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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

The hypothesis that Indian Ocean sea surface temperatures are linked to the intensity of the Indian summer monsoon was tested using sea temperature, rainfall and sea level pressure data for the period 1949–72. The data sets were compacted using empirical orthogonal function analyses. The time coefficients of the most important functions were used to statistically test the general hypothesis. The results suggest that in summer a warmer Arabian Sea or Indian Ocean is weakly associated with decreased rainfall and increased sea level pressure over much of the Indian subcontinent. The relationship between the Indian summer monsoon and the sea surface temperatures of the eastern tropical Pacific Ocean was also investigated. This analysis suggests that higher than normal Indian sea level pressures are often associated with higher eastern Pacific sea temperatures one month later and higher Indian region sea temperatures another month later.

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Bryan C. Weare

Abstract

The El Niño index suggested by Weare, Navato and Newell is extended to include the period 1974–83.This index is based upon the most important empirical orthogonal function of interannual Pacific sea surface temperature (SST) departures for the period 1949–73. The index for the entire period 1949–83 is presented in both graphical and tabular forms. It is also compared with another El Niño index based upon tropical SST departures calculated by Wright.

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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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Yiping Zhao and Bryan C. Weare

Abstract

A simple shallow-water Kelvin wave-CISK model on the equatorial beta plane is used to investigate the effect of high-frequency variations, typically a diurnal cycle, of cumulus convection on large-scale low-frequency wave modes in the Tropics.

A high-frequency variation of cumulus convection, such as the diurnal cycle, significantly modifies the wave- CISK instability of large-scale wave modes. When the intensity of the diurnal variation is relatively small, a few of the largest-scale waves remain unstable and the rest are neutralized (growth rates are zero). When the intensity of the diurnal variation is relatively large, wavenumber-one mode may remain unstable with a substantially reduced growth rate, while all other waves are neutralized. These findings suggest that, in general, the weak diurnal variation of cumulus convection over the tropical oceans favors the planetary-scale selection for the intraseasonal oscillation. On the other hand, strong diurnal variation of cumulus convection as observed over equatorial continental areas such as the Amazon Basin and Africa may contribute to the observed weak signals of the intraseasonal oscillation.

As the period of cumulus convection variation increases from the diurnal one, there is a threshold value of the period for a specific large-scale wave (wavenumber is specified), such that cumulus convection with a period shorter than the threshold value stabilizes the large-scale wave modes. Others turn the monotonically growing mode in the traditional wave-CISK framework into an oscillatory growing mode. The intensity of cumulus convection variation has an inverse effect on the threshold period.

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Charles Jones and Bryan C. Weare

Abstract

This paper examines whether or not low-level moisture convergence and surface latent heat flux act as forcing mechanisms of the Madden and Julian oscillation (MJO), as it is proposed by the theories of wave-CISK (conditional instability of the second kind) and evaporation-wind feedback. The mean brightness temperature of cloudy pixels at 11 μm, obtained from five years of International Satellite Cloud Climatology Project data, is used as a proxy for tropical convective activity. Five years of European Centre for Medium-Range Weather Forecasts analyses are used to estimate surface latent heat fluxes and moisture divergence integrated in the low levels of the troposphere.

Spectral analysis of latent heat fluxes over the Indian and Pacific Oceans shows significant spectral peaks in the frequency band of the MJO. These peaks are due mainly to the oscillation in the surface wind speed rather than in the specific humidity difference. Principal component analysis and tagged correlation patterns of filtered time series 20–70 days are used to investigate the relationships between anomalies in convection, surface latent heat fluxes, and low-level moisture divergence. The correlation patterns show that negative anomalies of latent heat fluxes are systematically observed to the east, whereas positive anomalies are observed to the west of the region of convection. Positive anomalies of surface latent heat flux tag time variations in convection by about 4 days. This result contrasts with the basic requirement of the evaporation-wind feedback theory, which claims that evaporation anomalies are positive on the eastern side of the convective region. In contrast, tag correlation patterns indicate that the region of maximum low-level moisture convergence is located to the east of the region of convection, and low-level moisture convergence leads time variations in convective activity by about 2 days. This observational result supports the frictional wave-CISK theory as a mechanism of the MJO.

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Bryan C. Weare and Igor I. Mokhov

Abstract

Total cloudiness of 29 models participating in the Atmospheric Model Intercomparison Project is compared with the ISCCP C2 as well as the Nimbus-7 and Meteor observational estimates. The root-mean-square differences between the annual means of the model calculations and the C2 observations after global means are removed vary from about twice to nearly four times the difference between the C2 and Meteor observations. The large differences are in some cases due to the fact that although a model qualitatively has patterns of spatial variations similar to those of the observations, the magnitude of those variations is much too small. In other cases the models have produced the approximate magnitude of the spatial variability of the observations but display sizable errors in the pattern of that variability.

Deficiencies with respect to the model simulations of the mean seasonal cycle are also pronounced. For instance, the differences between the zonal averages of total cloudiness for contrasting seasons suggest that near 60° most models predict minima in cloudiness in summer, whereas observations strongly suggest the opposite. In addition, smoothed seasonal cycle analyses suggest that a portion of these deficiencies in some models is the result of a simulated seasonal cycle that leads that of the observations by about two months. However, some models, which appear to have the proper phase of the seasonal cycle, still show large root-mean-square differences and small correlations when compared with the smoothed seasonal cycle of the C2 observations. The C2 and Meteor observations show a modest signal in total cloudiness for the only important interannual variation during the July 1983 through June 1988 observation period—the 1986/87 ENSO event. A few models reproduce this event about as well as do the Meteor observations, whereas many models fail to show any evidence of it.

Overall, models that better reproduce the ENSO results also tend to do well with seasonal variations. No specific differences are evident in the physical characteristics of models that are relatively adept at reproducing seasonal and interannual variations and those that perform more poorly. However, there is the general conclusion that models that have more sophisticated physical processes tend to better simulate the cloud observations.

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Bryan C. Weare and Amip Modeling Groups

Abstract

Estimates of zonally averaged cloudiness at each pressure level in 24 models participating in the Atmospheric Model Intercomparison Project are compared with the ISCCP C2 as well as the Nimbus 7 (N7) and Warren et al. (hereafter WH) observations. The global means of model high cloudiness are about two to five times greater than the C2 satellite observations. The large differences are probably related to excessive high, thin cloud in most models. Nearly all of the models have the observed maximum in high cloud at the equator, but also maxima near 60°N and 60°S, which are not observed. The globally averaged annual mean low cloud in most models is generally 15%–20% less than the WH observations and 10%–15% less than the C2 observations. The meridional structure of model annual mean low cloud both as observed from below and as observed from above show excesses north of about 50°N and deficits south of about 40°S when compared with WH and C2 observations, respectively.

The amplitude of the model seasonal cycle of high cloud in most cases is comparable to that of the C2 observations. However, nearly all models have tropical peaks in seasonal variability that are poleward of those in the observations. In most models the variation of the seasonal cycle of low cloud as observed from above differs considerably in both temporal phase and spatial pattern from that of the C2 observations.

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Craig E. Motell and Bryan C. Weare

Abstract

Statistical models that estimate tropical Pacific rainfall from the National Oceanic and Atmospheric Administration's global archive of polar-orbiter satellite data have been derived and tested. These rainfall models are based on the assumptions that rainfall is linearly related to bright visible and cold infrared radiation (IR) satellite. The models were derived by using measured monthly rainfall from small, flat, tropical islands with elevations less than 30 m together with digital IR and visible satellite data.

Three models were derived: one used visible and nighttime IR data (NIRVISQ); the second used only visible data (VISQ), and the third used an average of daytime and nighttime IR data (AVEIR). These models were found to predict between 62% and 67% of the variance of 1051 station-months of hindcast rainfall data measured from June 1974 through mid-March 1978 (J74M78). However, rainfall was found to be underpredicted on relatively high mean rainfall islands and vice versa. Similar prediction accuracies were found when the rainfall models were used to estimate rainfall on new low-latitude island stations during the J74M78 period. All three models showed a decrease in predictive skill during time periods after J74M78.

Tropical Pacific annual rainfall maps, estimated using the rainfall models and satellite data from June 1974 through May 1977, showed that NIRVISQ and VISQ may greatly overpredict rainfall in regions where stratus clouds are common such as in the eastern Pacific Ocean, but AVEIR appeared to predict reasonable rainfall amounts throughout the tropical Pacific. The AVEIR is thus the preferred model for predicting tropical oceanic rainfall.

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Bryan C. Weare and Fred M. Snell

Abstract

No abstract available.

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