Engineering topics

Valdes, Juan B.

Publications and source records attributed to Valdes, Juan B..

5 records

NASA NTRS1994

Multivariate space - time analysis of PRE-STORM precipitation

This paper presents the methodologies and results of the multivariate modeling and two-dimensional spectral and correlation analysis of PRE-STORM rainfall gauge data. Estimated parameters of the models for the specific spatial averages clearly indicate the eastward and southeastward wave propagation of rainfall fluctuations. A relationship between the coefficients of the diffusion equation and the parameters of the stochastic model of rainfall fluctuations is derived that leads directly to the exclusive use of rainfall data to estimate advection speed (about 12 m/s) as well as other coefficients of the diffusion equation of the corresponding fields. The statistical methodology developed here can be used for confirmation of physical models by comparison of the corresponding second-moment statistics of the observed and simulated data, for generating multiple samples of any size, for solving the inverse problem of the hydrodynamic equations, and for application in some other areas of meteorological and climatological data analysis and modeling.

Polyak, Ilya↗

NASA NTRS1994

The ground-truth problem for satellite estimates of rain rate

In this paper a scheme is proposed to use a point raingage to compare contemporaneous measurements of rain rate from a single-field-of-view (FOV) estimate based on a satellite remote sensor such as a microwave radiometer. Even in the ideal case the measurements are different because one is at a point and the other is an area average over the field of view. Also the point gage will be located randomly inside the field of view on different overpasses. A space-time spectral formalism is combined with a simple stochastic rain field to find the mean-square deviations between the two systems. It is found that by combining about 60 visits of the satellite to the ground-truth site, the expected error can be reduced to about 10% of the standard deviation of the fluctuations of the systems alone. This seems to be a useful level of tolerance in terms of isolating and evaluating typical biases that might be contaminating retrieval algorithms.

North, Gerald R.↗

NASA NTRS1993

International Conference on Mesoscale Precipitation: Hydrologic and Meteorological Aspects, 3rd, College Station, TX, Feb. 27-Mar. 1, 1991, Proceedings

Papers presented in this volume discuss a mechanism causing mesoscale organizations of precipitation in midlatitude cyclones, the spatial variability of summer Florida precipitation and its impact on microwave radiometer rainfall-measurement systems, the relation between the mean areal rainfall and the fractional area where it rains above a given threshold, and a possible explanation for low correlation dimension estimates for the atmosphere. Attention is also given to a physically based simulation of radar rainfall data using a space-time rainfall model, the predictability of mesoscale rainfall in the tropics, and an intercomparison of tree satellite infrared rainfall techniques over Japan and surrounding waters. Other papers are on an evaluation of sampling errors of precipitation from spaceborne and ground sensors, an approach to the estimation of the areal rain-rate distribution from spaceborne radar by the use of multiple thresholds, and a comparison of two satellite-based rainfall algorithms using Pacific atoll raingage data.

Valdes, Juan B.↗

NASA NTRS1993

Evaluation of sampling errors of precipitation from spaceborne and ground sensors

The spatial and temporal characteristics of rainfall over Oklahoma and Kansas are analyzed using the raingage data collected during the Preliminary Regional Experiment for STORM-Central (PRE-STORM). The spectra obtained are compared with those obtained from the oceanic precipitation in the GARP and with that obtained from analyzing raingage records in east Texas. In addition, the spectra are used to evaluate the sampling errors that are due to the spatial gaps in measurements. It was found that the temporal spectra from PRE-STORM had a spectral shape similar to that obtained in GARP, except that the tail of the PRE-STORM spectra has a lower slope than in GARP spectra, suggesting that the largest difference between tropical oceanic rainfall and convective land precipitation is at the highest frequencies.

Graves, Charles E.↗

NASA NTRS1990

Frequency-wavenumber spectrum for GATE phase I rainfields

The oceanic rainfall frequency-wavenumber spectrum and its associated space-time correlation have been evaluated from subsets of GATE phase I data. The records, of a duration of four days, were sampled at 15 minutes intervals in 4 x 4 km grid boxes over a 400 km diameter hexagon. In the low frequencies-low wavenumber region the results coincide with those obtained by using the stochastic model proposed by North and Nakomoto (1989). From the derived spectrum the inherent time and space scales of the stochastic model were determined to be approximately 13 hours and 36 km. The formalism proposed by North and Nakamoto was taken together with the derived spectrum to compute the mean square sampling error due to intermittent visits of a spaceborne sensor.

Nakamoto, Shoichiro↗