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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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Bryan C. Weare

Abstract

The role of moisture variations in the initiation of Madden–Julian oscillation (MJO) variability is examined. Composite singular value decomposition (CSVD) analyses at various time lags are carried out to discern the complex space–time relationships between convection, low-level specific humidity and divergence, and surface evaporation. The utilized data are low-level moisture and winds and surface evaporation from the NCEP–NCAR reanalysis and outgoing longwave radiation (OLR) from NOAA satellite observations for the period 1981–2000. These data are filtered at each point using a 150-point Lanczos filter capturing well the 20–100-day periodicities.

The two dominant CSVDs describe an eastward-propagating quasi-wavenumber-1 system in all of the analyzed variables. The dominant low-level divergence, moisture, and latent heat flux variations lead those of OLR by approximately 10 days. The low-level convergence and positive moisture anomalies develop near and to the east of the ensuing convective perturbations; positive latent heat flux variations generally occur farther to the east. Moisture variations develop at least 15 days prior to a convective event and have stronger correlations with OLR than those of simultaneous low-level divergence. Near the centers of convection the low-level moisture increases at the same time that the 1000-hPa flow is becoming slightly more divergent. This implies that the moisture preconditioning of convective events is not driven totally by moisture convergence. This may require a modification of the frictional wave conditional instability of the second kind (CISK) hypothesis for MJO development.

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Bryan C. Weare

Abstract

Comparisons are made between the spatial patterns of solar and thermal fluxes of radiation calculated by the UCLA general circulation model and observations. The latter include estimates at the Pacific Ocean surface of the climatological averages of net solar and thermal radiation, satellite observations of the climatological averages of planetary albedo and outgoing longwave radiation and satellite observation of the anomalies of albedo and outgoing longwave, for the December–February period of the 1982/83 El Niño.

The results show differences between model and observed climatological net solar radiation at the surface, and planetary albedo are up to about 25% of the mean with these differences closely tied to errors in the specification of model cloudiness. Model climatological net longwave radiation at the surface is systematically larger than the estimates of the observations by up to about 50% of the mean. The differences seem to he relatively unrelated to errors in model generated total cloudiness Systematic differences of around 5% of the mean are also evident between model climatological outgoing longwave at the top of the atmosphere and satellite observation. Model generated El Niño anomalies of planetary albedo and outgoing longwave radiation generally agree with observations as to position but underestimate magnitudes by up to a factor of 4. The potential causes and significance of them results are briefly discussed.

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Bryan C. Weare

Abstract

Centered composite analysis is described and applied to gain a better understanding of the initial phases of the Madden–Julian oscillation (MJO). Centered composite analysis identifies the dates and central locations of key events. The elements of the composite means are centered on these central locations before averages are calculated. In this way much of the spatial fuzziness, which is inherent in traditional composite analysis, is removed. The results for the MJO, based on MJO-filtered outgoing longwave radiation for the reference data and 40-yr ECMWF Re-Analysis (ERA-40) and NCEP–NCAR reanalysis products for the composites, show highly significant composites of unfiltered data for not only zero lag, but also lags back to 20 days before the target events. These composites identify propagating patterns of surface pressure, upper- and lower-troposphere zonal winds, surface temperature, and 850-hPa specific humidity associated with MJO convective events in the Indian Ocean. The propagation characteristics of important features, especially surface pressure, differ substantially for MJO convective anomalies centered over the Indian or western Pacific Oceans. This suggests that distinctly different mechanisms may be dominant in these two regions, and that many earlier analyses may be mixing properties of the two.

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Bryan C. Weare

Abstract

Longwave and shortwave cloud radiative forcing from the recently released National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses are compared to Earth Radiation Budget Experiment (ERBE) observations. The observed differences are analyzed utilizing concurrent International Satellite Cloud Climatology Project (ISCCP) estimates of cloudiness and other satellite observations.

The results show that the NCEP–NCAR longwave cloud forcing agrees well with that of ERBE not only for the annual means but also for seasonal and climatic variations. Areas of disagreement are generally related to disagreements between NCEP–NCAR high cloudiness and observations. Overall, the NCEP–NCAR shortwave cloud forcing is in poorer agreement with ERBE observations. NCEP–NCAR annual means in the Tropics are often 20–30 W m−2 too negative. On the other hand the NCEP–NCAR total cloud cover in this region is 10%–20% less than the ISCCP observations, which should lead to less, rather than more, negative shortwave cloud forcing. Thus the primary error in the mean shortwave cloud forcing is likely due to specification of clouds that are too reflective in the NCEP analysis model. Moderate errors in the variability of NCEP–NCAR SWCF are apparently related to errors in the analyzed seasonal variability of total cloudiness, which are exacerbated by NCEP model specification of clouds that are too bright and underestimates of the seasonal variability of the clear-sky fluxes.

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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

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

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

Zonal averages of low, middle and total cloud amount estimates derived from measurements from Nimbus-7 have been analyzed for the six-year period April 1979 through March 1985. The globally and zonally averaged values of six-year annual means and standard deviations of total cloud amount and a proxy of cloud-top height are illustrated. Separate means for day and night and land and sea are also shown. The globally averaged value of intra-annual variability of total cloud amount is greater than 7%, and that for cloud height is greater than 0.3 km. Those of interannual variability are more than one-third of these values. Important latitudinal differences in variability are illustrated. The dominant empirical orthogonal analyses of the intra-annual variations of total cloud amount and heights show strong annual cycles, indicating that in the tropics increases in total cloud amount of up to about 30% are often accompanied by increases in cloud height of up to 1.2 km. This positive link is also evident in the dominant empirical orthogonal function of interannual variations of a total cloud/cloud height complex. This function shows a large coherent variation in total cloud cover of about 10% coupled with changes in cloud height of about 1.1 km associated with the 1982–83 El Niño–Southern Oscillation event.

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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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