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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Dynamical response of the tropical total ozone to sea surface temperature changes

Interannual variations of total ozone in the equatorial region are discussed in terms of the lower-stratospheric dynamical processes associated with sea surface temperature (SST) changes on an El Nino-Southern Oscillation time scale. Tropospheric diabatic heating by the latent heat release in convective systems drives the interannual variations of tropopause height and atmospheric vertical motions. The adiabatic cooling associated with the dynamically forced ascending motion is subject to radiative heating, which further extends the upwelling motion into the stratosphere. SST variations can thus influence total ozone amounts by changing the tropopause height and vertical advection through modulation of tropospheric diabatic heating. Mechanistic relationships that describe total ozone fluctuations through the advection effect and the tropopause effect of SST variations are derived using the equivalent potential temperature. Total ozone fluctuations are estimated using a quantitative formulation of these mechanistic equations together with the observed SST data. These results indicate that the interannual variations of the zonal wave component of equatorial total ozone are governed by the tropopause effect of SST variations. Preliminary investigation suggests that the annual cycle of tropical total ozone is coupled to the tropopause height changes under the influence of the planetary wave-induced lower-stratospheric temperature rather than local SST.

Hasebe, Fumio↗
NASA NTRS1992

On the microwave measurement of precipitation in tropics

The difference between the brightness temperature structure over a typical tropical rain system and that of the midlatitude thunderstorms is examined. It is also determined how the brightness temperature and the rain rate relationship at the microwave frequencies deviates from that in the thunderstorms. It is shown, in particular, that the theoretical brightness temperature and rain rate relationship at 19.35 GHz provides good estimates of the surface rain rates for either convective or stratiform precipitation, but the relationship at 37 and 85.5 GHz may result in a significant overestimation of surface rain rate, especially for stratiform precipitation.

Weng, Fuzhong↗
NASA NTRS1992

A comparison of cirrus clouds determined by ISCCP and SAGE-II and their relation to convection in the tropics

Results of tropical thin cirrus cloud retrievals using International Satellite Cloud Climatology Project (ISCCP) and Stratospheric Aerosol and Gaseous Experiment (SAGE-II) data from January 1985 are presented. A preliminary analysis of the results shows that thin cirrus increases with increasing height in both data sets, and SAGE-II exhibits a high frequency of occurrence. The thin cirrus extinction coefficient shows maxima around the convective regions of South America and the western Pacific Ocean.

Christopher, Sundar A.↗
NASA NTRS1992

The Tropical Rainfall Measuring Mission (TRMM) update and its role in EOS and GEWEX

Updated information is presented on the U.S./Japan Tropical Rainfall Measuring Mission (TRMM), which is a relatively low-budget earth-probe satellite with a secondary objective of measuring upwelling radiation from the clouds and the surfaces below the satellite. Particular attention is given to the TRMM rain measurements and the characteristics of the three TRMM rain measuring instruments: the microwave radiometer, the radar, and the visible/IR radiometer. Also discussed are the TRMM contributions to EOS and GEWEX.

Simpson, Joanne↗
NASA NTRS1993

Mesoscale disturbances in the tropical stratosphere excited by convection - Observations and effects on the stratospheric momentum budget

Aircraft temperature and pressure measurements as well as satellite imagery are used to establish the amplitudes and the space and time scale of potential temperature disturbances over convective systems. A conceptual model is proposed for the generation of mesoscale gravity waves by convection. The momentum forcing that a reasonable distribution of convection might exert on the tropical stratosphere through convectively excited mesoscale gravity waves of the observed amplitudes is estimated. Aircraft measurements show that presence of mesoscale disturbances in the lower stratospheric temperature, disturbances that appear to be associated with underlying convection. If the disturbances are convectively excited mesoscale gravity waves, their amplitude is sufficient that their breakdown in the upper stratosphere will exert a zonal force comparable to but probably smaller than the planetary-scale Kelvin waves.

Pfister, Leonhard↗
NASA NTRS1993

Upper-level eddy angular momentum fluxes and tropical cyclone intensity change

The eddy flux convergence of relative angular momentum (EFC) at 200 mb was calculated for the named tropical cyclones during the 1989-1991 Atlantic hurricane seasons. A period of enhanced EFC within 1500 km of the storm center occurred about every five days due to the interaction with upper-level troughs in the midlatitude westerlies or upper-level, cold lows in low latitudes. Twenty-six of the 32 storms had at least one period of enhanced EFC. In about one-third of the cases, the storm intensified just after the period of enhanced EFC. In most of the cases in which the storm did not intensify the vertical shear increased, the storm moved over cold water, or the storm became extratropical just after the period of enhanced EFC. A statistically significant relationship was found between the EFC within 600 km of the storm center and the intensity change during the next 48 h. The EFC was also examined for the ten storms from the 1989-1991 sample that had the largest intensification rates. Six of the ten periods of rapid intensification were associated with enhanced EFC. In the remaining four cases the storms were intensifying rapidly in a low shear environment without any obvious interaction with upper-level troughs.

Demaria, Mark↗
NASA NTRS1993

Water vapor and cloud water measurements over Darwin during the STEP 1987 tropical mission

Measurements of stratospheric and upper tropospheric cloud water plus water vapor (total water) and water vapor were made with two Lyman alpha hygrometers as part of the STEP tropical experiment. The in situ measurements were made in the Darwin, Australia, area in January and February of 1987 on an ER-2 aircraft. Average stratospheric water vapor at a potential temperature of 375 K (the average value of Theta at the tropopause) was 2.4 parts per million by volume (ppmv). This water mixing ratio is below the 3.0 to 4.0 ppmv necessary to be consistent with the observed upper stratospheric dryness. Saturation with respect to ice and the potential for dehydration was observed up to Theta = 402 K.

Kelly, K. K.↗
NASA NTRS1993

The meteorological environment of the tropospheric ozone maximum over the tropical South Atlantic Ocean

Atmospheric flow patterns are examined over the South Atlantic Ocean where a maximum of tropospheric ozone has been observed just west of southern Africa. We investigate the flow climatology during October and perform a case study for six days during October 1989. Horizontal and vertical motions are examined and used to prepare 3D backward trajectories from the region of greatest ozone. An initially zonally symmetric distribution of ozone is treated as a passive tracer and advected by 3D flows forecast by the global model. Results from the passive tracer simulation indicate that 3D advection alone can produce a maximum of tropospheric ozone in the observed location. In addition, the trajectories suggest that by-products of biomass burning could be transported to the area of maximum ozone. Low-level flow from commonly observed regions of burning in Africa streams westward to the area of interest. Over Brazil, if the burning by-products are carried into the upper troposphere by convective process, they then could be transported eastward to the ozone feature in approximately five days. There is considerable subsidence over the tropical southern Atlantic, such that stratospheric influences also are a factor in producing the ozone maximum. Both planetary-scale and transient synoptic-scale circulation features play major roles in the various transport processes that influence the region. In summary, the observed tropospheric ozone maximum appears to be caused by a complex set of horizontal and vertical advections, transport from regions of biomass burning, and stratospheric influences.

Krishnamurti, T. N.↗
NASA NTRS1993

Assessing fire emissions from tropical savanna and forests of central Brazil

Wildfires in tropical forest and savanna are a strong source of trace gas and particulate emissions to the atmosphere, but estimates of the continental-scale impacts are limited by large uncertainties in the rates of fire occurrence and biomass combustion. Satellite-based remote sensing offers promise for characterizing fire physical properties and impacts on the environment, but currently available sensors saturate over high-radiance targets and provide only indications of regions and times at which fires are extensive and their areal rate of growing as recorded in ash layers. Here we describe an approach combining satellite- and aircraft-based remote sensing with in situ measurements of smoke to estimate emissions from central Brazil. These estimates will improve global accounting of radiation-absorbing gases and particulates that may be contributing to climate change and will provide strategic data for fire management.

Riggan, Philip J.↗
NASA NTRS1992

Multiparameter radar and advanced microwave precipitation radiometer observations of tropical convection

Combinations of both active and passive microwave sensors have been proposed for experiments such as the tropical rainfall measuring mission (TRMM). During the summer 1991 Convection and Precipitation/Electrification Experiment (CaPE) in central Florida, both cold- and warm-rain precipitation processes were studied with the NCAR CP-2 multiparameter radar, operating at 3 and 10 GHz. The Advanced Microwave Precipitation Radiometer (AMPR), which operates at 10.7, 19.35, 37.1, and 85.5 GHz, was flown aboard NASA's ER-2 high-altitude aircraft over storms being scanned by the CP-2 radar. Top-of-atmosphere multifrequency TB from the AMPR is presented along with coincident CP-2 radar observations. Joint analysis of both radar and radiometer data sets allows refinement of new and existing precipitation retrieval techniques which will utilize the multifrequency TB from an integrated spaceborne microwave radiometer/radar system.

Vivekanandan, J.↗
NASA NTRS1993

Nondestructive examination of the Tropical Rainfall Measuring Mission (TRMM) reaction control subsystem (RCS) propellant tanks

This paper assesses the feasibility of using eddy current nondestructive examination to determine flaw sizes in completely assembled hydrazine propellant tanks. The study was performed by the NASA Goddard Space Flight Center for the Tropical Rainfall Measuring Mission (TRMM) project to help determine whether existing propellant tanks could meet the fracture analysis requirements of the current pressure vessel specification, MIL-STD-1522A and, therefore be used on the TRMM spacecraft. After evaluating several nondestructive test methods, eddy current testing was selected as the most promising method for determining flaw sizes on external and internal surfaces of completely assembled tanks. Tests were conducted to confirm the detection capability of the eddy current NDE, procedures were developed to inspect two candidate tanks, and the test support equipment was designed. The non-spherical tank eddy current NDE test program was terminated when the decision was made to procure new tanks for the TRMM propulsion subsystem. The information on the development phase of this test program is presented in this paper as a reference for future investigation on the subject.

Free, James M.↗
NASA NTRS1991

Precipitation rates in the tropics - June 1984-May 1987

A study aimed at gaining better understanding of the heating potential of the tropics by examining the 'apparent' heat source (Q1), i.e. dry static energy budget, is presented. The Q1 budget and precipitation rates over a three year period are analyzed using a routine data set: the European Center for Medium-range Weather Forecasts analyses. A noticeable increase in the values of Q1 during years 2 and 3 compared to year 1 was observed, which is considered to be due to the changes in the ECMWF model physics that produced larger values of upper level divergence after May 1985. There is a slow eastward progression of maximum values across the Western Pacific during the calendar year 1986 and into early 1987.

North, Keith H.↗
NASA NTRS1991

Atmospheric weather regimes over tropical South America

Infrared radiance measurements by the GOES-6 satellite during April 1986 through April 1987 were used to characterize and identify distinct regimes of persistent large-scale cloudiness patterns over the Amazon Basin. It is suggested that the energetics of the tropical troposphere over the Amazon Basin can be directly related to the GOES large-scale cloudiness patterns. The geometry and persistence of the cloud patterns are influenced by shifts in general circulation features and are likely modulated by 4- to 5-day and 40- to 60-day waves. Diurnal forcing effects are more pronounced during weather regimes characterized by prominently clear skies over land areas.

Connors, Vickie S.↗
NASA NTRS1993

Circulation anomaly mechanisms in the tropical Atlantic sector during the northeast Brazil rainy season - Results from the GISS general circulation model

The paper examines the effect of the SST pattern in the tropical Atlantic on the overlying atmosphere from a 7-yr run of the GCM of the Goddard Institute for Space Studies (GISS) (period 1980-1986) and in comparison to upper air analyses of the ECMWF, surface ship observations and wind fields, and rainfall measurements in northeast Brazil. The GCM simulation reproduces the thermal and hydrostatic forcing of SST on the meridional gradients of lower-tropospheric thickness and pressure in the proper sense, albeit more weakly than observed. The modeled meridional wind component follows the meridional pressure gradient in the proper sense, but mean and range are smaller than observed, and the modeled interannual variability is not entirely consistent with observational evidence. Overall, the GISS GCM seems to reproduce reasonably well the response of lower-tropospheric thickness and near-surface pressure to SST, while the simulation of wind and rainfall is somewhat deficient.

Hastenrath, Stefan↗
NASA NTRS1985

The impact of tropical wind data on the analysis and forcasts of the GLA GCM for the global weather experiment

It is well-known that divergent wind estimates are much more dependent upon the analysis system than are estimates of the rotational wind. This conclusion is supported in recent analyses of FGGE SOP1 data produced by the Goddard Laboratory for Atmospheres (GLA), the Geophysical Fluid Dynamics Laboratory (GFDL) and the European Center for Medium Range Weather Forecasting (ECMWF). These analyses differ in the forecast models that are used for the four-dimensional assimilation, in the data rejection criteria, and, to a certain extent, in the data density. Because the final divergent wind is a product of both model constraints and observation, it is relevant to inquire how much of each goes into the final product. We presently investigate this question through a systematic analysis of tropical data that are sampled at different densities by the GLA GCM.

Paegle, Jan↗
NASA NTRS1985

Pathways of nitrogen loss following land clearing in a humid tropical forest

Tropical deforestation generally leads to large losses of carbon and nitrogen. The Premontane Wet Forest Life Zone is subject to the highest rate of deforestation in Central America, and carbon and nutrient losses in from these fertile soils is very rapid and extreme. Losses of 2000 to 3000 kgN/ha have been reported. Losses of this magnitude could be extremely significant on a regional or global scale if even a small proportion of this nitrogen is lost as nitrous oxide to the atmosphere or through leaching of nitrate to rivers. This study seeks to measure the rates and regulation of nitrogen transformations, and the pathways of nitrogen losses following land clearing and burning at a site in the Premontane Wet Forest Life Zone near Turrialba, Costa Rica.

Matson, Pamela↗
NASA NTRS1993

Remote sensing estimates of cirrus particle size for tropical and midlatitude cirrus: Hexagonal crystals and ice spheres

A large discrepancy exists in current estimates of a mean cirrus particle size appropriate for calculations of the effects of these ice clouds on solar and thermal infrared radiative fluxes. For spheres with large size parameter (x = (2 pi r / lambda) is greater than 30, where r is particle radius), and moderate absorption (n(sup i) x less than 1, where n(sup i) is imaginary index of refraction for ice), the optimal effective particle radius is given by: r(sub e) = integral of r(exp 3)n(r)dr / integral of r(exp 2)n(r)dr. For the remote sensing of cirrus particle size at wavelengths of 0.83, 1.65, and 2.21 mu m, a 50 mu m ice sphere would have a size parameter of about 200, and values of n(sup i) x of 0, 0.045, and 0.06, satisfying the above conditions. However, while r(sub e) is a well-defined parameter for spheres, this cross-section area-weighted particle radius can only be extended to non-spherical particles by defining some equivalent sphere, typically an equivalent volume or equivalent cross-section area sphere. Using equivalent volume spheres, values of r(sub e) obtained over Lake Michigan on October 28, 1986, during FIRE phase I varied from 200 mu m (King Air 2D Imaging probes) to 60 mu m (Landsat reflectances at 0.83, 1.65, and 2.2 mu m), to 25 mu m (HIS spectrometer thermal emission between 8 and 12 mu m). Three major uncertainties were identified in this comparison: small ice particles missed by the 2D-C aircraft probes, uncertain ice refractive index, and uncertainties in the single scatter albedos and scattering phase functions used in the radiative calculations. Since the first FIRE cirrus results, advances have been made in all three areas. The present paper reports on improvements in the radiative modeling of ice particles at 0.83, 1.65, and 2.21 mu m wavelengths appropriate for comparisons to Landsat Thematic Mapper data. The paper also includes new results for Landsat observations of ice clouds in the eastern and western tropical Pacific.

Wielicki, Bruce A.↗
NASA NTRS1993

Are tropical cirrus brighter than mid-latitude cirrus?

Recent measurements during FIRE II, in the tropics and elsewhere, support an emerging hypothesis about the role of stratospheric mixing in determining the microphysical and radiative properties of cirrus clouds. This is only a working hypothesis, and may change as new measurements become available. This report reviews the conditions under which certain types of ice crystals form.

Mitchell, David L.↗