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NASA NTRS1993

Recent advances in radar remote sensing of forest

On a global scale, forests represent most of the terrestrial standing biomass (80 to 90 percent). Thus, natural and anthropogenic change in forest covers can have major impacts not only on local ecosystems but also on global hydrologic, climatic, and biogeochemical cycles that involve exchange of energy, water, carbon, and other elements between the earth and atmosphere. Quantitative information on the state and dynamics of forest ecosystems and their interactions with the global cycles appear necessary to understand how the earth works as a natural system. The information required includes the lateral and vertical distribution of forest cover, the estimates of standing biomass (woody and foliar volume), the phenological and environmental variations and disturbances (clearcutting, fires, flood), and the longer term variations following deforestation (regeneration, successional stages). To this end, seasonal, annual, and decadal information is necessary in order to separate the long term effects in the global ecosystem from short term seasonal and interannual variations. Optical remote sensing has been used until now to study the forest cover at local, regional, and global scales. Radar remote sensing, which provides recent SAR data from space on a regular basis, represents an unique means of consistently monitoring different time scales, at all latitudes and in any atmospheric conditions. Also, SAR data have shown the potential to detect several forest parameters that cannot be inferred from optical data. The differences--and complementarity--lie in the penetration capabilities of SAR data and their sensitivity to dielectric and geometric properties of the canopy volume, whereas optical data are sensitive to the chemical composition of the external foliar layer of the vegetation canopy.

Letoan, Thuy↗

NASA NTRS1990

Microwave (SSM/I) Estimates of the Precipitation Rate to Improve Numerical Atmospheric Model Forecasts

Delay in the spin-up of precipitation early in numerical atmospheric forecasts is a deficiency correctable by diabatic initialization combined with diabatic forcing. For either to be effective requires some knowledge of the magnitude and vertical placement of the latent heating fields. Until recently the best source of cloud and rain water data was the remotely sensed vertical integrated precipitation rate or liquid water content. Vertical placement of the condensation remains unknown. Some information about the vertical distribution of the heating rates and precipitating liquid water and ice can be obtained from retrieval techniques that use a physical model of precipitating clouds to refine and improve the interpretation of the remotely sensed data. A description of this procedure and an examination of its 3-D liquid water products, along with improved modeling methods that enhance or speed-up storm development is discussed.

Raymond, William H.↗

NASA NTRS1990

Forecast model applications of retrieved three dimensional liquid water fields

Forecasts are made for tropical storm Emily using heating rates derived from the SSM/I physical retrievals described in chapters 2 and 3. Average values of the latent heating rates from the convective and stratiform cloud simulations, used in the physical retrieval, are obtained for individual 1.1 km thick vertical layers. Then, the layer-mean latent heating rates are regressed against the slant path-integrated liquid and ice precipitation water contents to determine the best fit two parameter regression coefficients for each layer. The regression formulae and retrieved precipitation water contents are utilized to infer the vertical distribution of heating rates for forecast model applications. In the forecast model, diabatic temperature contributions are calculated and used in a diabatic initialization, or in a diabatic initialization combined with a diabatic forcing procedure. Our forecasts show that the time needed to spin-up precipitation processes in tropical storm Emily is greatly accelerated through the application of the data.

Raymond, William H.↗

NASA NTRS1993

Orbit and sampling requirements: TRMM experience

The Tropical Rainfall Measuring Mission (TRMM) concept originated in 1984. Its overall goal is to produce datasets that can be used in the improvement of general circulation models. A primary objective is a multi-year data stream of monthly averages of rain rate over 500 km boxes over the tropical oceans. Vertical distributions of the hydrometers, related to latent heat profiles, and the diurnal cycle of rainrates are secondary products believed to be accessible. The mission is sponsored jointly by the U.S. and Japan. TRMM is an approved mission with launch set for 1997. There are many retrieval and ground truth issues still being studied for TRMM, but here we concentrate on sampling since it is the single largest term in the error budget. The TRMM orbit plane is inclined by 35 degrees to the equator, which leads to a precession of the visits to a given grid box through the local hours of the day, requiring three to six weeks to complete the diurnal cycle, depending on latitude. For sampling studies we can consider the swath width to be about 700 km.

North, Gerald↗

NASA NTRS1993

Atmospheric moisture fields derived by satellite observations over the tropical Pacific Ocean

Values of precipitable water are retrieved over the tropical and subtropical Pacific Ocean from TOVS infrared and microwave channel brightness temperature and OLR observations by means of stepwise linear regression. The most useful temperature and moisture sensing channels are pre-selected from sensitivity tests of a radiative transfer model. Numerous models are developed and tested against collocated radiosonde observations and Nimbus-7 SMMR precipitable water estimates. For RAOB comparisons, the best estimator used 15 TOVS predictors and captured 71.1 deg percent of the variance (+0.62 g/sq cm standard error) for column precipitable water; for precipitable water of 700-500 mb bulk layer, these values were 71.7 percent and +/- 0.17 g/sq cm. Little skill of estimated precipitable water was obtained for moisture above 500 mb. Regressions were less skillful against SMMR, unless collocation parameters were tightly controlled; SMMR was less acceptable than RAOB's because of observational drift and errors. Generally, the most skillful predictors were boundary layer brightness temperatures of TOVS channels and satellite estimated stability indices. 'Moisture channels' were hardly useful except for estimating middle and upper tropospheric moisture. Additional regression models were constructed testing the sensitivity to different observational and meteorological characteristics. Models which used some in situ observations surface observations or stabilities calculated from RAOB, were the most successful. The best of these explained 87.5 percent of the variance but the regression selected almost no TOVS channels, relying instead on conventional RAOB and surface observations. A set of four regression models were developed, stratifying atmospheric characteristics on the basis of collocated OLR values. These models improved the variance explained by 5.0 percent; the model associated with the highest OLR values (275 W/sq m less than or equal to OLR; that is, no cloud) showed only marginal skill. Precipitable water fields were generated from the best TOVS-only model for seven days in January 1983 and compared with SMMR-estimated fields. OLR fields and ECMWF precipitable water analysis. The TOVS regression model compared favorably to the SMMR analysis in amplitudes and features. It revealed more evolving synoptic signal than the ECMWF analysis. In synoptically active regions, it differed with respect to the OLR analysis, primarily because of actual differences in the vertical distribution of water vapor.

Chung, Hyosang↗

NASA NTRS1994

Specific features of space-time variations of ozone during the development of intensive tropical disturbances

An analysis of specific features of space-time variations of ozone in the tropical areas which has been performed on the basis of processing of the results of special expedition studies in the Atlantic and Pacific in 1987-1990 and the data of observations at the stations of the world ozonometric network over the 25-year period. The existence of a cause-and-effect relation has been revealed between the processes determining tropical cyclone (TC) development, and specific features of variations of the total content of ozone (TCO) and the vertical distribution of ozone (VDO) in the regions of TC action. Characteristic features of day-to-day and daily variations of TCO during TC development have been found. On the periphery of a developing TC, 1-4 days before it reaches the stage of storm, TCO increases, on average, by 5-8 percent, and a substantial increase in the concentration of ozone occurs in the middle and upper troposphere. The most probable physical mechanisms relating the observed specific features of ozone variations to TC evolution have been suggested. A hypothesis of the possibility of using ozone as an indicator for early prediction of TC development has been substantiated.

Nerushev, Alexander F.↗

NASA NTRS1994

Long-term observed ozone trends in the free troposphere and lower stratosphere

The vertical distributions of ozone trends in the free troposphere and lower stratosphere were derived from ozonesonde observations taken over an average period of approximately 20 years. The results for the annual trends show a consistent pattern of increased ozone of approximately 1 percent/yr to 2 percent/yr up to approximately 300 mb and decreased ozone of approximately -0.6 percent/yr from approximately 100 to 50 mb. Statistically significant positive trends found in midtroposphere (approximately 500 mb) at a set of representative stations in the Northern Hemisphere have little apparent seasonal variation. Negative trends are generally strongest at 50-70 mb with a tendency to be larger during spring. A highly significant negative trend of approximately -5 percent/yr is found near 100 mb over Syowa (69 deg S) during spring.

London, Julius↗

NASA NTRS1994

On ozone correlation with meteofields in the Northern Hemisphere

The correlation coefficients of temperature and geopotential heights at various levels with total ozone and its vertical distribution have been analyzed, using the ground based and ozone sounding data. Two independent groups of factors affect total ozone. The first group - the geopotential values of the troposphere - stratosphere border (100-500 mb) manifest themselves most of all in the middle latitudes. Pertaining to this group is the total ozone correlation with the tropopause height and temperature at 500 mb. The correlation coefficients are negative (-0.55 -0.65) and little depend on the season. Related to this factor is a high (up to 0.8) correlation of ozone partial pressure with the temperature in the lower stratosphere. The second group is the geopotential and temperature values at the 10-30 mb coefficients (up to 0.6) are observed in winter in the subpolar latitudes. In summer they are substantially lower - about 0.1.

Kadygrova, Tatiana V.↗

NASA NTRS1994

Climate-chemical interactions and greenhouse effects of trace gases

A completely coupled one-dimensional radiative-convective (RC) and photochemical-diffusion (PC) model has been developed recently and used to study the climate-chemical interactions. The importance of radiative-chemical interactions within the troposphere and stratosphere has been examined in some detail. We find that increases of radiatively and/or chemically active trace gases such as CO2, CH4 and N2O have both the direct effects and the indirect effects on climate change by changing the atmospheric O3 profile through their interaction with chemical processes in the atmosphere. It is also found that the climatic effect of ozone depends strongly on its vertical distribution throughout the troposphere and stratosphere, as well on its column amount in the atmosphere.

Shi, Guang-Yu↗

NASA NTRS1994

Column amounts of trace gases from ground based FTIR measurements in the late north polar winters 1990 and 1991

Two FTIR spectrometers were employed in the late winters 1990 and 1991 in Esrange, North Sweden, and in Ny Aalesund, Spitsbergen to detect zenith column amounts of several trace gases. Time series of column amounts of the trace gases O3, N2O, CH4, HNO3, NO2, CHl, and HF have been derived from the measured spectra. Additionally, some information on the vertical distribution of HCl could be obtained by analyzing the spectral line shapes. The results are interpreted in terms of dynamical and chemical processes.

Adrian, Gabriele↗

NASA NTRS1994

Measurements of the vertical profile, diurnal variation, and secular change of ClO in the stratosphere over Thule, Greenland, February-March, 1992

We report observations of stratospheric chlorine monoxide over the altitude range approx. 16 to 50 km at Thule, Greenland from Feb. 8 to Mar. 24, 1992. A new, more sensitive ground-based mm-wave spectrometer was employed for these measurements, similar in principle to that used earlier for the discovery of low altitude ClO in the Antarctic springtime. In this report, we discuss different aspects of vertical distribution, secular trends, and diurnal variation of ClO in the Arctic stratosphere, based on a preliminary analysis of our Thule data. We see no evidence for large (approx. 1.2-1.5 ppb) amounts of ClO in the lower stratosphere at any time during February or March, in agreement with UARS-MLS findings for this period, and in marked contrast to findings reported for the Arctic in January. We have some evidence for small enhancements (approx. 0.2-0.5 ppb) in the 18-30 km range in late February-early March, which might be associated with volcanic aerosol, rather than PSC, processing.

Dezafra, Robert L.↗

NASA NTRS1994

External comparisons of reprocessed SBUV/TOMS ozone data

Ozone Retrievals from the Solar Backscatter Ultraviolet (SBUV) Instrument on-board the Nimbus-7 Satellite have been reprocessed using an improved internal calibration. The resulting data set covering November, 1978 through January, 1987 has been archived at the National Space Science Data Center in Greenbelt, Maryland. The reprocessed SBUV total ozone data as well as recalibrated Total Ozone Mapping Spectrometer (TOMS) data are compared with total ozone measurements from a network of ground based Dobson spectrophotometers. The SBUV also measures the vertical distribution of ozone, and these measurements are compared with external measurements made by SAGE II, Umkehr, and Ozonesondes. Special attention is paid to long-term changes in ozone bias.

Wellemeyer, C. G.↗

NASA NTRS1994

The Tropical Rainfall Measuring Mission (TRMM)

The Tropical Rainfall Measuring Mission (TRMM) is a cooperative effort between National Space Development Agency (NASDA) of Japan and NASA of the USA. TRMM is designed to measure rain rates from space using a combination of high resolution radar, passive microwave radiometer and visible-infrared radiometer measurements from a spacecraft in a rapid precession, 350 km orbit inclined at 35 deg. These measurements, averaged over a 500 km grid for a month, are expected to provide monthly mean rainfall to an accuracy of 10 to 15 percent. TRMM will also make it possible to analyze the diurnal component of the rainfall and to derive the vertical distribution of latent heating. An accurate measure of the oceanic rainfall distribution will provide unprecedented insight into the halocline distribution and the stability of the oceanic surface layers. A status of the program is presented.

Theon, John S.↗

NASA NTRS1994

Passive microwave remote sensing of rainfall with SSM/I: Algorithm development and implementation

A physically based algorithm sensitive to emission and scattering is used to estimate rainfall using the Special Sensor Microwave/Imager (SSM/I). The algorithm is derived from radiative transfer calculations through an atmospheric cloud model specifying vertical distributions of ice and liquid hydrometeors as a function of rain rate. The algorithm is structured in two parts: SSM/I brightness temperatures are screened to detect rainfall and are then used in rain-rate calculation. The screening process distinguishes between nonraining background conditions and emission and scattering associated with hydrometeors. Thermometric temperature and polarization thresholds determined from the radiative transfer calculations are used to detect rain, whereas the rain-rate calculation is based on a linear function fit to a linear combination of channels. Separate calculations for ocean and land account for different background conditions. The rain-rate calculation is constructed to respond to both emission and scattering, reduce extraneous atmospheric and surface effects, and to correct for beam filling. The resulting SSM/I rain-rate estimates are compared to three precipitation radars as well as to a dynamically simulated rainfall event. Global estimates from the SSM/I algorithm are also compared to continental and shipboard measurements over a 4-month period. The algorithm is found to accurately describe both localized instantaneous rainfall events and global monthly patterns over both land and ovean. Over land the 4-month mean difference between SSM/I and the Global Precipitation Climatology Center continental rain gauge database is less than 10%. Over the ocean, the mean difference between SSM/I and the Legates and Willmott global shipboard rain gauge climatology is less than 20%.

Ferriday, James G.↗

NASA NTRS1995

The fate of atmospheric phosgene and the stratospheric chlorine loadings of its parent compounds: CCl4, C2Cl4, C2HCL3, CH3CCl3, and CHCl3

A study of the tropospheric and stratospheric cycles of phosgene is carried out to determine its fate and ultimate role in controlling the ozone depletion potentials of its parent compounds. Tropospheric phosgene is produced from the OH-initiated oxidation of C2Cl4, CH3CCl3, CHCl3, and C2HCl3. Simulations using a two-dimensional model indicate that these processes produce about 90 pptv/yr of tropospheric phosgene with an average concentration of about 18 pptv, in reasonable agreement with observations. We estimate a residence time of about 70 days for tropospheric phosgene, with the vast majority being removed by hydrolysis in cloudwater. Only about 0.4% of the phosgene produced in the troposphere avoids wet removal and is transported to the stratosphere, where its chlorine can be released to participate in the catalytic destruction of ozone. Stratospheric phosgene is produced from the photochemical degradation of CCl4, C2Cl4, CHCl3, and CH3CCl3 and is removed by photolysis and downward transport to the troposphere. Model calculations, in good agreement with observations, indicate that these processes produce a peak stratospheric concentration of about 25-30 pptv at an altitude of about 25 km. In contrast to tropospheric phosgene, stratospheric phosgene is found to have a lifetime against photochemical removal of the order of years. As a result, a significant portion of the phosgene that is produced in the stratosphere is ultimately returned to the troposphere, where it is rapidly removed by clouds. This phenomenon effectively decreases the amount of reactive chlorine injected into the stratosphere and available for ozone depletion from phosgene's parent compounds. A similar phenomenon due to the downward transport of stratospheric COFCl produced from CFC-11 is estimated to cause a 7% decrease in the amount of reactive chlorine injected into the stratosphere from this compound. Our results are potentially sensitive to a variety of parameters, most notably the rate of reaction of phosgene with sulfate aerosols. However, on the basis of the observed vertical distribution of COCl2, we estimate that the reaction of COCl2 with sulfate aerosol most likely has a gamma less than 5 x 10(exp -5) and, as a result, has a negligible impact on the stratospheric chlorine loadings of the phosgene parent compounds.

Kindler, T. P.↗

NASA NTRS1995

Comparison of cloud fields from atmospheric general circulation model, in situ and satellite measurements

This paper focuses on the comparison of cloud amounts derived from an atmospheric general circulation model (AGCM), Satellite-observed clouds, and Ground-based cloud observations. Unlike Earth Radiation Budget Experiment (ERBE)-type comparisons it does not mix potential errors in the cloud amount with those in the radiation code embedded in the model. Long term cloud climatologies were used to compare global cloud amounts and regional seasonal cycles. The AGCM successfully reproduced the signatures of the warm pool and North Pacific seasonal cycle cloudiness but failed in the low stratus region off the coast of South America, a known problem for AGCMs. The data sets also reproduced the anomaly signature associated with El Nino in the warm pool region, but the model amounts were lower. Global results had a similar success rate, with the model generally producing lower total cloud compared to the satellite and in situ measurements. To compare cloud vertical distributions the cloud height may need to be validated using the corresponding radiation fields. Unfortunately there were also some large discrepancies between the two observed cloud data sets. While tremendously improved over the last decade the character of the observed cloud data sets, must be substantially enhanced before they will be useful in validating AGCMs by any but the crudest levels of comparison.

Haskins, Robert D.↗

NASA NTRS1995

A method for estimating vertical distibution of the SAGE II opaque cloud frequency

A method is developed to infer the vertical distribution of the occurrence frequency of clouds that are opaque to the Stratospheric Aerosol and Gas Experiment (SAGE) II instrument. An application of the method to the 1986 SAGE II observations is included in this paper. The 1986 SAGE II results are compared with the 1952-1981 cloud climatology of Warren et al. (1986, 1988)

Wang, Pi-Huan↗

NASA NTRS1994

Windblown sand on Venus: The effect of high atmospheric density

The high density of the venusian atmosphere significantly affects aeolian saltation transport and may also allow rapid alterations in the radar backscatter cross-section of the surface. Saltation flux experiments were conducted under terrestrial and simulated venusian conditions to assess the effect of atmospheric density on aeolian transport of sediment. Total lane flux, the vertical distribution of saltating particles, and the average horizontal component of particle speed were measured and used to estimate the volumetric concentration of particles above the surface. Results show that so many particles are set into motion when wind speeds exceed a critical value that mid-air collisions are a common result, a condition termed 'choked' saltation. When choking occurs, there is a reduction in the rate at which the total saltation flux increases with increasing wind speed at least in part due to a reduction in the mean horizontal speed of the particles. Choked saltation on Venus may result in a reduction of the local aeolian erosion rate, the obliteration of aeolian bedforms, and rapid alteration of surface radar backscatter characteristics.

Williams, Steven H.↗