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  • 1
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    Comparison of monthly rain rates derived from GPI and SSM/I using probability distribution functions (1993)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Three years of monthly rain rates over 5 deg x 5 deg latitude-longitude boxes have been calculated for oceanic regions 50 deg N-50 deg S from measurements taken by the Special Sensor Microwave/Imager on board the Defense Meteorological Satellite Program satellites using the technique developed by Wilheit et al. (1987, 1991). The annual and seasonal zonal-mean rain rates are larger than Jaeger's (1983) climatological estimates but are smaller than those estimated from the GOES precipitation index (GPI) for the same period. Regional comparison with the GPI showed that these rain rates are smaller in the north Indian Ocean and in the southern extratropics where the GPI is known to overestimate. The differences are also dominated by a jump at 170 deg W in the GPI rain rates across the mid-Pacific Ocean. This jump is attributed to the fusion of different satellite measurements in producing the GPI.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Applied Meteorology (ISSN 0894-8763); 32; 2; p. 323-334.
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  • 2
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    Wind field model-based estimation of Seasat scatterometer winds (1993)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A model-based approach to estimating near-surface wind fields over the ocean from Seasat scatterometer (SASS) measurements is presented. The approach is a direct assimilation technique in which wind field model parameters are estimated directly from the scatterometer measurements of the radar backscatter of the ocean's surface using maximum likelihood principles. The wind field estimate is then computed from the estimated model parameters. The wind field model used in this approach is based on geostrophic approximation and on simplistic assumptions about the wind field vorticity and divergence but includes ageostrophic winds. Nine days of SASS data were processed to obtain unique wind estimates. Comparisons in performance to the traditional two-step (point-wise wind retrieval followed by ambiguity removal) wind estimate method and the model-based method are provided using both simulated radar backscatter measurements and actual SASS measurements. In the latter case the results are compared to wind fields determined using subjective ambiguity removal. While the traditional approach results in missing measurements and reduced effective swath width due to fore/aft beam cell coregistration problems, the model-based approach uses all available measurements to increase the effective swath width and to reduce data gaps. The results reveal that the model-based wind estimates have accuracy comparable to traditionally estimated winds with less 'noise' in the directional estimates, particularly at low wind speeds.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 98; C8; p. 14,651-14,668.
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  • 3
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    Retrieval of monthly rainfall indices from microwave radiometric measurements using probability distribution functions (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: An algorithm for the estimation of monthly rain totals for 5 deg cells over the ocean from histograms of SSM/I brightness temperatures has been developed. There are three novel features to this algorithm. First, it uses knowledge of the form of the rainfall intensity probability density function to augment the measurements. Second, a linear combination of the 19.35 and 22.235 GHz channels has been employed to reduce the impact of variability of water vapor. Third, an objective technique has been developed to estimate the rain layer thickness from the 19.35- and 22.235-GHz brightness temperature histograms. Comparison with climatologies and the GATE radar observations suggest that the estimates are reasonable in spite of not having a beam-filling correction. By-products of the retrievals indicate that the SSM/I instrument noise level and calibration stability are quite good.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Atmospheric and Oceanic Technology (ISSN 0739-0572); 8; 118-136
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    Standort Signatur Erwartet Verfügbarkeit
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  • 4
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    An analysis of the threshold method for measuring area-average rainfall (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Experimental evidence shows that the area-average rain rate and the fractional area covered by rain rate exceeding a fixed threshold are highly correlated; that is, are highly linearly related. A precise theoretical explanation of this fact is given. The explanation is based on the observation that rain rate has a mixed distribution, one that is a mixture of a discrete distribution and a continuous distribution. Under a homogeneity assumption, the slope of the linear relationship depends only on the continuous part of the distribution and as such is found to be markedly immune to parameter changes. This is illustrated by certain slope surfaces obtained from three specific distributions. The threshold level can be chosen in an optimal way by minimizing a certain distance function defined over the threshold range. In general, the threshold level should be not too far from the mean rain rate conditional on rain. The so-called threshold method advocates measuring rainfall from fractional area exploiting the observed linear relationship of the later with the area average rain rate. The method is potentially useful for the estimation of rainfall from space via satellites.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Applied Meteorology (ISSN 0894-8763); 29; 3-20
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    Standort Signatur Erwartet Verfügbarkeit
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  • 5
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    Estimation of mean rain rate - Application to satellite observations (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A method for the estimation of the mean area average rain rate from dependent data is developed and applied to the GARP Atlantic Tropical Experiment data. The method consists of fitting a mixed distribution, containing an atom at zero, by minimum chi-square in combination with certain time-space sampling designs. In modeling the continuous component of the mixed distribution, it is shown that the lognormal distribution provides a very close fit for the nonzero area average rainrates. A comparison with the gamma distribution shows that the lognormal distribution is a better choice as expressed by the minimum chi-square criterion. Some of the time-space sampling designs correspond to satellite sampling. The results indicate that a satellite visiting an area of about 350 x 350 sq km in the tropics approximately every 10 hours over a period can provide a rather close estimate for the mean area average rain rate.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 1965-197
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    Standort Signatur Erwartet Verfügbarkeit
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  • 6
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    The global atmospheric response to low-frequency tropical forcing: Zonally averaged basic states (1994)
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    Publikationsdatum: 2019-07-13
    Beschreibung: The extratropical response to localized, low-frequency tropical forcing is examined using a linearized, non-divergent barotropic model on a sphere. Zonal-mean basic states characterized by solid-body rotation or critical latitudes are considered. An analytical analysis based on WKB and ray tracing methods shows that, in contrast to stationary Rossby waves, westward moving, low-frequency Rossby waves can propagate through the tropical easterlies into the extratropics. It is shown analytically that the difference between the stationary and low-frequency ray paths is proportional to the forcing frequency and inversely proportional to the zonal wavenumber cubed. An expression for the disturbance amplitude is derived that shows the ability of the forced waves to maintain their strength well into middle latitudes depends on their meridional wave scale and northward group velocity, both of which are functions of the slowly varying background flow. A local energetics analysis shows that the combination of energy dispersion from the forcing region and energy extraction from the equatorward flank of the midlatitude jet produces disturbances that have the greatest impact on the extratropical circulation. Under the assumption that the forcing amplitude is independent of frequency, this impact is largest when the tropical forcing period is in the range 10-20 days.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of the Atmospheric Sciences (ISSN 0022-4928); 51; 23; p. 3412-3426
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
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    Random errors of oceanic monthly rainfall derived from SSM/I using probability distribution functions (1993)
    In:  Other Sources
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    Publikationsdatum: 2019-08-27
    Beschreibung: Global averages and random errors associated with the monthly oceanic rain rates derived from the Special Sensor Microwave/Imager (SSM/I) data using the technique developed by Wilheit et al. (1991) are computed. Accounting for the beam-filling bias, a global annual average rain rate of 1.26 m is computed. The error estimation scheme is based on the existence of independent (morning and afternoon) estimates of the monthly mean. Calculations show overall random errors of about 50-60 percent for each 5 deg x 5 deg box. The results are insensitive to different sampling strategy (odd and even days of the month). Comparison of the SSM/I estimates with raingage data collected at the Pacific atoll stations showed a low bias of about 8 percent, a correlation of 0.7, and an rms difference of 55 percent.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Monthly Weather Review (ISSN 0027-0644); 121; 8; p. 2351-2354.
    Format: text
    Standort Signatur Erwartet Verfügbarkeit
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  • 8
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    The wavenumber spectra of scatterometer-derived winds (1992)
    In:  Other Sources
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    Publikationsdatum: 2019-08-28
    Beschreibung: The accuracy of scatterometer-derived winds using a frequency-domain analysis and simulation is studied. The wavenumber spectra of the Seasat-A-scatterometer (SASS)-derived wind fields have been observed to be accentuated relative to the input wind field. The results of extensive simulations designed to test this observation are reported. Actual SASS measurements of the normalized radar backscatter (NRB) over an orbit (rev) are used as a template to generate simulated NRB measurements. Pointwise estimation of winds from the simulated NRB measurements is accomplished with the ambiguity closest to the true wind selected as the unique wind vector estimate. For comparison, winds are also estimated using a new model-based approach. After wind retrieval, the spectra of the estimated wind fields are computed and compared to the input wind field. The high wavenumber portion of the spectra of the pointwise estimated winds was higher than the spectra of the true winds by an amount which depends on the wind speed variance.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: In: IGARSS '92; Proceedings of the 12th Annual International Geoscience and Remote Sensing Symposium, Houston, TX, May 26-29, 1992. Vol. 2 (A93-47551 20-43); p. 1005-1007.
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  • 9
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    Ozone heating and the destabilization of traveling waves during summer (1994)
    In:  Other Sources
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    Publikationsdatum: 2019-08-28
    Beschreibung: The effects of ozone heating on the linear stability of lower stratospheric traveling waves of the summertime, extratropical circulation are examined. Based on coupled equations for the quasigeostrophic potential vorticity and ozone volume mixing ratio, it is shown that the diabatic heating arising from ozone advection can offset the damping due to Newtonian cooling, leading to wave amplification and significant changes in the structure and zonally rectified fluxes of the wave fields in both the lower stratosphere and troposphere. The vertical profile of the zonal mean wind plays a crucial role in determining whether the ozone heating destabilizes eastward and/or westward traveling disturbances.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 21; 14; p. 1531-1534
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  • 10
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    Estimating the exceedance probability of rain rate by logistic regression (1990)
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    Publikationsdatum: 2019-07-12
    Beschreibung: Recent studies have shown that the fraction of an area with rain intensity above a fixed threshold is highly correlated with the area-averaged rain rate. To estimate the fractional rainy area, a logistic regression model, which estimates the conditional probability that rain rate over an area exceeds a fixed threshold given the values of related covariates, is developed. The problem of dependency in the data in the estimation procedure is bypassed by the method of partial likelihood. Analyses of simulated scanning multichannel microwave radiometer and observed electrically scanning microwave radiometer data during the Global Atlantic Tropical Experiment period show that the use of logistic regression in pixel classification is superior to multiple regression in predicting whether rain rate at each pixel exceeds a given threshold, even in the presence of noisy data. The potential of the logistic regression technique in satellite rain rate estimation is discussed.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 2217-222
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