Abstract

Stratospheric wind data for the Marshall Islands region during 1 April–1 July, 1958, are analyzed for contributions in the 4–6 day period range. It is shown that, excluding waves with vertical wavelengths <2 km from the data, 4–6 day power in the equatorial stratosphere during this period must be due to some combination of Kelvin, mixed gravity-Rossby, and n=1 Rossby waves with zonal wavenumbers <7–10. It is further shown that a theoretical model wherein each of the above three wave types is associated with a zonal wavenumber of either 3 or 4 is consistent with the data. The resulting observationally calibrated model is used to calculate the acceleration of the mean flow by wave absorption, which is then compared with the observed acceleration. In general, the waves satisfactorily account for accelerations above 23 km. Below 23 km there is a need for an additional source of easterly momentum with a specific vertical distribution which we show could be provided by an n=1 easterly gravity wave whose vertical wavelength, however, would be too short for the wave to be seen in radiosonde data. We also show that if a mean flow together with 4–6 day waves is spectral analyzed, there will be power at periods >4–6 days due to the acceleration of the mean flow by waves, and there may also be power at periods <4–6 days due to the modification of the waves by the changing mean flow. We finally examine what the theory suggests is happening at levels where wave absorption is altering the mean flow, and show, some of the difficulties in relating such behavior to data averaged over a three-month period.

Abstract

The wavenumber-frequency spectra of the meridional transport of angular momentum at 100, 200 and 500 mb, at 20, 40, 60 and 80N, show that there exist definite spectral domains of wave interactions between the zonal and meridional velocities at various latitudes. In the middle latitudes near 40N, the spectral band of the meridional transport of angular momentum is oriented from a region of low wavenumbers and low frequencies to a region of high wavenumbers and negative frequencies designated for eastward-moving waves. In low latitudes, however, the spectral band is confined to a narrow band centered near zero frequency.

An analysis of the linear and nonlinear contributions to the meridional transport of angular momentum in various wavenumber-frequency domains indicates that in the mid-troposphere the primary contribution to the nonlinear interactions always involves the interactions of the spectral domain of concern with the mean zonal flow and the stationary planetary waves. It is also found that except in the domain of low-frequency, eastward-moving cyclone waves the following characteristics are in common. 1) the meridional transport of angular momentum is directed toward the north pole; 2) the resultant of the nonlinear interactions due to the longitudinal convergence of the transport provides a poleward flux of angular momentum in the domains of eastward-moving waves, but provides an equatorward transport in the domains of westward-moving waves; 3) the resultant of the nonlinear interactions due to the latitudinal convergence of the transport generally contributes a poleward transport of angular momentum in the domains of westward-moving waves, but contributes an equatorward transport in the domains of eastward-moving waves; 4) the ageostrophic effect always counteracts the nonlinear interactions due to the longitudinal convergence of the transport of angular momentum; and 5) the effects of eddy and molecular stress forces generally work against the ageostrophic effect.

The frequency spectra of the meridional transport of angular momentum indicate that: 1) in the summer most of the transport is accomplished by the moving waves, the eastward-moving waves contributing to most of the poleward transport, and the westward-moving waves to the equatorward transport; 2) in the winter most of the transport is accomplished by the stationary waves, and both the eastward- and westward- moving waves contribute to the poleward transport of angular momentum.

The wavenumber spectra of the transport of angular momentum indicate that in both the summer and winter seasons waves of practically all wavelengths in low and middle latitudes contribute to the poleward transport of angular momentum. In high latitudes, however, only the very long waves contribute to the equatorward transport of angular momentum.

Abstract

The three-dimensional equation of radiative transfer is formally solved using a Fourier-Riccati approach while calculations are performed on cloudy media embedded in a two-dimensional space. An extension to Stephens’ work, this study addresses the coupling between space and angle asserted by the equation of transfer. In particular, the accuracy of the computed radiation field as it is influenced by the angular resolution of the phase function and spatial discretization of the cloudy medium is discussed. The necessity of using a large number of quadrature points to calculate fluxes even when the phase function is isotropic for media exhibiting vertical and horizontal inhomogeneities is demonstrated. Effects of incorrect spatial sampling on both radiance and flux fields are also quantified by example. Radiance and flux comparisons obtained by the Fourier-Riccati model and the independent pixel approximation for inhomogeneous cloudy media illustrate the inadequacy of the latter even for tenuous clouds.

Abstract

Brightness temperature difference (BTD) values are calculated for selected Geostationary Operational Environmental Satellite (GOES-6) channels (3.9, 12.7 µm) and Advanced Very High Resolution Radiometer channels (3.7, 12.0 µm). Daytime and nighttime discrimination of particle size information is possible given the infrared cloud extinction optical depth and the BTD value. BTD values are presented and compared for cirrus clouds composed of equivalent ice spheres (volume, surface area) versus randomly oriented hexagonal ice crystals. The effect of the hexagonal ice crystals is to increase the magnitude of the BTD values calculated relative to equivalent ice sphere (volume, surface area) BTDs. Equivalent spheres (volume or surface area) do not do a very good job of modeling hexagonal ice crystal effects on BTDs; however, the use of composite spheres improves the simulation and offers interesting prospects. Careful consideration of the number of Legendre polynomial coefficients used to fit the scattering phase functions is crucial to realistic modeling of cirrus BTDs. Surface and view-angle effects are incorporated to provide more realistic simulation.

Abstract

In September 2006, NASA Goddard’s mobile ground-based laboratories were deployed to Sal Island in Cape Verde (16.73°N, 22.93°W) to support the NASA African Monsoon Multidisciplinary Analysis (NAMMA) field study. The Atmospheric Emitted Radiance Interferometer (AERI), a key instrument for spectrally characterizing the thermal IR, was used to retrieve the dust IR aerosol optical depths (AOTs) in order to examine the diurnal variability of airborne dust with emphasis on three separate dust events. AERI retrievals of dust AOT are compared with those from the coincident/collocated multifilter rotating shadowband radiometer (MFRSR), micropulse lidar (MPL), and NASA Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) sensors. The retrieved AOTs are then inputted into the Fu–Liou 1D radiative transfer model to evaluate local instantaneous direct longwave radiative effects (DRE_LW) of dust at the surface in cloud-free atmospheres and its sensitivity to dust microphysical parameters. The top-of-atmosphere DRE_LW and longwave heating rate profiles are also evaluated. Instantaneous surface DRE_LW ranges from 2 to 10 W m⁻² and exhibits a strong linear dependence with dust AOT yielding a DRE_LW of 16 W m⁻² per unit dust AOT. The DRE_LW is estimated to be ∼42% of the diurnally averaged direct shortwave radiative effect at the surface but of opposite sign, partly compensating for the shortwave losses. Certainly nonnegligible, the authors conclude that DRE_LW can significantly impact the atmospheric energetics, representing an important component in the study of regional climate variation.

Abstract

The authors investigate the extent to which the contrast brightness of ship tracks, that is, the relative change in observed solar reflectance, in visible and near-infrared imagery can be explained by the microphysics of the background cloud in which they form. The sensitivity of visible and near-infrared wavelengths for detecting reflectance changes in ship tracks is discussed, including the use of a modified cloud susceptibility parameter, termed the “contrast susceptibility,” for assessing the sensitivity of background cloud microphysics on potential track development. It is shown that the relative change in cloud reflectance for ship tracks is expected to be larger in the near-infrared than in the visible and that 3.7-μm channels, widely known to be useful for detecting tracks, have the greatest sensitivity. The usefulness of contrast susceptibility as a predictor of ship track contrast is tested with airborne and satellite remote sensing retrievals of background cloud parameters and track contrast. Retrievals are made with the high spatial resolution Moderate Resolution Imaging Spectroradiometer Airborne Simulator flown on the National Aeronautics and Space Administration’s high-altitude ER-2 aircraft, and with the larger-scale perspective of the advanced very high resolution radiometer. Observed modifications in cloud droplet effective radius, optical thickness, liquid water path, contrast susceptibility, and reflectance contrast are presented for several ship tracks formed in background clouds with both small and large droplet sizes. The remote sensing results are augmented with in situ measurements of cloud microphysics that provide data at the smaller spatial scales.

Abstract

An airborne scanning spectrometer was developed for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.2 µm. The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. The spectrometer scans a swath width of 37 km, perpendicular to the aircraft flight track, with a 2.5-mrad instantaneous field of view. Images are thereby produced with a spatial resolution of 50 m at nadir from a nominal aircraft altitude of 20 km. Nineteen of the spectral bands correspond closely to comparable bands on the Moderate Resolution Imaging Spectroradiometer (MODIS), a facility instrument being developed for the Earth Observing System to be launched in the late 1990s. This paper describes the optical, mechanical, electrical, and data acquisition system design of the MODIS Airborne Simulator and presents some early results obtained from measurements acquired aboard the National Aeronautics and Space Administration ER-2 aircraft that illustrate the performance and quality of the data produced by this instrument.

Aerosol- and moonsoon-related droughts and floods are two of the most serious environmental hazards confronting more than 60% of the population of the world living in the Asian monsoon countries. In recent years, thanks to improved satellite and in situ observations, and better models, great strides have been made in aerosol and monsoon research, respectively. There is now a growing body of evidence suggesting that interaction of aerosol forcing with monsoon dynamics may alter the redistribution of energy in the atmosphere and at the Earth s surface, thereby influencing monsoon water cycle and climate. In this article, the authors describe the scientific rationale and challenges for an integrated approach to study the interactions between aerosol and monsoon water cycle dynamics. A Joint Aerosol-Monsoon Experiment (JAMEX) is proposed for 2007–11, with enhanced observations of the physical and chemical properties, sources and sinks, and long-range transport of aerosols, in conjunction with meteorological and oceanographic observations in the Indo-Pacific continental and oceanic regions. JAMEX will leverage on coordination among many ongoing and planned national research programs on aerosols and monsoons in China, India, Japan, Nepal, Italy, and the United States, as well as international research programs of the World Climate Research Program (WCRP) and the World Meteorological Organization (WMO).