Search Results

You are looking at 1 - 10 of 43 items for

  • Author or Editor: Bryan C. Weare x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
Bryan C. Weare

Abstract

Diabatic heating rates calculated in the UCLA general circulation model are analyzed. The heating rates are for the December–January–February season for the mean of four climatological runs and the mean of four runs with observed 1982/83 sea surface temperatures as boundary conditions. Vertically integrated total diabatic heating for the climatological calculations agrees well with observations. Unfortunately, the available observations of zonally averaged vertical structure differ from each other so substantially that comparisons with the model are inconclusive. Nevertheless, the vertical structure of the model seems quite realistic and in general agreement with analyses of tropical cloud clusters or middle latitude cyclones.

The model diabatic heating is stratified in a number of ways. Zonal average height-latitude cross sections of the heating due to cumulus and large-scale precipitation and radiation show complex profiles for the rates associated with precipitation and much less structure with those associated with radiation except near the ground. Sample vertical profiles for regions in the tropics and new 45°N suggest that the zonal averages are generally representative of the individual heating profiles except near the surface.

Anomalous heating rates associated with the model 1982/83 season, in which a a strong El Niño was observed, show a moderate alteration in diabatic heating over the breadth of the tropical Pacific. In the eastern equatorial Pacific, increases are evident for the heating rates associated with cumulus and large-scale precipitation and radiation. Reductions in cumulus and radiative heating over other parts of the tropics are also apparent.

Full access
Bryan C. Weare

Abstract

A linear shallow water model on an equatorial β-plane is driven by three different estimates of the heating rate perturbation during seven seasons of a composite El Niño episode. In two cases the heating perturbations for this model are assumed to be closely linked to the pattern of sea surface temperature anomalies. In the, third the heating is related to direct estimates of precipitation departures. The model-derived winds based upon these three estimates are compared to one another and to data available from marine weather reports. An analysis of area averaged winds, surface latent heat fluxes and moisture convergences shows general agreement with observations over the composite El Niño seasons for the results of all three forcing estimates, although the model results using the direct precipitation estimates seem generally to be the poorest. A second analysis explores the similarity of smaller-scale features of the model results to the observations. This suggests that the simple parameterized heating using sea surface temperature estimates gives the only solutions which fairly well identify smaller-scale, variations.

Full access
Bryan C. Weare

Abstract

Zonal average temperature for each 15° latitude band of the globe for six atmospheric levels is computed for crush mouth between May 1958 and December 1974. The averages are based on reports at a total of 190 WMO stations. Monthly and annual means, interannual variances and one-month lag correlations are presented. The correlations between zonal mean temperatures at different levels within the same latitudinal zone and different zones at the same level are also illustrated. Sample power spectra are presented for middle-latitude and subtropical zonal mean temperatures.

Full access
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.

Full access
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.

Full access
Bryan C. Weare

Abstract

Interannual variations of net surface heating of the tropical Pacific Ocean are analyzed for the period 1957–76. Special emphasis is given to exploring the relationship between these variations and those in sea temperature associated with El Niño/Southern Oscillation. The analyses include eigenvector analysis, composites of the net heating for various phase of El Niño, and time series analysis of various measures of the variability. The results indicate large-scale patterns of variability dominated by time scales greater than one year. A link between the large-scale variations of sea temperature and net surface heating is evident from each set of analyses. In general, anomalously high heating appears to be associated with cold water. However, it also seems apparent that greater than average heating of the ocean persists for several months into the pe6ods of El Niño when sea temperatures are rising rapidly. Thus it is hypothesized that net surface heating contributes to the development of the early stages of an El Niño warm period.

Full access
Bryan C. Weare

Abstract

A shallow water equations model of the tropical atmosphere is developed and tested. The model includes a simple parameterization of convective rainfall so that the model heating is a function of fixed sea surface temperatures and model dependent circulation. To achieve quasi-steady states the model is iterated a number of steps.

For equatorial sea temperature perturbations imposed on an idealized Pacific Ocean, the model shows two distinct modes of response. In one mode local evaporation is slightly more important than moisture convergence. This results in a relatively weak, equatorially trapped perturbation circulation. In the other mode moisture convergence is dominant and the associated circulation is stronger and less equatorially trapped. The necessary condition for the second “enhanced” mode is that the positive sea temperature perturbations overlap sufficiently with the climatologically warm waters of the western ocean.

Subtropical sea temperature perturbations generally result in a steady state heating with a relatively broad meridional scale and moderate magnitude. The final states are relatively insensitive to the position of the sea temperature perturbations relative to the mean sea temperature field.

While these results must be viewed as quite preliminary due to the simple nature of the dynamical model, they do suggest that actual atmospheric heating perturbations may be the result of complex interactions between oceanic and atmospheric conditions by way of both the surface evaporation and moisture convergence processes.

Full access
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.

Full access