Barbara E. Carlson

Nonsphericity of dust‐like tropospheric aerosols: Implications for aerosol remote sensing and climate modeling

T-matrix computations of light scattering by polydispersions of randomly oriented nonspherical aerosols and Mie computations for equivalent spheres are compared. Findings show that even moderate nonsphericity results in suubstantial errors in the retrieved aerosol optical thickness if satellite reflectance measurements are analyzed using Mie theory. On the other hand, the use of Mie theory for nonspherical aerosols produces negligible errors in the computation of albedo and flux related quantities, provided that the aerosol size distribution and optical thickness are known beforehand. The first result can be explained by large nonspherical-spherical differences in scattering phase function, while the second result follows from small nonspherical-spherical differences in single-scattering albedo and asymmetry parameter. No cancellation of errors occurs if one consistently uses Mie theory in the retrieval algorithm and then in computing the albedo for the retrieved aerosol optical thickness.

Long-term satellite record reveals likely recent aerosol trend.

Analysis of the long-term Global Aerosol Climatology Project data set reveals a likely decrease of the global optical thickness of tropospheric aerosols by as much as 0.03 during the period from 1991 to 2005. This recent trend mirrors the concurrent global increase in solar radiation fluxes at Earths surface and may have contributed to recent changes in surface climate.

Forcings and chaos in interannual to decadal climate change

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well-mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.

Absorption within Inhomogeneous Clouds and Its Parameterization in General Circulation Models

Abstract The effect on absorption in clouds of having an inhomogeneous distribution of droplets is shown to depend on whether one replaces a homogeneous cloud by an inhomogeneous cloud that has the same mean optical thickness, or one that has the same spherical albedo. For the purposes of general circulation models (GCMs), the more appropriate comparison is between homogeneous and inhomogeneous clouds that have the same spherical albedo, so that the radiation balance of the planet with space is maintained. In this case it is found, using Monte Carlo and independent pixel approximation calculations, that inhomogeneous clouds can absorb more than homogeneous clouds. It is also found that because of the different effects of cloud inhomogeneity on absorption and on the transmission of the direct beam the absorption efficiency of an inhomogeneous cloud may be either greater (for low and high optical depths) or lesser (for intermediate optical depths) than that for a homogeneous cloud of the same mean optical d...

Tropospheric gas composition and cloud structure of the Jovian north equatorial belt

High spatial resolution Voyager infrared interferometer spectrometer spectra of the North Equatorial Belt (NEB) reveal longitudinal variability of 5-μm brightness temperatures of order 100°C. These observations are used to investigate spatial variations in the gas composition and cloud structure of the NEB. We use an anisotropic multiple scattering radiative transfer model to calculate synthetic spectra for comparison with the IRIS observations. The spectral dependence of cloud extinction from 180 to 2300 cm−1 is modeled using Mie theory. The entire spectral range of the IRIS observations (180–2300 cm−1) is used to constrain the cloud properties and vertical structure of the NEB. Within the model, cloud base locations vary with assumed gas abundances according to thermochemical equilibrium. We find that spatial variations in the abundance profiles of the condensible species, para hydrogen profiles and cloud optical depths can be used as tracers of the local and large-scale dynamics. Based on the spectral dependence of NH3 cloud extinction that is required to fit the IRIS observations, we conclude that the bulk of the NH3 cloud extinction is provided by large particles, effective radii ≈100 μm; however, a small particle mode may also be present. We find that the observed 5-μm brightness temperature structure can be reproduced by spatial variations in cloud opacity and water relative humidity. NEB hot spots, due to their low cloud opacity, provide a unique opportunity to study the deep cloud structure in the Jovian atmosphere. Cloud opacity is required at P > 4 bars (coincident with the location of the thermochemically predicted H2O cloud) to reproduce the observed continuum level near 2130 cm−1, as well as to model the overall shape of the continuum between 2100 and 2300 cm−1. Water relative humidity is found to vary spatially above the base of the water cloud increasing from ≈15% in hot spots to 100% in colder spectral ensembles. The variation of relative humidity is strongly correlated with the variation of cloud opacity, suggesting dynamic depletion of water vapor above the cloud forming level as the most plausible model to explain the spatial variation in the water profile within the NEB.

Scattering of light by large nonspherical particles: ray-tracing approximation versus T-matrix method

We report, for the f irst time to our knowledge, comparisons of light-scattering computations for large, randomly oriented, moderately absorbing spheroids based on the geometric-optics approximation and the exact T-matrix method. We show that in most cases the geometric-optics approximation is (much) more accurate for spheroids than for surface-equivalent spheres and can be used in phase function computations (but not in polarization computations) for nonspherical particles with size parameters as small as 60. Differences in the single-scattering albedo between geometric-optics and T-matrix results are surprisingly small, even for small size parameters.

Remote Sensing of Atmospheric Aerosols and Trace Gases by Means of Multifilter Rotating Shadowband Radiometer. Part I: Retrieval Algorithm

Abstract A retrieval algorithm for processing multifilter rotating shadowband radiometer (MFRSR) data from clear and partially cloudy days is described and validated. This method, while complementary to the Langley approach, uses consistency between the direct normal and diffuse horizontal measurements combined with a regression technique to simultaneously retrieve daily time series of column mean aerosol particle size, aerosol optical depth, NO2, and ozone amounts along with the instruments calibration constants. Comparison with the traditional Langley calibration method demonstrates two advantages of the approach described here: greater calibration stability and a decreased sensitivity of retrievals to calibration errors.

The Spatiotemporal Structure of Twentieth-Century Climate Variations in Observations and Reanalyses. Part II: Pacific Pan-Decadal Variability

Abstract The spatiotemporal structure of Pacific pan-decadal variability (PDV) is isolated in global long-term surface temperature (ST) datasets and reanalysis atmospheric parameter fields from which El Nino–Southern Oscillation (ENSO) effects have been removed. Empirical orthogonal function (EOF) and combined EOF analysis of the resulting time series identify PDV as one of two primary modes of long-term variability, the other being a global warming (GW) trend, which is addressed in a companion paper (Part I). In this study, it is shown that one of several PDV interdecadal regime shifts occurred during the 1990s. This significant change in the Pacific basin is comparable but antiphase to the well-known 1976 climate regime shift and is consistent with the observed changes in biosystems and ocean circulation. A comprehensive picture of PDV as manifested in the troposphere and at the surface is described. In general, the PDV spatial patterns in different parameter fields share some similarities with the patt...

The Spatiotemporal Structure of Twentieth-Century Climate Variations in Observations and Reanalyses. Part I: Long-Term Trend

Abstract The dominant interannual El Nino–Southern Oscillation (ENSO) phenomenon and the short length of climate observation records make it difficult to study long-term climate variations in the spatiotemporal domain. Based on the fact that the ENSO signal spreads to remote regions and induces delayed climate variation through atmospheric teleconnections, an ENSO-removal method is developed through which the ENSO signal can be approximately removed at the grid box level from the spatiotemporal field of a climate parameter. After this signal is removed, long-term climate variations are isolated at mid- and low latitudes in the climate parameter fields from observed and reanalysis datasets. This paper addresses the long-term global warming trend (GW); a companion paper concentrates on Pacific pan-decadal variability (PDV). The warming that occurs in the Pacific basin (approximately 0.4 K in the twentieth century) is much weaker than in surrounding regions and the other two ocean basins (approximately 0.8 K...

Cloud microphysics of the giant planets

Abstract The predominant cloud microphysical processes for the atmospheres of the giant planets are determined by a comparison of their characteristic time constants. These results are an extension of the earlier microphysical modeling by Rossow to other atmospheres and by a more thorough exploration of thermal structure and compositional effects. The NH3 clouds on Jupiter and Saturn are found to be weakly precipitating systems, similar to the thicker cirrus clouds on Earth, while the H2O clouds are much more massive than water clouds on Earth. The NH4SH clouds are very tenuous on Jupiter and Saturn but may produce precipitation on Uranus and Neptune. The CH4 and H2O clouds on Uranus and Neptune are completely unlike any cloud on Earth, being 10–100 times more massive than the densest water clouds on Earth. Horizontal contrasts in weakly precipitating systems, Such as the NH3 cloud on Jupiter may be muted and the vertical extent may be small in a weak dynamic regime or large in a multilayered haze complex...