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Bryan C. Weare

Abstract

Regional estimates of low, middle, high, and total cloud amounts derived from bispectral measurements from Nimbus-7 have been analyzed for the six-year period April 1979 through March 1985. Fractional cloud cover for the three height categories was used to calculate a proxy mean cloud-top height. Intra- and interannual standard deviations of total cloud amount and mean cloud height show realistic patterns throughout most of the globe except at very high latitudes. Over much of the cash, intra-annual and interannual variations in total cloud amount are strongly positively correlated with variations in cloud height. Furthermore, both total cloud amount and cloud height variations are moderately correlated with sea surface temperature variations. The strongest correlations are positive in the tropics for both intra-annual and interannual variations. In middle latitudes, moderate negative correlations are positive with intra-annual variations, whereas moderate positive correlations occur on interannual time frames. In the tropics 1°C changes in temperature are statistically related to a change of total cloudiness of at least 2% and a change in cloud height of more than 0.5 km.

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Bryan C. Weare

Abstract

A new method for combining satellite and surface-based cloud observations into a self-consistent three-dimensional field is presented. This method derives the probabilities of the cloud states, which are most consistent with all of the observations and assumptions concerning the nature and relative uncertainties of the observations. It is applied to a three-layer atmosphere using monthly satellite- and surface-based cloud observations. The reconstructions of the observed fields usually lead to modifications of the surface-observed low cloud amount of less than 0.008 fractional cloud cover. Over the ocean the satellite-view low cloud amounts are usually decreased by between 0.06 and 0.12 for most of the middle latitudes and southeastern tropical Pacific. Over land the adjustments in the satellite low cloud amounts are generally smaller. The method leads to increases in satellite high cover of between 0.03 and 0.09 over most regions, and increases in middle cloud cover of up to around 0.03 over the subtropical oceans. Comparisons between derived total cloud cover and that calculated with the commonly used random and mixed overlap assumptions suggest that the mixed assumption generally better fits the results. On the whole there is overall fairly good agreement between the percent low cloud relative to total cloud cover in the reconstructed observations and a global climate model, but the model has a far larger percentage of high clouds nearly everywhere, especially in the tropical convective regions and over the Indian subcontinent.

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Bryan C. Weare

Abstract

The relationships between the patterns of monthly sea surface temperature (SST) variations and those of precipitation in the tropical Pacific Ocean region are examined. The rainfall data utilized are derived from satellite observations of outgoing longwave radiation. A composite empirical orthogonal function (EOF) analysis of the SST and precipitation indicates a dominant mode of variation linking SST variations in the eastern equatorial region with those of MNWI about 30° westward. One-point correlation analyses show that this general relationship is present for all points in the eastern and central ocean, but that no significant SST-rainfall correlations are evident for SST points wen of about the dateline. The one-point correlation analyses also suggest that during time periods outside of El Niñ's, the rainfall response to SST changes is largely confined to the areas of climatological precipitation maxima, suggesting only a minor alteration in the large-scale circulation. On the other hand, during El Niñ there is the strong suggestion that circulation changes give rise to complete shifts in rainfall zones. The possible influence of the strong spatial autocorrelations of SST on these results are also explored using the one-point correlations. It is concluded that the observed SST-rainfall teleconnections cannot be wholly explained by the large-scale nature of the SST variations.

The inferences derived from these analyses are utilized in formulating linear regression (LR) models to specify tropical precipitation anomalies based upon a knowledge of concurrent SST perturbations. Preliminary analyses suggest that while relatively large hindcast skills are evident in various LR models, the apparent skills decrease substantially when the models are applied to new data. While these results do not prove that such models can never be very useful, they do project difficulties in greatly increasing the skill, especially as long as the available data periods are relatively short

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Bryan C. Weare

Abstract

An additionally constrained multiple linear regression technique is outlined for use with the prediction or specification of fields of geophysical variables. The additional constraints are the requirement that the predictions or specifications have spatial interrelationships which are similar to the dominant empirical orthogonal functions of the dependent variables. Since the traditional multiple linear regression estimates by definition minimize the mean square errors for the data used to develop the model, the utility of the additional constraint can be evaluated only when the regression models are tested on new data. A so-called jackknife technique is used in this regard.

This additionally constrained multiple linear regression technique is tested on two examples of the specification of monthly values using independent data for the same month. The first is the specification of North Pacific sector 700-mb geopotential heights using the time coefficients of the first two empirical orthogonal functions of Pacific sea surface temperature. The second is the specification of monthly precipitation totals at 36 stations in the western United States using 700-mb heights. Use of the additional constraint leads to average increases in observed-predicted correlations of between 5 and 30% when used with new data. These improvements are quite evenly distributed over nearly all of the points of the fields of the dependent variable.

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Bryan C. Weare

Abstract

Generalized maximum covariance analysis (GMCA) has been developed and applied to diagnosing the relationships between ENSO tropospheric heating variations and tropical stratospheric waves. GMCA identifies the most important patterns of covariability between interannual tropospheric heating variations and eddy zonal and meridional velocities, temperatures, and ozone mixing ratios in the tropics between 200 and 10 hPa. The first two sets of GMCA time coefficients have variations that are strongly related to ENSO and are highly correlated at a lag of about a year. The diagnosed spatial patterns have broad wavenumber 1 characteristics, which are associated with ENSO. These dominant modes of heating variations are linked to a rich three-dimensional pattern of stratospheric eddy perturbations over a wide range of lags. Generally, all major features propagate slowly to the east along with the tropospheric heating anomaly. In addition there is strong vertical coherence such that the strongest anomalies tilt westward from the bottom to the top of the domain. This tilt is associated with propagating wavenumber-1 gravity waves. The patterns are such that, in the lower stratosphere, regions of divergence, corresponding to upward motion, are associated with lower temperatures and reduced ozone mixing ratios and vice versa. These findings are consistent with adiabatic cooling of rising low–ozone concentration tropospheric air. Evidence suggests that the analyzed eddy variations of temperatures and winds are contributing to systematic changes in the zonal mean circulation.

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Bryan C. Weare

Abstract

Monthly means of selected variables of the 2.5° International Satellite Cloud Climatology Project (ISCCP) C2 total cloud cover (CC), cloud-top pressure (CTP), and cloud water (CW) are statistically related to sea surface temperature (SST). The statistical tools utilized include intra- and interannual correlation, regression, and composite empirical orthogonal function (EOF) analyses.

The dominant intra- and interannual composite EOFs all show that CC, CTP, and CW departures have spatially coherent links with those of SST. The second most important intra-annual functions also show coherent relations, which are about three months out of phase with those of the dominant functions. The regression analysis suggests that this phase relation may be explained by significant correlations of the cloud variables with not only SST, but also with the time derivative of SST (dSST/dt). For instance, in the tropical Pacific increased CC is accompanied by increases in SST but decreases in dSST/dt, and increased CTP is associated with decreases in SST. However, at middle and high latitudes other relationships exist, such that larger CCs may be associated with decreased SSTs, or higher CTPs may be related to higher SSTs. These diagnosed relationships have important implications for understanding cloud and cloud radiative feedbacks in weather and climate.

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Bryan C. Weare

Abstract

Multilag singular value decomposition (MLSVD) analysis is developed and applied to diagnosing the impact of interannual variations of outgoing longwave radiation (OLR) on tropical stratospheric temperature changes. MLSVD is designed to analyze simultaneously variations at multiple levels and for a large number of temporal lags and leads. The two dominant MLSVDs are strongly related to El Niño–Southern Oscillation (ENSO). The associated patterns of tropical OLR are similar to the canonical ENSO SST patterns with strong negative sign regions stretching along the equator in the eastern and central Pacific. These dominant modes are strongly linked to temperature perturbations at a wide range of lags. At the lowest analyzed level (200 hPa) and zero lag positive temperatures anomalies are in the region of low OLR. In the lower stratosphere near 100 hPa, strong negative temperature perturbations replace the positive values of the lowest level. Higher in the stratosphere near 20 hPa, equatorial temperature perturbations are again positive, but with a more zonally elongated spatial pattern. Overall, the equatorial temperature anomalies propagate slowly to the east, at a speed strongly related to ocean–atmosphere coupling of less than 1 m s−1, and vertically and westward into the stratosphere by Rossby waves with a speed in the range of 30 m s−1.

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Bryan C. Weare

Abstract

The relationships between the net radiation at the surface and the top of the atmosphere in the UCLA general circulation model are investigated. These suggest that it may be possible to formulate statistical models from limited observations relating satellite derived net radiation at the top of the atmosphere to net radiation at the earth's surface. However, the results suggest it may not be possible to develop comparable statistical models to infer net infrared radiation at the surface or the vertically integrated radiative heating of an atmospheric column.

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Bryan C. Weare

Abstract

This paper analyzes several near-global datasets of low cloud cover, including the the International Satellite Cloud Climatology Project (ISCCP) satellite observations, C. J. Hahn et al. surface-derived observations, and the National Centers for Environmental Prediction (NCEP) and ECMWF reanalysis products (ERA). The magnitudes of annual-mean ISCCP and C. J. Hahn observations of low cloud fraction are found to differ by up to about 0.4 for a number of locations. These differences are largely attributable to the fact that ISCCP low clouds are only those low clouds that are not obstructed by higher cloud. Those of both the NCEP and ERA low clouds, which should be comparable to the Hahn low cloud dataset, have magnitudes up to about 0.3 less than the latter. The dominant EOFs of the seasonal variation of ISCCP and Hahn observations low cloud differ substantially over much of the Northern Hemisphere, where there is a sizable number of observations in Hahn. The pattern of the dominant seasonal EOF of NCEP low clouds has a number of qualitative similarities with that of Hahn between approximately 10° and 40°N. That of the ECMWF low clouds is less similar and has much larger amplitudes in the high latitudes of both hemispheres than any of the other datasets. The calculated regression coefficients between interannual variations of Niño-3 SST variations and low cloud departures in the equatorial central Pacific have positive magnitudes of about 0.02 (°C)−1 for the C. J. Hahn et al. and NCEP data, but negative values of similar magnitudes for the ISCCP and ECMWF low cloud fractions. These results suggest a need for improved observational estimates and model specifications of the three-dimensional structure of clouds.

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Bryan C. Weare

Abstract

The factors controlling the Earth Radiation Budget satellite (ERBS) longwave clear sky and cloud-forcing fluxes are investigated using statistical analyses of the ERBS fluxes with International Satellite Cloud Climatology Project (ISCCP) cloud and ECMWF thermodynamic variables. For both land and ocean between 60°S and 60°N statistically significant models exist relating interannual variations of clear sky fluxes and surface temperature, precipitable water, tropospheric temperature, total cloud amount, and cloud-top pressure. An analysis of mean clear sky fluxes suggests that mean ERBS longwave clear sky fluxes are overestimates of the “true” values by between +2 and +10 W m−2 over the area investigated. The biases appear to be most related to errors in the detection or exclusion of low clouds in the Earth Radiation Budget Experiment clear sky determination algorithm. Other statistical models show that variations in total cloud amount and cloud-top pressure control cloud forcing over oceans, but that total cloud amount and clear sky flux are most important over land. Thus over land, especially at higher latitudes, care must be taken not to interpret changes in cloud forcing solely in terms of variations in cloud parameters.

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