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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Geosat crossover analysis in the tropical Pacific. II - Verification analysis of altimetric sea level maps with expendable bathythermograph and island sea level data

Altimetric sea level time series for the tropical Pacific have been generated from the Geosat crossover differences during the classified era of the Geosat mission from April 1985 to September 1986. These are compared with the expendable bathythermograph (XBT)-derived dynamic height and island sea level observations, yielding good agreement. A complex empirical orthogonal function analysis is also applied to the altimetric and XBT results. The analysis demonstrates the dominance of the annual signal in this relatively short time series and the westward propagation of major features away from the equator.

Tai, Chang-Kou↗
NASA NTRS1989

AVHRR for monitoring global tropical deforestation

Advanced Very High Resolution Radiometer (AVHRR) data have been used to assess the dynamics of forest trnsformations in three parts of the tropical belt. A large portion of the Amazon Basin has been systematically covered by Local Area Coverage (LAC) data in the 1985-1987 period. The analysis of the vegetation index and thermal data led to the identification and measurement of large areas of active deforestation. The Kalimantan/Borneo forest fires were monitored and their impact was evaluated using the Global Area Coverage (GAC) 4 km resolution data. Finally, High Resolution Picture Transmission (HRPT) data have provided preliminary information on current activities taking place at the boundary between the savanna and the forest in the Southern part of West Africa. The AVHRR approach is found to be a highly valuable means for carrying out deforestation assessments in regional and global perspectives.

Malingreau, J. P.↗
NASA NTRS1989

The mechanism for the scale selection of tropical intraseasonal oscillations. I - Selection of wavenumber 1 and the three-scale structure

The mechanism for the scale selection of tropical intraseasonal oscillations (ISO), was investigated using two models which differed only in the cumulus parameterization. In the first, more complicated model, moisture budgets are treated explicitly, and the cumulus parameterization of Kuo (1974) is used. Using this model, the ISO with the wavenumber 1 structure was successfully reproduced, and some possible nonlinear causes for the selection of wavenumber 1 were specified. The second model does not employ moisture; in this model, cumulus convection is related to low-level convergence only. The three possible causes obtained in the first experiment were parameterized, and could be added to or eliminated from the second model, making it possible to determine which nonlinear property was essential for the mechanism of the wavenumber selection.

Itoh, Hisanori↗
NASA NTRS1989

Tropical synoptic composites in VAS, OLR, and TOVS radiance data

The horizontal structure of specific tropical synoptic features is compared with that of a zonal ITCZ. Consideration is given to the two different appearances of the ITCZ: a weakly convective zonally symmetric state, and a locally synoptically-active convective state. Particular attention is given to the ways in which various satellite systems view these distinctive states.

Mcguirk, James P.↗
NASA NTRS1989

Tropical squall lines of the Arizona monsoon

Three cases of squall lines composed of strong to severe thunderstorms that formed over Arizona, and Sonora (Mexico) on July, 16-17 and 17-18, 1984, and August 2-3, 1986, are examined. Data, which included satellite imagery, VISSR-derived fields, surface data, and records or cloud-to-ground lightning strikes, indicate that the initiation, growth, and dissipation of all three squall lines were very similar. Results indicate that these mesoscale convective systems developed in an environment of relatively strong low-level shear with very weak shear aloft and that they possessed almost all the properties of a typical tropical squall line.

Smith, Walter P.↗
NASA NTRS1988

Relationships between sea surface temperature and outgoing longwave radiation over the tropical oceans

The spatial relationships between convection and SST at intraseasonal, seasonal, and interannual timescales over the tropical oceans are studied. The dependence of these local relationships on large-scale surface convergence/divergence is determined. The data used are day and night averaged NOAA outgoing longwave radiation (OLR) and Climate Analysis Center monthly SSTs based on in situ measurements. Below 28 C, the SST and OLR have an approximately linear relationship. Above 28 C, the relationship between the SST and convection may be much more complex.

Chan, Paul H.↗
NASA NTRS1989

Tropical Pacific moisture variability

Research objectives are to: (1) describe the synoptic scale variability of moisture over the tropical Pacific Ocean; (2) describe the systems leading to this variability; and (3) develop and implement satellite analysis procedures to facilitate (1) and (2) over the data sparse Pacific.

Mcguirk, James P.↗
NASA NTRS1989

Monthly surface thermal forcing in the tropical Pacific from 1980 to 1983

Monthly distributions of surface latent heat flux and solar irradiance in the tropical Pacific were computed from observations of the scanning multichannel microwave radiometer on Nimbus-7 and the visible infrared spin scan radiometer on GOES-W. They are the dominant variable components of the surface heat flux, the sum of which gives the approximate thermal forcing on the ocean. Monthly maps of this sum, from January 1980 to September 1983, and within 20 deg N and 20 deg S, 180 deg and 80 deg W, are presented.

Liu, W. Timothy↗
NASA NTRS1989

A day-to-day comparison study of Seasat scatterometer winds with winds observed from islands in the tropical Pacific

The winds derived from the Seasat-A Satellite Scatterometer (SASS) measurements have been the subject of great interest since the 1978 mission, because of the promise of radically improved wind observations over the world ocean. Due to the early end of the mission, only a few of the planned ground truth validation experiments could be made, and the subsequent lack of sufficient high quality independent wind data for comparison has limited the ability to resolve critical issues regarding the scatterometer's performance and the correct interpretation of its signal. Operational weather observations were made of ocean winds independent of Seasat mission plans during the Seasat mission period; the results are reported of a comparison study using such observations. Previous verification with in situ winds has been primarily in middle latitudes (GOASEX, JASIN, and NDBO buoys); winds observed from nine tropical Pacific islands are compared with nearly contemporaneous measurements taken by SASS during overpasses of the islands.

Davison, Jerry↗
NASA NTRS1989

The annual and interannual variabilities of precipitable water, surface wind speed, and sea surface temperature over the tropical Pacific

The Nimbus-7 Scanning Multichannel Microwave Radiometer (SSMR) provided simultaneous measurements of three geophysical parameters, each of which describing a certain aspect of the evolution of the 1982-1983 ENSO: the sea-surface temperature (T), precipitable water (W), and surface-wind speed (U). In this paper, values derived from the SSMR were compared with in situ measurements from ships, research buoys, and operational island stations in the tropical Pacific between January 1980 and October 1983, demonstrating the temporal and spatial coherence of the SSMR measurements. The results show that the variabilities of the surface convergence, sea surface temperature, and precipitable water are related. It was found that W anomalies were not always colocated with T anomalies, and that W anomalies were often associated with negative U anomalies, interpreted as surface convergence.

Liu, W. Timothy↗
NASA NTRS1989

The radiative budget of tropical clouds - A case study

Data collected during flight 9 of the Stratosphere-Troposphere Exchange Project (STEP), in Australia (1987), are used in an attempt to define the nature of one particular cirrus outflow region and its radiative impact. To explore the mechanisms by which air is exchanged between the stratosphere and troposphere, a series of instruments was flown on the NASA ER-2 high-altitude research aircraft to make measurements relevant to the radiative budget of tropical clouds. The data acquired included upwelling infrared radiance at 10.5 and 6.5 microns (spectral bandwidth of 1 micron); net infrared flux (spectral bandwidth of 3 to 40 microns); cloud particle size distribution; water-vapor and total water (vapor plus condensed phase); aerosol particle size distribution; and ambient temperature.

Ackerman, T. P.↗
NASA NTRS1989

Post World War II trends in tropical Pacific surface trades

Multidecadal time series of surface winds from central tropical Pacific islands are used to compute trends in the trade winds between the end of WWII and 1985. Over this period, averaged over the whole region, there is no statistically significant trend in speed or zonal or meridional wind (or pseudostress). However, there is some tendency, within a few degrees of the equator, toward weakening of the easterlies and increased meridional flow toward the equator. Anomalous conditions subsequent to the 1972-73 ENSO event make a considerable contribution to the long-term trends. The period 1974-80 has been noted previously to have been anomalous, and trends over that period are sharply greater than those over the longer records.

Harrison, D. E.↗
NASA NTRS1990

Surface winds from tropical Pacific islands - Climatological statistics

Variability space- and time-scales are presently investigated on the basis of multidecadal time series of surface wind observations from islands in the tropical Pacific. The frequency distribution of variance, derived from daily-averaged data, exhibits considerable geographical variation. Poleward of about 5 deg of latitude, the interannual variability in zonal wind diminishes sharply and the zonal and meridional wind variances become increasingly comparable. The zonal wind energy level in the 3- to 60-day band decreases with distance from the equator.

Harrison, D. E.↗
NASA NTRS1990

The wet tropospheric range correction - Product intercomparisons and the simulated effect for tropical Pacific altimeter retrievals

Four data sets used to correct satellite altimetry for the effects of tropospheric water vapor over the tropical Pacific Ocean are evaluated statistically, with a focus on the period 1979-1983. Satellite altimeter measurements are simulated by degrading model sea-level solutions with selected water-vapor data, and seasonal and interannual variations are investigated. The results are presented in extensive contour maps and characterized in detail. The standard deviation between two satellite-based data sets (Nimbus-7 SMMR and TOVS) was found to be 2-4 cm, while both the rms difference between seasonal cycles of the two sets and the difference between the mean fields were 1-2 cm. Larger differences (4-8 cm) were found between these data sets and those based on operational analyses (ECMWF and Fleet Numerical Oceanography Center). The maximum interannual anomaly in the water-vapor correction was 12 cm, for the eastern Pacific at the peak of the 1982-1983 ENSO event.

Zimbelman, Darrell F.↗
NASA NTRS1990

Comparison of Seasat scatterometer winds with tropical Pacific observations

The Seasat-A satellite scatterometer (SASS) observed near-surface vector winds over the world ocean from an 800-km orbit by measuring radar backscatter from the wind-roughened surface. To increase the geographical range of SASS and in situ comparison experiments, data from nine islands in the tropical Pacific and contemporaneous scatterometer winds were compared. The SASS-2 algorithm due to Wentz provides clearly better agreement in wind speed than the earlier SASS-1 algorithm. SASS-1 speeds tend to be higher than the island measurements by about 1 m/s, while daily mean SASS-2 minus island wind speed differences average -0.07 m/s. The rms differences between SASS-2 and the daily mean island data average 1.7 m/s, the SASS-1 rms differences average 2.2 m/s.

Davison, Jerry↗
NASA NTRS1990

Tropical precipitation rates during SOP-1, FGGE, estimated from heat and moisture budgets

Using the NASA Goddard Laboratory analyses collected during the first FGGE Special Observing Period, global estimates of precipitation rates were derived for the domain between 30-deg N to 30-deg S, and the results were compared to OLR patterns and to each other to evaluate their consistency and reliability. Regional averages are presented to examine the variability of rainfall rates among selected regions of the Southern Hemisphere tropics. Finally, precipitable water was computed and compared to results derived from SMMR estimates and to the precipitation patterns. Results show that the heat and moisture budget estimates of precipitation compare favorably. Vertical profiles reveal that maximum convective heating occurs in the middle troposphere. The profile of the South Pacific convergence zone region compares best with profiles obtained over the western North Pacific.

Pedigo, Catherine B.↗
NASA NTRS1989

Tropical synoptic composites in VAS, OLR and TOVS radiance data

The horizontal structure of specific tropical synoptic features are compared with the features of a zonal mean ITCZ using four different forms of satellite radiance data: daily gridded OLR data, GOES VAS water vapor imagery, brightness temperatures from TOVS water vapor channels (6.7 and 7.3 microns), and brightness temperature vectors for a complete set of TOVS channels. All of the data are composited over many events to minimize the effects of missing data, sampling errors, and system variability. The major differences between the observing systems are examined, focusing on resolution and clouds. It is found that the ITCZ exhibits a zonally symmetric convective phase and an enhanced regional convective phase (plumes), both of which can be differentiated from climatological mean behavior.

Mcguirk, James P.↗
NASA NTRS1990

Sources of variability in satellite-derived estimates of phytoplankton production in the eastern tropical Pacific

As a proxy for satellite CZCS observations and concurrent measurements of primary production rates, data from 138 stations occupied seasonally during 1967-1968 in the offshore eastern tropical Pacific were analyzed in terms of six temporal groups and our current regimes. Multiple linear regressions on column production Pt show that simulated satellite pigment is generally weakly correlated, but sometimes not correlated with Pt, and that incident irradiance, sea surface temperature, nitrate, transparency, and depths of mixed layer or nitracline assume little or no importance. After a proxy for the light-saturated chlorophyll-specific photosynthetic rate P(max) is added, the coefficient of determination ranges from 0.55 to 0.91 (median of 0.85) for the 10 cases. In stepwise multiple linear regressions the P(max) proxy is the best predictor for Pt.

Banse, Karl↗