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Limited Time Period (LTP) running trends are used to evaluate Microwave Sounding Unit (MSU) derived tropospheric temperature trend methods in an attempt to alleviate documented considerable disagreements between tropospheric datasets so investigation into the atmospheric variability is able to move forward.Regression derived coefficients were used to combine lower stratosphere (LS) and mid-troposphere to lower stratosphere (MT) simulated MSU channels from RATPAC radiosonde data. This protocol is used to estimate tropospheric temperature trends and compared to actual RATPAC derived tropospheric temperature trends. It is found that the statistical LS/MT combination results in greater than 50% error over some LTP. These errors are found to exist when strong cooling in the stratosphere is coincident with periods when the level separating cooling from warming is above the tropopause.LTP trends are also created from various MSU difference time series between the University of Alabama in Huntsville (UAH) and Remote Sensing System (RSS) group's lower troposphere (LT) and MT channels. Results suggest the greatest discrepancies over time periods where NOAA-11 through NOAA-15 adjustments was applied to the raw LT data over land. Discrepancies are shown to be dominated by differences in diurnal correction methods due to orbital drift. Comparison of MSU data with radiosonde data indicate that RSS's method of determining diurnal effects is overestimating the correction in the LT channel. Diurnal correction signatures still exist in the RSS LT time series and are likely affecting the long term trend with a warm bias.These findings suggest atmospheric amplification is not happening in the atmosphere using globally averaged data over the MSU era. There is evidence however from the radiosonde data that shows greater warming in the ~300-500 hPa layer than at the surface during some LTP in the complete radiosonde database. This temporal change in temperature trends warrants further stu

Precipitation (P) and precipitable water (W) are important components of the hydrological cycles in the earth system, and their seasonal cycles are closely related to monsoon circulations over monsoon regions. Through theoretical analyses and extensive analysis of data from in-situ measurements, satellite remote sensing, and regional reanalysis, significant progress has been made (via four peer-reviewed publications) in four areas related to P, W, and monsoon onset and retreat. First, based on the normalized W index, a novel unified method is proposed to determine global monsoon onset and retreat dates. The results are consistent with those obtained from different local criteria. Second, theoretical and data analyses demonstrate that, because of the large annual range of temperature, W can increase from winter to summer anywhere except in the tropics, including both monsoon and nonmonsoon regions. Third, while the seasonal variation of P is, in general, caused by complex processes (e.g., atmospheric circulations), thermodynamic derivations and data analysis demonstrate that the variation of P from winter to summer can be easily understood from the comparative strength between the change of water vapor and the change of temperature. In monsoon regions, the change of water vapor from winter to summer is much greater than the change of temperature, so P has an in-phase relation with W. While in some of the nonmonsoon regions, where winter is the rainy season, the change of temperature is much greater than the change of water vapor, leading to an out-of-phase relation between P and W, and, relative to summer, the coldness of the winter air is much more significant than its dryness. Finally, the satisfactory performance of the globally unified monsoon index can be understood by comparing the seasonal cycles of P and W. The significant positive correlations between P and W at seasonal and synoptic scales imply that W has the ability to indicate both the means and the inte

The in-flight calibration of satellite radiometers using ground truth measurements relies on the use of an atmospheric radiative transfer code. The accuracy of the calibration depends largely on the aerosol model used in the radiative transfer code. In order to improve the calibrations, a camera system has been developed for the determination of the aerosol size distribution, index of refraction, and scattering phase function. In addition, the camera can be used to measure ozone and water vapor content. The camera uses a two dimensional silicon CCD array to image the sun and the solar aureole. A filter wheel provides sixteen spectral bands from 380 nm to 1045 nm. The camera is mounted on an altitude-azimuth mount for tracking the sun. An external computer allows automatic or manual data acquisition. The aerosol size distribution retrieval is based on the combined inversion of solar extinction and solar aureole data. The real part of the aerosol refractive index is determined using scattering measurements in the near-backward direction, while diffuse-to-global measurements provide the imaginary part. The performance of the inversion schemes is illustrated for simulated in-flight satellite signal predictions over both high and low reflectance targets.

A new iterative technique to find both the real and imaginary index of refraction of atmospheric aerosols has been developed which utilizes polarized sky radiance measurements in the solar principal plane. The technique requires that the aerosol size distribution, Rayleigh optical depth, aerosol optical depth, surface albedo, and preferably the surface anisotropic characteristics be known from correlative measurements. The calculated optimal radiance ratio, which is the ratio of maximum sensitivity to the index being retrieved with minimum sensitivity to other parameters, is compared with the measured optimal radiance ratio. The retrieved index is found through iterative steps to minimize the difference between the measurement ratio and calculated ratio. The degree to which the uncertainties of the ensemble of parameters that affect retrieved index is assessed. Mie single scattering sensitivity simulations and radiative transfer simulations using a Gauss-Sidel routine are made to determine the sensitivity of polarized sky radiance to index, size distribution, surface albedo, and surface reflectance anisotropy. The results indicate that this inversion technique is viable, especially to retrieve the effective real index of refraction. While the retrieval of imaginary index did not work as consistently well as the retrieval of real index, the imaginary index retrieval yielded promising results with larger values of imaginary index (∼0.020). This technique also offers new insight into the effect surface anisotropy has on sky radiance.

Spectra from 1.7-3.3 μm acquired at the NASA Kuiper Airborne Observatory include 2 of Saturn's near-IR atmospheric transmission windows that are at least partially obscured by telluric H₂O and CO₂ absorptions at ground-based telescopes. This entire spectral region was fitted to a model that included gaseous absorption by H₂, CH₄, NH₃, and PH₃ and the effects of multiple scattering by haze. The objectives were to determine accurate elemental abundance ratios (e.g., C/H, P/H, etc) and to characterize the size, distribution, and composition of the haze particles in Saturn's atmosphere. The results for C/H and P/H are 8.5 x 10⁻⁴ and 4.3 x 10⁻⁷, respectively. No evidence of gaseous NH₃ was found. The upper limit to the NH₃ mixing ratio at Saturn's radiative-convective boundary is ≈ 10⁻⁹. Ammonia is decidedly undersaturated at atmospheric pressures lower than ≈ 1 bar. The upper limit to gaseous NH₃ at 3 μm is extremely low compared to detected amounts derived from observations at visible, mid-IR, and microwave wavelengths. These differences can be reconciled on the basis of different mechanisms for spectral line formation in these disparate spectral regions. A search for solid phase NH₃ was also negative. From thermochemical arguments it has been widely assumed that NH₃ ice crystals comprise the upper clouds on Saturn, although no incontrovertible spectroscopic proof has ever been presented. Strong bands of solid NH₃ at 3 μm therefore offer an important test of this assumption. Saturn's observed spectrum was placed on an absolute reflectivity scale which then could be compared with synthesized spectra of candidate haze particles. The calculations demonstrated that the reflectances of pure, polydisperse NH₃ ice crystals with effective radii ranging from 0.1 to 2.25 μm are not compatible with Saturn's 3 μm spectrum. A reasonable fit to Saturn's continuum spectrum can only be achieved by using bright, micron-sized scattering haze particles mixed-in with H₂, CH₄, and PH₃ in S

Numerical simulations using the Penn State University/NCAR MM4 model are performed to examine a stably stratified, zonal easterly flow past large scale three-dimensional mountain ranges in a rotating, initially barotropic, atmosphere. Upstream blocking by the mountain range diverts the flow primarily to the south and around the mountain. Conservation of potential vorticity results in the formation of a horizontal jet at low levels south of the mountain. This jet is barotropically unstable and leads to a continuous production of synoptic scale vorticity maxima which separate from the mountain and propagate downstream. Numerical simulations using topography representative of the Sierra Madre in Mexico imply that this mechanism may be important in providing some of the initial disturbances which grow into tropical cyclones in the eastern North Pacific Ocean. The wave train produced in the simulations corresponds to waves with 3-7 day periods which have been identified observationally in the eastern North Pacific region. The sensitivity of this effect to the stability of the basic state and the upstream wind speed is investigated. Simulations are also performed which show that the Hoggar and Atlas mountains of west-central Africa block the low-level easterlies resulting in a barotropically unstable jet and a train of vorticity maxima which separate from the mountain and propagate downstream. The spacing of these disturbances is roughly 1600 km and they propagate to the east with a period of about 2.5 days. These characteristics correspond to those of observed waves in the Africa/Atlantic region. It will also be shown that the unique topography of north-central Africa results in a mid-tropospheric easterly jet which has a maximum between 0-10°E and 15-20°N. The location and magnitude of this jet correspond to the so-called African easterly jet which is usually attributed to the strong surface temperature gradients over the continent of Africa. The numerical simulations p

An interactive computer program .has been developed to compute accurate values of lightning-caused changes in the cloud electric field (ΔE). The ΔE' s for individual discharges in eight Florida thunderstorms have been analyzed using a nonlinear, least-squares minimization procedure and point charge (Q) and point dipole (P) models of the change in cloud charge. The results indicate that the temporal and spatial behavior of the Q- and P- model parameters are similar to those reported previously by Koshak and Krider [1989]. In all storms, the high altitude P-vectors tend to point downward toward a narrow altitude band of Q-solutions that is centered at about 8 km; low altitude P-vectors tend to point upward toward the Q-region: and the P-vectors that are at the same altitude as the Q-solutions tend to be horizontal. Because there are inherent limitations in the above least-squares analysis method and models, a new, fundamentally different approach for analyzing lightning field changes has been developed. This method finds an optimum volume charge distribution on a grid of finite dimensions and resolution. with this linear approach, we now have the ability to describe complex field change patterns subject to a variety of external constraints. We also have a framework in which a standard eigenanalysis can be used to access the general information content of data and the effects of measurement errors. Tests of the linear method with simulated lightning sources show that a centroid of the lightning charge distribution can be retrieved to within the grid resolution (2 km) when a Landweber iterative algorithm is used. Tests on three natural lightning events show that there is good agreement with previous Q- and P- model solutions and a resonable result for one event that could not be described with either a Q- or a Pmodel.

The purpose of this dissertation is to further explore results obtained by Gall, Williams, and Clark (1988) in a series of numerical frontogenesis experiments. In the course of those simulations, gravity waves were produced which grew in intensity as the fronts (one at each of the two horizontal planes bounding the model domain) increased in strength. Waves formed ahead (upwind) of and directly above the surface front, but not behind it, which suggests a connection with the banded cloud patterns often seen ahead of cold fronts (Fig. 1). In addition, structures resembling standing waves were observed in the "wedge" between the frontal surface and the ground. The question of the asymmetrical distribution of waves relative to the surface front was investigated using the method of ray-tracing. The frontal fields which served as the mean state were obtained from the anelastic model developed by T. L. Clark at NCAR. Hypothetical sources of various frequencies were place in the vicinity of both frontal zones and the energy followed through the fluid. It was found that the energy density of those waves moving toward the center of the model domain (which includes waves moving behind the surface front) decreased with time. This decrease was due mainly to geometrical spreading of the rays and the loss of energy to the mean flow via the action of Reynolds stresses. By contrast, waves moving away from the center of the domain (including waves propagating ahead of the surface front) gained energy from the mean flow and appeared to experience a much smaller ray divergence. The single most important factor in producing this asymmetry was found to be vertical shear of the frontogenesis-induced cross-front circulation. The first attempts to analyze the waves underneath the surface front using linear theory produced a surprising result: If waves impinge upon a horizontal surface at a certain angle a reflected wave of infinite amplitude is produced. It is suggested that this may be a m

A ground-based near-infrared spectroradiometer was built and used to measure relative spectral reflectance from cumulus congestus and cumulonimbus clouds during the 1985 and 1986 Arizona summer monsoon seasons. Thermodynamic phase was inferred from spectral features in the regions between 1.55-1.75μm and 2.1-2.3μm where there are distinct differences between absorption in liquid water and ice and absorption by water vapor is very weak. Although liquid water and ice are nearly transparent in the visible, they absorb weakly in the near-infrared and that absorption is amplified by multiple scattering in clouds. Reflectance measurements are simple to make, requiring neither high spectral resolution nor absolute detector response. Three distinct aspects of differences between absorption in liquid water and ice were used to infer phase: (a) Ratio of the signal at 1.65 μm to that at 2.2 μm; (b) Wavelength of peak signal in the 1.65 μm water vapor transmission window; (c) Half-bandwidth of the 2.1-2.3 μm feature. Representative spectra are presented and analyzed on the basis of the predicted behavior of liquid water and ice cloud absorption. The results are consistent with young cumuli rapidly glaciating as they reach cooler levels, well before evidence of anvil formation or fibrous structure, contrary to the notion that phase can be inferred from visible cloud features.

The Arizona Monsoon Boundary is defined as the boundary separating two distinctly different air masses over Mexico, the southwestern United States, and the adjacent Pacific during the summer. The structure and dynamics of this boundary are examined by cross-sectional analysis using three different data sources: (1) a time-height cross section, constructed using radiosonde observations, at the time the boundary initially passed through Tucson in 1984; (2) a composite cross section through the boundary, constructed from the Fleet Numerical Oceanography Center analysis; and (3) a cross section through the boundary using high-resolution fields of temperature, moisture, and geopotential height obtained from the VISSR Atmospheric Sounder (VAS). All three cross sections showed similar structure. In some respects, the Arizona monsoon boundary resembles a mid-latitude front with a distinct and relatively sharp air mass change across the boundary, forced almost entirely by confluence. A direct ageostrophic circulation is produced by this forcing, giving weak ascent on the warm, moist side of the boundary. The gradients and flow associated with the composite boundary are weaker, by a factor of four, than those associated with strong mid-latitude fronts. However, the VAS cross section suggests that, at times, the strength of the boundary approaches that of middle-latitude fronts. The wind shear suggested by the composite boundary ought to be unstable to baroclinic or barotropic processes. Disturbances developing along the boundary have been observed. One example of such a disturbance is examined using GOES imagery, lightning strike data, cloud track winds, and VAS data. Satellite images show the disturbance resembling a mid-latitude occluded cyclone, with an apparent low pressure center over northern Baja California and front-like cloud features extending eastward and southward from the low. Lightning strike data show convective activity occurring along the front-like features.

Calculations were made of the back-scattering of 3.21-cm waves from water bubbles ranging in diameter, D, from 0.2 to 5.0 cm. Film thicknesses between 2 and 1000 microns were considered. As the diameter increases the back-scattering crosssection shows a gradual upward trend but sharp minima occur at intervals of D which are at integral values of 1/2 wave length. As the film thickness increases there is a general increase of the back-scattering cross-sections with the increase being most pronounced at small film thicknesses. Except at intervals of D close to 1/2 wave length, the back-scattering cross-sections of bubbles are higher than that from water spheres of the same mass. (Author)