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  • 1
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    Principal modes of variation of rain-rate probability distributions (1994)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Radar or satellite observations of an area generate sequences of rain-rate maps. From a gridded map a histogram of rain rates can be obtained representing the relative areas occupied by rain rates of various strengths. The histograms vary with time as precipitating systems in the area evolve and decay and amounts of convective and stratiform rain in the area change. A method of decomposing the histograms into linear combinations of a few empirical distributions with time-dependent coefficients is developed, using principal component analysis as a starting point. When applied to a tropical Atlantic dataset (GATE), two distributions emerge naturally from the analysis, resembling stratiform and convective rain-rate distributions in that they peak at low and high rain rates, respectively. The two 'modes' have different timescales and only the high-rain-rate mode has a statistically significant diurnal cycle. The ability of just two modes to describe rain variabiltiy over an area can explain why methods of estimating area-averaged rain rate from the area covered by rain rates above a certain threshold are so successful.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Applied Meteorology (ISSN 0894-8763); 33; 9; p. 1067-1078
    Format: text
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  • 2
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    Detecting the diurnal cycle of rainfall using satellite observations (1993)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The diurnal cycle in rainfall varies considerably from region to region in the tropics. Determining this variability is important both for comparing predictions of atmospheric models to real atmospheric behavior and for making sure that estimates of total rainfall from low-altitude satellites are not biased because of their infrequent observations of a given region of the earth. Although there are no data from the proposed Tropical Rainfall Measuring Mission (TRMM) satellite to work with yet, we can ask how well the diurnal cycle in rainfall will be detected when the satellite is eventually collecting data, given the satellite's proposed sampling characteristics. Data analyses for the diurnal cycle are discussed, taking into account the fact that the satellite visits will be irregularly spaced in time. The amplitudes of the first few harmonics will be determined by least-squares fits to the satellite observations, and the tests needed to establish the statistical significance of the fitted amplitudes are discussed. The accuracy with which the first few harmonics of the diurnal cycle can be detected is estimated from several months of satellite data using rainfall statistics observed during the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE).
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Applied Meteorology (ISSN 0894-8763); 32; 2; p. 311-322.
    Format: text
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  • 3
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    The relation of radar to cloud area-time integrals and implications for rain measurements from space (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The relationships between satellite-based and radar-measured area-time integrals (ATI) for convective storms are determined, and both are shown to depend on the climatological conditional mean rain rate and the ratio of the measured cloud area to the actual rain area of the storms. The GOES precipitation index of Arkin (1986) for convective storms, an area-time integral for satellite cloud areas, is shown to be related to the ATI for radar-observed rain areas. The quality of GPI-based rainfall estimates depends on how well the cloud area is related to the rain area and the size of the sampling domain. It is also noted that the use of a GOES cloud ATI in conjunction with the radar area-time integral will improve the accuracy of rainfall estimates and allow such estimates to be made in much smaller space-time domains than the 1-month and 5-deg boxes anticipated for the Tropical Rainfall Measuring Mission.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Monthly Weather Review (ISSN 0027-0644); 120; 9, Se
    Format: text
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  • 4
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    A space-time stochastic model of rainfall for satellite remote-sensing studies (1987)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A model of the spatial and temporal distribution of rainfall is described that produces random spatial rainfall patterns with these characteristics: (1) the model is defined on a grid with each grid point representing the average rain rate over the surrounding grid box, (2) rain occurs at any one grid point, on average, a specified percentage of the time and has a lognormal probability distribution, (3) spatial correlation of the rainfall can be arbitrarily prescribed, and (4) time stepping is carried out so that large-scale features persist longer than small-scale features. Rain is generated in the model from the portion of a correlated Gaussian random field that exceeds a threshold. The portion of the field above the threshold is rescaled to have a lognormal probability distribution. Sample output of the model designed to mimic radar observations of rainfall during the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE), is shown. The model is intended for use in evaluating sampling strategies for satellite remote-sensing of rainfall and for development of algorithms for converting radiant intensity received by an instrument from its field of view into rainfall amount.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 92; 9631-964
    Format: text
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  • 5
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    Theory of optimal weighting of data to detect climatic change (1986)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A search for climatic change predicted by climate models can easily yield unconvincing results because of 'climatic noise,' the inherent, unpredictable variability of time-average atmospheric data. A weighted average of data that maximizes the probability of detecting predicted climatic change is presented. To obtain the optimal weights, an estimate of the covariance matrix of the data from a prior data set is needed. This introduces additional sampling error into the method. This is presently taken into account. A form of the weighted average is found whose probability distribution is independent of the true (but unknown) covariance statistics of the data and of the climate model prediction.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of the Atmospheric Sciences (ISSN 0022-4928); 43; 1694-171
    Format: text
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  • 6
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    Sampling errors for satellite-derived tropical rainfall - Monte Carlo study using a space-time stochastic model (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Estimates of monthly average rainfall based on satellite observations from a low earth orbit will differ from the true monthly average because the satellite observes a given area only intermittently. This sampling error inherent in satellite monitoring of rainfall would occur even if the satellite instruments could measure rainfall perfectly. The size of this error is estimated for a satellite system being studied at NASA, the Tropical Rainfall Measuring Mission (TRMM). First, the statistical description of rainfall on scales from 1 to 1000 km is examined in detail, based on rainfall data from the Global Atmospheric Research Project Atlantic Tropical Experiment (GATE). A TRMM-like satellite is flown over a two-dimensional time-evolving simulation of rainfall using a stochastic model with statistics tuned to agree with GATE statistics. The distribution of sampling errors found from many months of simulated observations is found to be nearly normal, even though the distribution of area-averaged rainfall is far from normal. For a range of orbits likely to be employed in TRMM, sampling error is found to be less than 10 percent of the mean for rainfall averaged over a 500 x 500 sq km area.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 2195-220
    Format: text
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  • 7
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    Climate sensitivity from fluctuation dissipation - Some simple model tests (1980)
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    Publikationsdatum: 2011-08-18
    Beschreibung: Leith has suggested that climatic response to change in external forcing parameters of the climate system may be estimated via the fluctuation-dissipation theorem (FDT). The method, which uses the natural fluctuations of the atmosphere to probe its dynamics, is tested here using a twenty-variable truncation model of the barotropic vorticity equation. Dissipative terms are added to the equations, so that the model is pushed away from the region where it is expected to satisfy the FDT. It is found that, even though the FDT is no longer satisfied in every detail, the FDT continues to provide an excellent estimate of the climatic sensitivity of the model.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Journal of the Atmospheric Sciences; 37; Aug. 198
    Format: text
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  • 8
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    Sampling problems (1981)
    In:  Other Sources
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    Publikationsdatum: 2011-08-18
    Beschreibung: Precipitation estimates from satellites are subject to a number of uncertainties involving design characteristics, satellite positioning, natural variability of precipitation, and the noncontinuous acquisition of data. The sources and sizes of these uncertainties are in need of proper evaluation and estimation. The present sampling and estima-theory seems to be adequate for some measurement problems (e.g., determining precipitation at a point), while others require further theoretical work (e.g., determining the time history of precipitation over large areas).
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Precipitation Meas. from Space:; 5 p
    Format: text
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  • 9
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    Sampling errors in the estimation of empirical orthogonal functions (1982)
    In:  Other Sources
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    Publikationsdatum: 2011-08-18
    Beschreibung: Empirical Orthogonal Functions (EOF's), eigenvectors of the spatial cross-covariance matrix of a meteorological field, are reviewed with special attention given to the necessary weighting factors for gridded data and the sampling errors incurred when too small a sample is available. The geographical shape of an EOF shows large intersample variability when its associated eigenvalue is 'close' to a neighboring one. A rule of thumb indicating when an EOF is likely to be subject to large sampling fluctuations is presented. An explicit example, based on the statistics of the 500 mb geopotential height field, displays large intersample variability in the EOF's for sample sizes of a few hundred independent realizations, a size seldom exceeded by meteorological data sets.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Monthly Weather Review; 110; July 198
    Format: text
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  • 10
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    The latitude dependence of the variance of zonally averaged quantities (1982)
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    Publikationsdatum: 2011-08-18
    Beschreibung: Geometric characteristics of the spherical earth are shown to be responsible for the increase of variance with latitude of zonally averaged meteorological statistics. An analytic model is constructed to display the effect of a spherical geometry on zonal averages, employing a sphere labeled with radial unit vectors in a real, stochastic field expanded in complex spherical harmonics. The variance of a zonally averaged field is found to be expressible in terms of the spectrum of the vector field of the spherical harmonics. A maximum variance is then located at the poles, and the ratio of the variance to the zonally averaged grid-point variance, weighted by the cosine of the latitude, yields the zonal correlation typical of the latitude. An example is provided for the 500 mb level in the Northern Hemisphere compared to 15 years of data. Variance is determined to increase north of 60 deg latitude.
    Schlagwort(e): METEOROLOGY AND CLIMATOLOGY
    Materialart: Monthly Weather Review; 110; May 1982
    Format: text
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