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Scott Hottovy and Samuel N. Stechmann

Abstract

A linear stochastic model is presented for the dynamics of water vapor and tropical convection. Despite its linear formulation, the model reproduces a wide variety of observational statistics from disparate perspectives, including (i) a cloud cluster area distribution with an approximate power law; (ii) a power spectrum of spatiotemporal red noise, as in the “background spectrum” of tropical convection; and (iii) a suite of statistics that resemble the statistical physics concepts of critical phenomena and phase transitions. The physical processes of the model are precipitation, evaporation, and turbulent advection–diffusion of water vapor, and they are represented in idealized form as eddy diffusion, damping, and stochastic forcing. Consequently, the form of the model is a damped version of the two-dimensional stochastic heat equation. Exact analytical solutions are available for many statistics, and numerical realizations can be generated for minimal computational cost and for any desired time step. Given the simple form of the model, the results suggest that tropical convection may behave in a relatively simple, random way. Finally, relationships are also drawn with the Ising model, the Edwards–Wilkinson model, the Gaussian free field, and the Schramm–Loewner evolution and its possible connection with cloud cluster statistics. Potential applications of the model include several situations where realistic cloud fields must be generated for minimal cost, such as cloud parameterizations for climate models or radiative transfer models.

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Samuel N. Stechmann and Scott Hottovy

Abstract

In the tropics, rainfall is coupled with waves in the form of, for example, convectively coupled equatorial waves (CCEWs) and the Madden–Julian oscillation (MJO). In perhaps the simplest viewpoint of CCEWs, the effects of moisture and convective adjustment can predict the basic aspects of their propagation and structure: reduced propagation speeds and reduced meridional length scales. Here, a similar simple viewpoint is investigated for the MJO’s propagation and structure. To do this investigation, budget analyses of a model MJO are first presented to illustrate and motivate the asymptotic scaling assumptions. Asymptotic models are then derived for the MJO. In brief, the structure of the asymptotic MJO is described by a tropical geostrophic balance, and the slow propagation arises from the dynamics of moist static energy. To be specific, if the moist static energy has a background vertical gradient that is asymptotically weak (i.e., a moist stability that is nearly neutral), then it supports a slowly propagating wave. Beyond these main aspects, other processes also have an influence, such as eddy diffusion of moisture. In comparing the simple viewpoints of CCEWs and the MJO, one main difference is in the propagation speeds: relative to a dry wave speed of 50 m s−1, the MJO has a speed of 5 m s−1, resulting from a reduction factor of 0.1 related to moist stability, whereas the basic CCEW speed is 15 m s−1, resulting from a reduction factor of the square root of 0.1, related to the square root of the moist stability.

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N. A. Scott and A. Chedin

Abstract

A computationally fast line-by-line method for the determination of atmospheric absorption is described. This method is based on the creation of an Automatized Atmospheric Absorption Atlas (4A) covering all possible plausible atmospheric conditions (temperature, mixing ratios of absorbing gases, zenith angle). It is applied to synthetic computations of atmospheric transmittances and radiant energies associated with three types of satellite observations: radiometric measurements made by HIRS/2 (High-Resolution Infrared Sounder) on TIROS-N; infrared images taken from the geostationary satellite METEOSAT; interferometric experiment IRIS (Infrared Interferometer Spectrometer) on VOYAGER (NASA's mission to Jupiter, Saturn and possibly Uranus). For all three experiments, comparisons were made with real observations and are presented associated with radiosonde data for the first. Concerning computation times, a gain of a factor varying between 15 and 40 is obtained when using the 4A line-by-line method rather than a standard line-by-line method.

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D. Imbault, N. A. Scott, and A. Chedin

Abstract

Synthetic computations of atmospheric transmissions and radiant energies are presented in order to simulate remote soundings of the sea surface temperature in two pairs of channels: ∼8.7 and 11.6 μm, on the one hand, and 11 and 12 μm, on the other. This study takes into account absorptions by the line spectra of the various absorbers and by the water vapor continuum, first separately and then simultaneously. In the latter case, the accuracy of the linear parametric retrieval scheme previously presented by Imbault et al. (1978) is shown to depend mostly on the choice of wavelengths for the two channels and two criteria for optimization are introduced. Applying this parametric scheme to the analysis of the experimental results obtained, with the help of an airborne scanning radiometer, over a relatively humid atmosphere within the midlatitudes has led us to conclude that the usual expression for the water vapor absorption coefficient seems to overestimate this phenomenon.

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E. Péquignot, A. Chédin, and N. A. Scott

Abstract

Atmospheric Infrared Sounder (AIRS; NASA Aqua platform) observations over land are interpreted in terms of monthly mean surface emissivity spectra at a resolution of 0.05 μm and skin temperature. For each AIRS observation, an estimation of the atmospheric temperature and water vapor profiles is first obtained through a proximity recognition within the thermodynamic initial guess retrieval (TIGR) climatological library of about 2300 representative clear-sky atmospheric situations. With this a priori information, all terms of the radiative transfer equation are calculated by using the Automatized Atmospheric Absorption Atlas (4A) line-by-line radiative transfer model. Then, surface temperature is evaluated by using a single AIRS channel (centered at 12.183 μm) chosen for its almost constant emissivity with respect to soil type. Emissivity is then calculated for a set of 40 atmospheric windows (transmittance greater than 0.5) distributed over the AIRS spectrum. The overall infrared emissivity spectrum at 0.05-μm resolution is finally derived from a combination of high-spectral-resolution laboratory measurements of various materials carefully selected within the Moderate-Resolution Imaging Spectroradiometer/University of California, Santa Barbara (MODIS/UCSB) and Advanced Spaceborne Thermal Emission and Reflection Radiometer/Jet Propulsion Laboratory (ASTER/JPL) emissivity libraries. It is shown from simulations that the accuracy of the method developed in this paper, the multispectral method (MSM), varies from about 3% around 4 μm to considerably less than 1% in the 10–12-μm spectral window. Three years of AIRS observations (from April 2003 to March 2006) between 30°S and 30°N have been processed and interpreted in terms of monthly mean surface skin temperature and emissivity spectra from 3.7 to 14.0 μm at a spatial resolution of 1° × 1°. AIRS retrievals are compared with the MODIS (also flying aboard the NASA/Aqua platform) monthly mean L3 products and with the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies baseline-fit method (UW/CIMSS BF) global infrared land surface emissivity database.

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A. Chedin, N. A. Scott, and A. Berroir

Abstract

An experimental simulation of a single-channel, double-angle viewing technique for the determination of sea surface temperature from satellite is presented. This method relies upon the fact that the same area can be viewed simultaneously at two different angles (different air masses) by the geostationary satellite METEOSAT and by the polar orbiting satellite TIROS-N. Extrapolating the two air mass observations to zero air mass is shown to give a value of the temperature in good agreement with the true sea surface temperature. A discussion concerning the viewing angles is presented.

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C. Claud, N. A. Scott, and A. Chedin

Abstract

A 4½-yr monthly stratospheric temperature record derived from TIROS-N Operational Vertical Sounder satellite observations has been used to study the global variability of the stratosphere in the Tropics. A comparison with an independent set of temperatures (Free University of Berlin) is first discussed. Among the different parameters that influence the tropical stratosphere, 1) the regular seasonal cycle, 2) the quasi-biennal oscillation (QBO), and 3) the El Niño–Southern Oscillation (ENSO) effects are studied in detail. A transition level has been found at about 30 hPa. Below this level, the standard stratospheric seasonal cycle in the temperatures is modulated by ENSO and the QBO, while above, ENSO has no discernible influence. In addition, longitudinal variations of monthly mean temperatures show minima during northern winter months from the tropopause up to 50 hPa over some areas, in relation to convection. Results presented here are also discussed in the view of recently published studies based on either radiosonde reports or microwave satellite measurements. While there is a fair agreement with radiosonde-based studies, more finescale details on the horizontal are obtained due to a much better sampling. Differences with other satellite-based studies are due to a better description of the temperature behavior along the vertical.

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B. C. Scott and N. S. Laulainen

Abstract

Two case studies are used to examine the relationship of the sulfate concentration in surface precipitation to the microphysical characteristics of the precipitating cloud systems. The data from one case study support the contention that existing sulfate aerosol was incorporated into cloud water by the nucleation process and accounted for nearly all of the observed cloud and precipitation water sulfate concentration. These activated sulfate particles comprised nearly 60% of the clear-air sulfate mass concentration. Once nucleated, the sulfate particles accumulated water through the condensation process and were subsequently deposited at the surface after accretion on large snowflakes. The presumption of aqueous phase sulfate oxidation of SO2 was not necessary to account for the observed sulfate concentrations.

The data from the second case study are more limited and difficult to interpret. Nucleation and below cloud washout appeared to be the main contributors to the surface sulfate concentration in precipitation water.

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N. Beriot, N. A. Scott, A. Chedin, and P. Sitbon

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A method is presented for the calibration of infrared radiometers on geostationary satellites using calibrated infrared radiometers on an orbiting satellite. This method relies on similarities between the weighting functions corresponding to the radiometers on geostationary satellites like METEOSAT or the GOES series and the weighting functions of some of the channels on the TIROS-N Operational Vertical Sounder (TOVS). It makes use of iso-secant observations of the same scene from both satellites. Many such observations are available every day resulting in daily calibration curves defined by several hundred points. This calibration method is shown to be very sensitive, accurate and tractable. This method does not require the collection of radiosonde data or any kind of in situ experiments and may be completely automated.

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C. J. Stubenrauch, N. A. Scott, and A. Chedin

Abstract

Onboard the NOAA satellites, the High-Resolution Infrared Sounder (HIRS) with its 20 channels, combined with the Microwave Sounding Unit (MSU), provides a powerful tool for cloud field classification at a spatial resolution of about 100 km. The 3I (improved initialization inversion) algorithm-developed to obtain atmospheric temperature and water vapor profiles as well as cloud and surface properties-has been modified in order to extract more reliable information on cloud-top pressure and effective cloud amount. These cloud parameters have been compared to cloud types identified by an operationally working threshold algorithm based on Advanced Very High Resolution Radiometer measurements over the North Atlantic. The improved 3I cloud algorithm provides cloud parameters not only for high clouds but also greatly improves the determination of low clouds. The algorithm has also been extended to give cloud information over partly cloudy situations. The 3I cloud field classification yields 11 different cloud field types for spatial elements of 100 km according to cloud height, cloud thickness, and cloud cover. The radiative effects of these different cloud field types are studied by combining the 3I results with Earth Radiation Budget Experiment (ERBE) fluxes. A simple radiative transfer theory can relate the ERBE outgoing longwave flux to all 3I cloud field types to within 5 W m−2. This encourages a detailed analysis of cloud radiative effects on a global scale. Especially during night, as shown in this study, International Satellite Cloud Climatology Project (ISCCP) cloud information can be extended by the HIRS-MSU analysis, because the ISCCP provides information on cloud thickness only during day.

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