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At least 1,279 records · Page 71
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NASA NTRS1983

Forecasting tropical cyclone recurvature with upper tropospheric winds

Data from 17 tropical cyclones during the 1974 through 1979 hurricane seasons are used to investigate whether the high level winds far to the northwest, north and northeast of the hurricane center can be used to predict hurricane track recurvature. When the man 200-mb winds 1500 to 2000 km northwest and north of the storm center equal or exceed 20 m/s, 80 per cent of the storms recurved before traveling as much as 12 degrees of longitude farther west. The high winds were also used to predict change in direction of forward motion during the next 72 hours. The regression equations developed explain up to 41 per cent of the variance in future direction. In addition to the geostrophic winds used, winds were also obtained by tracking clouds with successive satellite imagery. The u-components of the satellite winds are highly correlated with the geostrophic winds at 200-mb and could probably be used instead of them when available. The v-components are less highly correlated.

Gentry, R. C.↗
NASA NTRS1983

A satellite stereoscopic technique to estimate tropical cyclone intensity

The techniques used to obtain the mean equivalent temperature of the eye wall for Hurricanes Frederic (1979) and Allen (1980) using GOES satellite IR and stereoscopic observations are described. The eye wall is the area of greatest convection near the center of the storm, and is bounded by the inner radius around the eye and the outer radius bounding the area of inner core convection. The stereoscopic capability afforded by the GOES West and East spacecraft permits simultaneous, two-view imagery of a tropical cyclone, yielding height measurement accuracies of 0.5 km and horizontal accuracies as small as 1 km. An airborne lidar unit was used to verify the height measurements made of Hurricane Frederic. At the same time, the GOES East Visible IR Spin Scan Radiometer (VISSR) provided the mean wall temperatures from the release of latent heat. The trials aided in identifying the assumptions and consequent inaccuracies introduced into the temperature sounding data during analysis. The satellite data is concluded useful for monitoring changes in storm intensity.

Rodgers, E. B.↗
NASA NTRS1983

Possible effects of the El Chichon volcanic cloud on the radiation budget of the northern tropics

A series of calculations with a one-dimensional, time-marching, radiative-convective model are performed to assess the impact of the El Chichon volcanic cloud on the radiation budget of the northern tropics during the 6-month period following the injection of volcanic material into the stratosphere. Extensive measurement of the cloud obtained from airborne, spacecraft, and ground platforms were used to define the model parameters and to test the predictions of the model. The El Chichon cloud is predicted to have caused an increase in planetary albedo of 10 percent, a decrease in total solar radiation of 2-3 percent at the ground on cloudless days, and an increase in temperature of 3.5 K at the 24-km (30-mb) level. These predictions are compatible with relevant observations, within their respective error bars.

Pollack, J. B.↗
NASA NTRS1983

Some features of the structure of the tropical tropopause

Statistics of the minimum saturation humidity mixing ratio are extracted from 2500 radiosonde ascents in the tropics (10 N to 10 S) and in the region of the Asian summer monsoon. Two periods of approximately 20 days are examined, one in the northern winter and one in the northern summer. The relation of the observations to an hypothesis, proposed by A. W. Brewer in 1949, of mass exchange between troposphere and stratosphere is briefly discussed.

Atticks, M. G.↗
NASA NTRS1984

An objective method for forecasting tropical cyclone motion using Nimbus and NOAA-2 infrared measurements

A statistical method for predicting tropical cyclone positions 12, 24, 48 and 72 hours after initial observation is described. Satellite measurements were at the 11.0 micron channel and at equivalent blackbody temperatures which give representations of the structure of the cyclones, as well as the circulation features surrounding them. In all, 197 individual measurements for blackbody temperature were made. Algorithms prepared from a single satellite image included 14 climatological and persistence type variables, and are based on eigenvector preprocessing and a series of independent screening tests. The independent tests showed that the average error made by the algorithms from the single satellite observation were comparable to the 48 hr Joint Typhoon Warning Center (JTWC) forecast and were about 10 percent better for 72 hr forecasts. Forecasts using only climatological persistence variables were found to be 20 percent worse than JTWC for 24 hr forecasts and 10 percent worse for 48 and 72 hr forecasts. When both satellite and nonsatellite variables were included, it was found that the method was comparable to JTWC for the 24 and 48 hr forecasts and about 25 percent better for the 72 hr forecasts.

Hunter, H. E.↗
NASA NTRS1984

Tropical Atlantic wind field variations during Sequal Preliminary results

The Sequal (Seasonal Response of the Equatorial Atlantic) program has the objective to model correctly the dominant physical processes producing the seasonal redistribution of heat, mass, and salt in the upper ocean. The efforts of Sequal are greatly enhanced by the Focal (Francais Ocean et Climat dans l'Atlantique Equatorial) program. A knowledge of fields of wind stress over the tropical Atlantic is needed for the achievement of the Sequal objectives. The Sequal program makes use of the Fleet Numerical Oceanography Central (FNOC) 'Nogaps' wind fields as a first-guess field which will subsequently be modified. Nogaps is a global weather prediction model consisting of independent global wind and mass analysis submodels. The present investigation is concerned with some preliminary comparisons between Nogaps predictions and surface observations, taking into account monthly mean Nogaps fields and monthly means from a century-long ship report-based climatology.

Harrison, D. E.↗
NASA NTRS1984

Interannual variability versus seasonal variability in the tropical Atlantic

Monthly fields of sea surface temperature and wind stress for the tropical Atlantic are constructed for January 1964 to December 1979. Inside the Gulf of Guinea the amplitude of the seasonal variability of SST ranges between 2.5 and 4 times greater than the interannual variability. In the remainder of the equatorial Atlantic this measure is less than 2. Standard deviations of wind stress anomalies indicate that, except near the Intertropical Convergence Zone, the magnitude of interannual fluctuations is greater than that of seasonal ones. The joint analyses of these SST and wind stress fields indicate the largest positive SST anomaly in the Gulf of Guinea (July, 1968) was immediately preceded by the only reversal of the trade winds west along the equator.

Picaut, J.↗
NASA NTRS1983

Diagnoses of tropical geopotential during the Southern Hemisphere summer

The effect of strong amplitude and high frequency oscillations of divergent circulations in the Southern Hemisphere on estimates of tropical geopotential during the summer is quantified. FGGE data are analyzed in order to determine the mass-flow balance in the upper troposphere, and outgoing longwave radiation averaged over two periods, January and February 1979, is given. It is found that the divergence terms for balance equations may represent geopotential perturbations on the order of 20 to 30 m, and that the influence of strong latent heat releases and divergent circulations in the Southern Hemisphere can invalidate analysis schemes based on quasi-nondivergent solutions.

Paegle, J. N.↗
NASA NTRS1985

A GCM Study of the Atmospheric Response to Tropical SST Anomalies

The purpose is to understand the evolution of the atmospheric anomalies associated with the most recent warm episode by the use of simulation studies with the UCLA general circulation model (GCM). The approach is to integrate the model using the observed sequence of Sea Surface Temperature (SST) anomalies during 1982 to 1983, and compare it with a control run in which all boundary conditions vary from month to month as in the climatology. The control for the experiment was a three year simulation using seasonally varying climatological SSTs. The anomaly calculation was initialized from 15 June of the first year of the control. From 15 June to 1 July, the SST was gradually modified by the anomalies observed during June and July of 1982. From 1 July on, the run was continued using the control's SST plus the 1982 to 1983 anomalies. SSTs were varied daily, interpolating between monthly means. This second integration as carried to the end of February of the second year (1983 in the anomaly). Only anomalies over the tropical Pacific were used.

Suarez, M. J.↗
NASA NTRS1984

Applications of VAS and TOVS to tropical cyclones

The results of the attempts to use VAS (visible IR spin scan radiometer atmospheric sounder) data for predicting tropical cyclones are reported. VAS, mounted on a GEO platform, has 14 detectors for upwelling radiance. The McIDAS software was, by the 1983 hurricane season, upgraded to providing imagery and mass, motion and moisture patterns using the satellite data, which also permitted determinations of accurate temperature and moisture profiles. Comparisons made between VAS and radiosonde data revealed fairly close agreement of satellite and in-situ conditions during the 1983 season. When combined with data from the Tiros operational vertical sounder (TOVS) to define wind streamlines and predict the path of a hurricane from data fed into numerical models, sufficiently good agreement was obtained to initiate plans to forecast a hurricane trajectory during the 1984 season.

Velden, C. S.↗
NASA NTRS1984

Temporal variations of the tropical 40-50 day oscillation

In recent years there has been a great deal of interest in a quasi-periodic tropical oscillation of zonal winds, which was first reported by Madden and Julian. An attempt to determine the temporal variation of the oscillation parameters is presented here. Using a 4-year duration global time series and a 25-year station time series, it is found that although the nonseasonal variations are large, any seasonal cycle in the oscillation amplitude and frequency must be very small. The small seasonal signal in the oscillation frequency seems to argue against explanations for the time scale based on Doppler-shifted traveling waves.

Anderson, J. R.↗
NASA NTRS1985

Surface radiation in the tropical Pacific

Monthly surface radiative fluxes have been calculated for the tropical Pacific between January 1970 and February 1978, using a radiative transfer parameterization. The radiative transfer parameterization included detailed treatments of the molecular and droplet absorptions, and surface and cloud reflections. The input data used in the calculations were obtained from the National Climatic Center (NCC), the National Center for Atmospheric Research (NCAR), and from the University of Hawaii. The results show that the distribution of surface radiation closely follows the distribution of cloudiness, and, to a lesser degree, humidity. The rms net error in the surface radiation estimates was about 15 W per sq m, with the largest contribution from uncertainties in the cloud cover and humidity data. The sensitivity of surface radiation parameterizations to input data errors is discussed, and some accuracy requirements for satellite retrievals of atmospheric and cloud parameters are proposed. The calculations are presented in the annually-averaged maps of surface radiation variations.

Chou, M.-D.↗
NASA NTRS1984

Vertical displacement of the mid-tropospheric water vapor boundary in the tropics derived from the VISSR Atmospheric Sounder (VAS) 6.7-micron channel

A technique for locating areas of upward or downward motion of the upper boundary of the midtropospheric water-vapor envelope over tropical oceans on the basis of GOES Visible/IR Spin-Scan Radiometer (VISSR) Atmospheric Sounder (VAS) 6.7-micron brightness temperatures is developed and demonstrated. The technique employs an analogy to the method developed by Schoeberl and Krueger (1983) for the bottom of the ozone layer and depends on the relationship investigated by Chesters and Uccellini (1982) between 6.7-micron brightness temperature and the pressure of the water-vapor upper boundary. The results of analysis of VAS data for the North Atlantic (20-40 deg N and 35-75 deg W) on September 5-7, 1982, are presented in maps which are shown to be physically consistent and in agreement with conventional upper-air measurements. Refinement of the method to account for horizontal advection and diffusion is suggested.

Stout, J.↗
NASA NTRS1985

Subseasonal and interannual variations of tropical convection and the El Nino/Southern Oscillation

It was found that during the northern winter, fluctuation in tropical convection is dominated by eastware propagating cloud clusters from the Indian Ocean. Most of these clusters reach as far as the central Pacific. Longitude time sections of outgoing longwave radiation (OLR) anomaly along the equator show that there are about 3 to 4 such episodes each winter. Also, simultaneous OLR anomalies over the equatorial central Pacific (ECP) and the maritime continent (MC) are negatively correlated and their variations are dominated by a 40 to 60 day period, suggesting the presence of a standing wave phenomenon. Such combination of propagating and standing phenomena can be captured by the extended empirical orthogonal function (EEOF) analysis. The EEOF method combines the temporal as well as the spatial correlation information to produce eigenmodes which have space time dependence. A 5 day and a 10 day lag in addition to the simultaneous spatial correlation is used.

Lau, K. M.↗
NASA NTRS1985

The tropical special observing system

The performance summaries of the components of the Tropical Observing System is covered. Some of the more important points concerning the quality of the data from each of the observing systems is emphasized, and examples of the assimilation of the Level IIb data are shown.

Julian, P. R.↗
NASA NTRS1985

Seasat microwave wind and rain observations in severe tropical and midlatitude marine storms

Initial results of studies concerning Seasat measurements in and around tropical and severe midlatitude cyclones over the open ocean are presented, together with an assessment of their accuracy and usefulness. Complementary measurements of surface wind speed and direction, rainfall rate, and the sea surface temperature obtained with the Seasat-A Satellite Scatterometer (SASS), the Scanning Multichannel Microwave Radiometer (SMMR), and the Seasat SAR are analyzed. The Seasat data for the Hurrricanes Fico, Ella, and Greta and the QE II storm are compared with data obtained from aircraft, buoys, and ships. It is shown that the SASS-derived wind speeds are accurate to within 10 percent, and the directions are accurate to within 20 percent. In general, the SASS estimates tend to measure light winds too high and intense winds too low. The errors of the SMMR-derived measurements of the winds in hurricanes tend to be higher than those of the SASS-derived measurements.

Black, P. G.↗
NASA NTRS1986

Diurnal variation of outgoing longwave radiation in the tropics

The diurnal harmonic in longwave emission in the tropical belt (30 deg N-30 deg S) is estimated from nine years of NOAA polar-orbiting satellite data. The results are compared successfully with Nimbus-7 ERB scanner data and with GOES-West geosynchronous satellite data. An interesting and consistent diurnal variation in longwave emission is found over the regions of intense oceanic convection, such as the ITCZ and SPCZ regions, with a peak-to-peak variation of 6-8 W/sq m and a maximum in the morning (0600-1200 LST). Histogram analysis indicates that this variation is associated with a diurnal variation in convective cloud (about 400 mb). Over regions of very intense convection, a diurnal variation of very high clouds (above 100 mb), which is out of phase with the variations at lower levels in the atmosphere, reduces the magnitude of the diurnal harmonic in longwave emission. It is interesting that histograms based on data averaged over 8-km and 250-km boxes give the same qualitative information about cloud and emission variability.

Hartmann, D. L.↗