Stephen Sylvain Leroy

A Technical Description of Atmospheric Sounding by GPS Occultation

Abstract In recent years, the global positioning system (GPS) has been exploited via radio occultation techniques to obtain profiles of refractivity, temperature, pressure and water vapor in the neutral atmosphere and electron density in the ionosphere. The GPS/MET experiment, which placed a GPS receiver in a low-Earth orbit, provided a wealth of data which was used to test this concept and the accuracy of the retrievals. Several investigations have already demonstrated that the retrieval accuracies obtained with GPS/MET is already comparable, if not better, than the more traditional atmospheric sensing techniques (e.g., radiosondes). Even though the concept of atmospheric profiling via radio occultation is quite a simple one, care must be taken to separate the numerous factors that can affect the occulted signal. These include the motion of the satellites, clock drifts, relativistic effects, the separation of the ionosphere and the neutral atmosphere, and the contribution of the upper atmosphere where sensitivity of the GPS signal is weak. In addition, care must be taken to use proper boundary conditions, use proper smoothing intervals and interpolation schemes to avoid retrieving artificial atmospheric structures, and most importantly detect and correct phase measurement errors introduced by sharp refractivity gradients in the atmosphere. This work describes in some detail the several steps involved in processing such data. In particular, it describes a system that was developed at the Jet Propulsion Laboratory and used to process the GPS/MET data. Several examples of retrieved refractivity, temperature and water vapor profiles are shown and compared to analyses from the European Center for Medium-range Weather Forecast (ECMWF). Statistical comparisons of GPS/MET and ECMWF temperatures for data collected during June 21–July 4, 1995, indicate that differences are of order 1– 2 K at northern latitudes where the ECMWF analyses are most accurate.

Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System

Recent radio occultation measurements using Global Positioning System satellite transmitters and an orbiting receiver have provided a globally distributed set of high-resolution atmospheric profiles, suggesting that the technique may make a significant contribution to global change and weather prediction programs. Biases in occultation temperatures relative to radiosonde and model data are about 1 kelvin or less in the tropics and are generally less than 0.5 kelvin at higher latitudes. Data quality is sufficient to quantify significant model errors in remote regions. Temperature profiles also reveal either an equatorial Rossby-gravity or an inertio-gravity wave. Such waves provide a fundamental source of momentum for the stratospheric circulation.

Temperature and circulation in the stratosphere of the outer planets

Abstract A zonally symmetric, linear radiative-dynamical model is compared with observations of the upper tropospheres and stratospheres of the outer planets. Seasonal variation is included in the model. Friction is parameterized by linear drag (Rayleigh friction). Gas opacities are accounted for but aerosols are omitted. Horizontal temperature gradients are small on all the planets. Seasonal effects are strongest on Saturn and Neptune but are weak even in these cases, because the latitudinal gradient of radiative heating is weak Seasonal effects on Uranus are extremely weak because the radiative time constant is longer than the orbital period. The one free parameter in the model is the frictional time constant. Within the context of this simple model, comparison with observed temperature perturbations over zonal currents in the troposphere shows that the frictional time constant is on the same order as the radiative time constant for all these planets. Finally, vertical motions predicted by the model are extremely weak. They are much smaller than one scale height per orbital period, except in the immediate neighborhood of tropospheric zonal currents.

Measurement of geopotential heights by GPS radio occultation

Geopotential heights of constant pressure surfaces are retrieved from global positioning system (GPS) radio occultation data. In order to assess accuracy a subset of data obtained by GPS/MET during spring 1995 and summer 1995 are compared to the output of the European Centre for Medium-Range Weather Forecasts (ECMWF) global model. The root-mean-square measurement error is 20 m throughout the upper troposphere and lower stratosphere. Furthermore, the ECMWF global model contains enhanced errors in the southeast Pacific. In probing the data for potential utility in climate studies, a Bayesian interpolation technique is used to map the geopotential height fields in the upper troposphere during the summer. Despite limitations of the GPS/MET data set the global average 300-mbar geopotential height over a 2-week period in summer 1995 is determined with an accuracy of 7 m. By obtaining greater coverage and partially resolving synoptic variability, a future constellation of 16 orbiting receivers could obtain global average geopotential height estimates in the upper troposphere with an accuracy of l m each day. Accuracy would be somewhat worse for regional studies, except in the tropics where synoptic variability is depressed.

Zonal motion and structure in Jupiter's upper troposphere from voyager infrared and imaging observations

Abstract Voyager infrared data are used to produce global cylindrical projection digital maps of temperature at the 270- and 150-mb pressure levels, infrared cloud optical depths at 45- and 5-μm wavelength, and the ammonia abundance near the 680-mb level. Voyager imaging data are used to produce maps of orange reflectivity and violet reflectivity (effective wavelengths of 0.58 and 0.41 μm, respectively). The zonal motions of the dominant structures in each of the quantities are evaluated using cross-correlation of the maps. Structures in the maps of cloud optical depths, ammonia abundance, and visible reflectivities show the same zonal motion at almost all latitudes. The dominant upper tropospheric thermal structures move at a rate which is far different than the cloud indicators, and they remain stationary with respect to the bulk rotation of the planet. Comparison of these motions with the zonally averaged thermal wind shear inthe upper troposphere indicates that the thermal structures move at a rate far different than the local fluid velocity. We present a simple theory which suggests that these slowly moving thermal structures could represent the influence of a disturbance seated in a slowly moving region deep in Jupiters atmospheres, although this explanation is not unique. Intercomparison of the motions and longitudinal alignment of the various cloud indicators with the thermal wind shear in the upper troposphere shows that the few latitudes at which differences exist correlate strikingly with latitudes of largest shear, suggesting that the observed differences may reflect height differences in observed cloud features. Fourier analysis of the maps of temperature at 270-mb show a robust stationary feature at a zonal wavenumber of 9 near 15° N latitude. The maps of cloud optical depth at 45 μm show a feature at zonal wavenumber 11 near 20° N. These periodicities may be related to similar structures recently detected in groundbased observations, and their longevity and robustness may indicate preferred wavemodes of the atmosphere.

Validating the microwave sounding unit stratospheric record using GPS occultation

[1]xa0We validate the temperature climatology recorded by the Microwave Sounding Unit with GPS occultation data collected by the GPS/MET experiment. We choose to validate only the lower stratospheric MSU climatology in order to circumvent the wet-dry ambiguity associated with GPS occultation in the mid- to lower troposphere. We simulate the lower stratospheric channels brightness temperature by convolving each GPS/MET temperature profile with a vertical weighting function and then map the irregularly gridded data using a Bayesian interpolation scheme. In northern polar night, the MSU Tls deviates from GPS occultation by as much as 10 K while occultation is consistent with the NCEP Reanalysis used for diagnostic purposes. NCEP and GPS occultation deviate by 1 K in the tropics, consistent with a warm bias in NCEP Reanalysis. GPS occultation renders the problems MSU encounters with inter-satellite calibration obsolete, because calibration by atomic clocks is free of systematic error and can completely cover the diurnal cycle.

Cloud variability as revealed in outgoing infrared spectra: Comparing model to observation with spectral EOF analysis

[1] Spectrally resolved outgoing radiance is a potentially powerful tool for testing climate models. To show how it can be used to evaluate the simulation of cloud variability, which is the principal uncertainty in current climate models, we apply spectral empirical orthogonal function (EOF) analysis to satellite radiance spectra and synthetic spectra derived from a general circulation model (GCM). We show that proper averaging over a correct timescale is necessary before applying spectral EOF analysis. This study focuses on the Central Pacific and the western Pacific Warm Pool. For both observation and GCM output, cloud variability is the dominant contributor to the first principal component that accounts for more than 95% of the total variance. However, the amplitude of the first principal component derived from the observations (2 � 3.4 Wm � 2 )i s 2� 6 times greater than that of the GCM simulation. This suggests that cloud variability in the GCM is significantly smaller than that in the real atmosphere. INDEX TERMS: 3359 Meterology and Atmospheric Dynamics: Radiative processes; 3360 Meterology and Atmospheric Dynamics: Remote sensing; 3337 Meterology and Atmospheric Dynamics: Numerical modeling and data assimilation; 3399 Meterology and Atmospheric Dynamics: General or miscellaneous

Convective Generation of Gravity Waves in Venus's Atmosphere: Gravity Wave Spectrum and Momentum Transport

The emission of internal gravity waves from a layer of dry convection embedded within a stable atmosphere with static stability and zonal winds varying in height is calculated. This theory is applied to Venus to investigate whether these waves can help support the westward maximum of angular momentum of Venuss middle atmosphere. The emission mechanism is similar to that suggested for driving the gravity modes of the Sun and relates the amplitude and spectrum of the waves to the amplitude and spectrum of the convection. Waves are damped by several mechanisms: wavebreaking in the stable atmosphere, critical layer absorption, reabsorption by the convection, and wave radiation to space. The authors use plane parallel geometry without rotation and assume sinusoidal wave fluctuations in the horizontal dimensions. The vertical dependence is determined using the WKBJ approximation. n nIt is found that convectively generated gravity waves do not exert an acceleration where the westward winds are greatest. Instead, they deposit westward momentum in a 1-km thick layer just above the convection. Other waves deposit eastward momentum far above the westward wind maximum where decelerations can exceed 20 m s^−1 day^−1, comparable to deceleration amplitudes in Earths mesosphere. Although the momentum fluxes by gravity waves are substantial, the vertical profile of acceleration does not match what is required for supporting Venuss atmospheric superrotation.

Initial Results of GPS-LEO Occultation Measurements of Earth’s Atmosphere Obtained with the GPS-MET Experiment

The radio occultation technique, which has been repeatedly proven for planetary atmospheres, was first utilized to observe Earth’s atmosphere by the GPS-MET experiment (launched in April 1995), in which a high performance GPS receiver was placed into a low-Earth orbit. During certain phases of the mission, more than 100 occultations per day are acquired. A subset of this occultation data is analyzed and temperature in the neutral atmosphere and electron profiles in the ionosphere are obtained. Comparing about 100 GPS-MET retrievals to accurate meteorological analyses obtained from the European Center for Medium-range Weather Forecasting at heights between 5–30 km, temperature differences display biases of less than 0.5K and standard deviations of 1–2K in the northern hemisphere, where the model is expected to be most accurate. Furthermore, electron density profiles obtained for different geodetic locations and times show the main features that are expected in the ionosphere.

Waves, Advection, and Cloud Patterns on Venus

Abstract The stable layers adjacent to the nearly neutral layer within the Venus clouds are found to be capable of supporting vertically trapped, horizontally propagating waves with horizontal wavelengths of about 10 km and speeds of a few meters per second relative to the mean wind in the neutral layer. These waves may possibly be excited by turbulence within the neutral layer. We examine the properties of the waves, and the patterns which they might produce within the visible clouds if excited near the subsolar point. The patterns can be in agreement with many features in images. The waves are capable of transferring momentum latitudinally to help maintain the general atmospheric spin, but at present we are not able to evaluate wave amplitudes. We also examine an alternative possibility that the cloud patterns are produced by advection and shearing by the mean zonal and meridional flow of blobs formed near the equator. We conclude that advection and shearing by the mean flow is the most likely explanati...