be of the same order as eddy heat and momentum flux forcing to the maintenance of the mean meridional circulation.

Surface–atmosphere forcing obtained by differentiating the cloud-induced effects from the measured radiative fluxes indicates that an east–west coupled North Africa–western Pacific energy transport dipole is maintained mainly by low-latitude land–ocean contrasts associated with shortwave radiation but supported by cloud controls on tropical longwave radiation. This implies that interannual variations in the net radiation balance associated with these two regions can give rise to fluctuations of the basic dipole structure and thus fundamental changes in low-latitude climate.

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Mickey M-K. Wai
and
Eric A. Smith

Abstract

Land–atmosphere interactions are examined for three different synoptic situations during a 21-day period in the course of the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment 1989 to better understand the relationship between biophysical feedback processes, boundary layer structure, and circulations in the boundary layer. The objective is to understand how the secondary circulation discussed in Part I of this paper was able to sustain itself throughout the duration of the 1989 intensive field campaign. The study is based on diagnostic analysis of measurements obtained from a network of surface meteorology and energy budget stations, augmented with high vertical resolution radiosonde measurements. Shallow convection associated with an undisturbed boundary layer situation and rainfall occurring during two different disturbed boundary layer situations—one associated with a surface trough, the other with the passage of a cold front—led to markedly different impacts on the surface layer and the boundary layer recovery timescale. In the undisturbed case, the growth of a cloud layer produced a negative feedback on the boundary layer by stabilizing the surface layer, and cutting off the turbulence transport of heat and moisture into the subcloud layer. The deficits in heat and moisture then led to cloud dissipation. During the surface trough development and cold front passage events, rainfall reaching the surface led to the collapse of the surface layer, decrease of surface and subsurface soil temperatures, depressed sensible heating, and a slow reduction and even temporary termination of evapotranspiration. After the rains subsided, the boundary layer recovery process began with vigorous evapotranspiration rates drying the upper soil layers on a timescale of 1–2 days. During this period, 55%–65% of the net surface available heating was used for evapotranspiration, whereas only 30%–35% went directly into boundary layer heating. As the near-surface soil moisture dropped, surface sensible heating became more important in influencing boundary layer energetics. The boundary layer required approximately two days to recover to its initial temperature in the case of the surface trough. After passage of the cold front, both the soil and boundary layer cooled and dried due to cold temperature advection. Evapotranspiration rates remained relatively large for about two days after the frontal passage. The boundary layer had not completely recovered by the end of the intensive data collection period after the frontal passage, so recovery time was at least a week. The analysis shows that with the exception of three days during the surface trough event, and two or three days during the frontal passage event, the surface-driven secondary circulation persisted.

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Graeme L. Stephens
,
Steven Ackerman
, and
Eric A. Smith

Abstract

We present a modification to the parameterization scheme of Stephens which improves on the estimation of shortwave absorption by cloud. In particular, the variation of cloud absorption with solar elevation angle is improved with the modified scheme.

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Ralph R. Ferraro
,
Eric A. Smith
,
Wesley Berg
, and
George J. Huffman

Abstract

The success of any passive microwave precipitation retrieval algorithm relies on the proper identification of rain areas and the elimination of surface areas that produce a signature similar to that of precipitation. A discussion on the impact of and on methods that identify areas of rain, snow cover, deserts, and semiarid conditions over land, and rain, sea ice, strong surface winds, and clear, calm conditions over ocean, are presented. Additional artifacts caused by coastlines and Special Sensor Microwave/Imager data errors are also discussed, and methods to alleviate their impact are presented. The strengths and weaknesses of the “screening” techniques are examined through application on various case studies used in the WetNet PIP-2. Finally, a methodology to develop a set of screens for use as a common rainfall indicator for the intercomparison of the wide variety of algorithms submitted to PIP-2 is described.

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