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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Precipitation rates in the tropics based on the Q1-budget method - 1 June 1984-31 May 1987

The 'apparent' heat source method (Q1 budget) is used to compute the total derivative of dry static energy from 30 deg N to 30 deg S for the period June 1, 1984-May 31, 1987. The dataset is produced from the ECMWF global analyses and consists of twice-daily values of temperature, geopotential height, horizontal wind components, and vertical velocity at increments of 2.5 x 2.5 deg lat/long at seven pressure levels. Vertically integrated values of ds/dt, which are equal to total diabatic heating, Q1, are combined with estimates of net columnar radiation and surface sensible heat exchange to compute mean monthly precipitation rates, P0, as the residual in the Q1 budget. The accuracy of these P0 values is thoroughly examined, and it is suggested that the technique produces reliable estimates of precipitation over tropical oceanic areas on a monthly basis. Time series of mean monthly P0 for several geographic regions of the Southern Hemisphere tropics and the equatorial western Pacific (TOGA-COARE region) reveal that (1) the South Pacific convergence zone has the highest precipitation rates in the Southern Hemisphere; (2) a clear and distinct seasonal cycle is prominent in all regions; and (3) the 1986-87 ENSO event is easily identified, particularly in the TOGA-COARE region.

Vincent, Dayton G.↗
NASA NTRS1992

Mesoscale air-sea interactions related to tropical and extratropical storms in the Gulf of Mexico

Observations of the lower atmosphere of the northwestern Gulf of Mexico from November 1982 to mid-February 1983 were studied in which seven significant cyclones were generated in the northwestern gulf. It was found that all seven storms occurred when the vorticity correlate of the horizontal air temperature difference was about 3-5 C above the climatological mean difference. It is shown that a maximum in the frequency of tropical storms within the Gulf of Mexico exists some 275 km south of the Mississippi delta at 27 deg N, 90 deg W. This maximum is a result of only those storms which originate within the gulf. Two plausible effects of the Loop Current and its rings on tropical storms are discussed. One is that these ocean features are large and consolidated heat and moisture sources from which a nearby slowly moving atmospheric disturbance can extract energy. The second is that of the cyclonic vorticity that can be generated in the lower atmosphere by such oceanographic features.

Lewis, James K.↗
NASA NTRS1992

Tropical rainfall measuring mission - US/Japan science operations

The Tropical Rainfall Measuring Mission (TRMM) is a joint-venture between the National Aeronautics and Space Administration (NASA) and the National Space Development Agency (NASDA) of Japan. This cooperative program provides the resources required to place in a low-earth orbit a 3500 kilogram (kg) satellite. The satellite carries a complement of instruments designed to measure the tropical and subtropical rainfall as required to provide data products which will lead to climate modeling. This paper describes the mission, the spacecraft and instrument complement, and the data processing, distribution, and archiving system. Included in the paper is the possible, but not approved, an experimental data relay link using a NASDA data relay satellite.

Keating, Thomas↗
NASA NTRS1990

An evaluation of the structure of tropical intraseasonal oscillations in three general circulation models

The attributes for the simulation of tropical intraseasonal oscillations in the 30 to 60 day range by three GCMs are compared with analyses of the European Center for Medium-Range Weather Forecasts. Models under consideration include the Goddard Laboratory of Atmospheric sciences (GLAS), Goddard Laboratory for Atmospheres (GLA), and UCLA GCMs. They are characterized by the same resolutions of four degrees latitude by five degrees longitude with 9 levels. Observations reveal that the eastward traveling planetary scale structure becomes more stationary over the Indonesian region and accelerates over the Central Pacific. In the tropics a Kelvin wave-type structure is found to be dominant near the center of the oscillation. The simulated winds are realistic although the meridional component is too strong, especially in the GLA GCM. The differences in the structure of the oscillation in the GLAS GCM and GLA GCM are considered to be a consequence of the different numerical schemes used. The GCMs are characterized by preferred zones for diabatic heating, with a turn-on heating occurring when the rising branch of the intraseasonal oscillation passes over these convective regions.

Park, Chung-Kyu↗
NASA NTRS1991

The tropical rainfall measuring mission (TRMM) and its role in studies of climate variations

Adequate measurement of rainfall over the global tropics can only be made from space, using an inclined low orbit and a combination of sensors. A relatively low-budget earth probe satellite called the tropical rainfall measuring mission (TRMM) is described. It is an approved joint project between the United States and Japan, with an intended launch by the Japanese in 1996. It will fly at an orbit elevation of 350 km and an inclination of 35 deg. Radiation instruments were added to the payload in late 1990. The TRMM rain package, its instruments, expected products, and some results of prelaunch research on the orbit sampling, rain retrievals, and ground validation procedures are discussed.

Simpson, Joanne↗
NASA NTRS1992

Impact of cumulus initialization on the spinup of precipitation forecasts in the tropics

To ameliorate the precipitation spinup problem, i.e., the prediction model's inability to produce realistic precipitation rates at the beginning of the forecast period, the impact of a tropical initialization procedure on precipitation forecasts is examined. Attention is focused on the examination of the time behavior of precipitation rates and the closely associated variables of horizontal divergence and moisture during the first 42 time steps corresponding to 10.5 h. It is determined that the originally analyzed tropical divergence fields have realistic geographical distributions but there intensities seem to be too low by a factor of two.

Kasahara, Akira↗
NASA NTRS1992

The Tropical Rainfall Measuring Mission and the development of its observatory

The paper describes a cooperative program between NASA and NASDA, the Tropical Rainfall Measuring Mission (TRMM), which is an earth system science mission designed to study characteristics of the rainfall occurring over the tropics and subtropics. The salient features of the TRMM are described, with special attention given to the mission parameters, the instrument complement, the TRMM observatory, and the command and data handling subsystem. Also discussed are the TRMM requirement factors and the resulting approach to the TRMM observatory design.

Lavigna, Thomas A.↗
NASA NTRS1993

Tropical Pacific moisture variability

The research objectives are the following: (1) to describe synoptic scale variability of moisture over the tropical Pacific Ocean and the systems leading to this variability; (2) to implement satellite analysis procedures to accomplish (1); (3) to incorporate additional satellite information into operational analysis/forecast systems at NMC; and (4) to synthesize knowledge gained from satellite observations through diagnosis and numerical models. Significant accomplishments in FY 91/92 are presented and include the following: (1) satellite forecast applications; (2) satellite data analysis; and (3) tropical plume mechanisms.

Mcguirk, James P.↗
NASA NTRS1992

Buffering of stratospheric circulation by changing amounts of tropical ozone - A Pinatubo case study

Stratospheric aerosol from Mount Pinatubo heated the tropical lower stratosphere by about 0.3 K/day mainly due to absorption of terrestrial infrared radiation. This heating was dissipated by: (1) an observed increase in stratospheric temperatures, which enhanced the radiation cooling; (2) additional mean upward motion, observed for the aerosol cloud, which led to adiabatic cooling; and (3) reductions in ozone concentrations resulting from enhanced upward motions. Each of these processes operated on a different time scale: maximum temperatures were observed after about 90 days; maximum ozone losses of about -1.5 ppm occurred after 140 days when the enhanced vertical velocities effectively lifted the ozone profile by about 2 km. We believe this shows that ozone plays an important role in buffering vertical motion in the tropical lower stratosphere, and hence the residual Brewer Dobson circulation of the whole stratosphere.

Kinne, S.↗
NASA NTRS1993

C-band attenuation by tropical rainfall in Darwin, Australia, using climatologically tuned Z(e)-R relations

The probability matching method (PMM) is used as a basis for estimating attenuation in tropical rains near Darwin, Australia. PMM provides a climatological relationship between measured radar reflectivity and rain rate, which includes the effects of rain and cloud attenuation. When the radar sample is representative, PMM estimates the rainfall without bias. When the data are stratified for greater than average rates, the method no longer compensates for the higher attenuation and the radar rainfall estimates are biased low. The uncompensated attenuation is used to estimate the climatological attenuation coefficient. The two-way attenuation coefficient was found to be 0.0085 dB/km ( mm/h) exp -1.08 for the tropical rains and associated clouds in Darwin for the first two months of the year for horizontally polarized radiation at 5.63 GHz. This unusually large value is discussed. The risks of making real-time corrections for attenuation are also treated.

Atlas, David↗
NASA NTRS1993

Heating, moisture, and water budgets of tropical and midlatitude squall lines - Comparisons and sensitivity to longwave radiation

A 2D time-dependent and nonhydrostatic numerical cloud model is presently used to estimate the heating, moisture, and water budgets in the convective and stratiform regions for both a tropical and a midlatitude squall line. The model encompasses a parameterized, three-class ice phase microphysical scheme and longwave radiative transfer process. It is noted that the convective region plays an important role in the generation of stratiform rainfall for both cases. While a midlevel minimum in the moisture profile for the tropical case is due to vertical eddy transport in the convective region, the contribution to the heating budget by the cloud-scale fluxes is minor; by contrast, the vertical eddy heat-flux is relatively important for the midlatitude case due to the stronger vertical velocities present in the convective cells.

Tao, W.-K.↗
NASA NTRS1993

Comparison of special sensor microwave imager vector wind stress with model-derived and subjective products for the tropical Pacific

An assessment of a new source of vector wind stress data relative to existing analyses of the surface wind field is presented. The large-scale variability of vector wind stress generated by Atlas et al. (1991) and based on the special sensor microwave imager (SSM/I) remotely sensed observations of surface wind speed is compared with five operational and subjectively analyzed wind products across the tropical Pacific basin for the first year of SSM/I, July 1987 through June 1988. The spatial and temporal variability of the zonal component, meridional component, and curl of the wind stress are examined relative to their future use in wind-driven ocean model studies of tropical Pacific Ocean circulation.

Busalacchi, Antonio J.↗
NASA NTRS1993

Impact of increased TOVS signal on the NMC global spectral model - A tropical-plume case study

Temperature and moisture signals are extracted from operationally available TOVS channel radiances for a tropical plume that developed over the North Pacific in January 1989, and are inserted into the NMC's operational medium-range forecast model. Several 48-h forecasts are made to test the impact of the additional information. It is shown, at least within a single tropical plume event, that there is still unutilized information within operational TOVS satellite observations. This information resides in the moisture channels and within small-amplitude, but horiontally coherent, thermal features. Perturbed upper-tropospheric relative humidity patterns are smoother and more consistent with satellite imagery than is the historical forecast.

Mcguirk, James P.↗
NASA NTRS1993

Intraseasonal interactions between the tropics and extratropics in the Southern Hemisphere

Observational evidence is presented of connections between tropical heating and circulation changes in subtropical and extratropical latitudes of the Southern Hemisphere (SH) in summer and winter, and the processes responsible for these interactions are described. The observational evidence suggests that, during SH summer (but not SH winter), both the acceleration of westerly winds south of the heating region through meridional overturnings and excitation of Rossby waves are possible. During winter, the enhanced tropical convective activity results again in local meridional overturning with zonal wind acceleration of Rossby waves farther south.

Berbery, Ernesto H.↗
NASA NTRS1993

Tropical deforestation and habitat fragmentation in the Amazon - Satellite data from 1978 to 1988

Landsat satellite imagery covering the entire forested portion of the Brazilian Amazon Basin was used to measure, for 1978 and 1988, deforestation, fragmented forest, defined as areas less than 100 square kilometers surrounded by deforestation, and edge effects of 1 kilometer into forest from adjacent areas of deforestation. Tropical deforestation increased from 78,000 square kilometers in 1978 to 230,000 square kilometers in 1988 while tropical forest habitat, severely affected with respect to biological diversity, increased from 208,000 to 588,000 square kilometers. Although this rate of deforestation is lower than previous estimates, the effect on biological diversity is greater.

Skole, David↗
NASA NTRS1993

Comparison of dynamic height variations in the tropical Atlantic during 1987-1989 as viewed in sections hydrography and Geosat altimetry

During the 1980s the Soviet Union conducted a series of 25 hydrographic cruises in the tropical Atlantic under the Sections (acronym for Energetically Active Zones of the Ocean and Climate Variability) program. As one of its objectives, Sections has produced the most extensive hydrographic data set ever collected in the tropical Atlantic. Here we examine dynamic topography from seven cruises during the years 1987 through 1989 and compare them to corresponding maps of sea level from the Geosat satellite mission. The two independent estimates of sea level variation are well correlated, with the sea level fields explaining approximately half of the variance of dynamic height fields. There is a seasonal variation to the agreement, so that the correlation increases somewhat in summer and fall, as the variability of the height field increases. Wavenumber spectra of both dynamic height and sea level are quite red for wavelengths greater than 400-600 km. Smaller wavelengths cannot be resolved by the hydrographic sampling program. A comparison of the dynamic height and sea level spectra indicates that sea level has somewhat less variability than dynamic height at all wavenumbers.

Carton, James A.↗
NASA NTRS1985

Tropical upper tropospheric motion field

Upper tropospheric cloud motion vectors and commercial aircraft wind reports have enabled a detailed definition of the motion field. These two data sets have enhanced our observational capability since the launch of 4 to 5 geostationary satellites around the globe and the implementation of the ASDAR program. A sample is illustrated of recent commercial aircraft wind reports at the 300, 250, and 200 mb surfaces over parts of the Pacific Ocean, North America, and Atlantic Ocean. This data coverage is impressive. The rms difference between these data and those obtained from rawinsonde ascents (colocated) is around + or - 4 m/s. In a recent study of the cloud winds, from geostationary satellites, Johnson (1984) noted almost 15 m/s vector wind differences between estimates from GMS, GOES, and METEOSAT with respect to the rawinsonde observations. These results are illustrated based on the assessment of Johnson (1984). Overall, the commercial aircraft winds are the more accurate of these two observing systems. However, it is apparent that because of the sparcity of airline routes across southern oceans, one has to rely rather heavily on the cloud winds as a primary data source over the tropical southern oceans. The operational rawinsonde coverage over the tropics is generally very poor. The overall data coverage based primarily on the collections at the ECWMF are shown.

Krishnamurti, T. N.↗
NASA NTRS1993

A regional analysis of cloudy mean spherical albedo over the marine stratocumulus region and the tropical Atlantic Ocean

Since clouds are the largest variable in Earth's radiation budget, it is critical to determine both the spatial and temporal characteristics of their radiative properties. The relationships between cloud properties and cloud fraction are studied in order to supplement grid scale parameterizations. The satellite data used is from three hourly ISCCP (International Satellite Cloud Climatology Project) and monthly ERBE (Earth Radiation Budget Experiment) data on a 2.5 deg x 2.5 deg latitude-longitude grid. Mean cloud spherical albedo, the mean optical depth distribution, and cloud fraction are examined and compared off the coast of California and the mid-tropical Atlantic for July 1987 and 1988. Individual grid boxes and spatial averages over several grid boxes are correlated to Coakley's theory of reflection for uniform and broken layered cloud and to Kedem, et al.'s findings that rainfall volume and fractional area of rain in convective systems is linear. Kedem's hypothesis can be expressed in terms of cloud properties. That is, the total volume of liquid in a box is a linear function of cloud fraction. Results for the marine stratocumulus regime indicate that albedo is often invariant for cloud fractions of 20% to 80%. Coakley's satellite model of small and large clouds with cores (1 km) and edges (100 m) is consistent with this observation. The cores maintain high liquid water concentrations and large droplets while the edges contain low liquid water concentrations and small droplets. Large clouds are just a collection of cores. The mean optical depth (TAU) distributions support the above observation with TAU values of 3.55 to 9.38 favored across all cloud fractions. From these results, a method based upon Kedem, et al's theory is proposed to separate the cloud fraction and liquid water path (LWP) calculations in a general circulation model (GCM). In terms of spatial averaging, a linear relationship between albedo and cloud fraction is observed. For tropical locations outside the Intertropical Convergence Zone (ITCZ), results of cloud fraction and albedo spatial averaging followed that of the stratus boxes containing few overcast scenes. Both the ideas of Coakley and Kedem, et al. apply. Within the ITCZ, the grid boxes tended to have the same statistical properties as stratus boxes containing many overcast scenes. Because different dynamical forcing mechanisms are present, it is difficult to devise a method for determining subgrid scale variations. Neither of the theories proposed by Kedem, et al. or Coakley works well for the boxes with numerous overcast scenes.

Ginger, Kathryn M.↗