Search Results

You are looking at 1 - 10 of 22 items for :

  • Author or Editor: Bryan C. Weare x
  • Journal of Climate 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

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

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.

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
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.

Full access
Bryan C. Weare

Abstract

The role of moisture variations in the initiation of Madden–Julian oscillation (MJO) variability is reexamined through composite singular value decomposition (CSVD) analyses using the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr Re-Analyses (ERA-40) data. The CSVD analyses at various time lags are carried out to discern the complex space–time relationships between convection identified using outgoing longwave radiation and 1000-hPa divergence, 850-hPa specific humidity, and surface evaporation. The most striking difference from the earlier analyses using NCEP–NCAR reanalysis data is that the observed relations between 20–100-day filtered OLR and · V 1000 anomalies are weaker and less significant in the current analyses. On the other hand, both analyses show increasing low-level moisture near and to the east of the developing convection. Thus, both results imply that moisture preconditioning of convective events is not totally driven by boundary layer moisture convergence.

Full access
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.

Full access