Observations of planet Earth and especially all climate system components and forcings are increasingly needed for planning and informed decision making related to climate services in the broadest sense. Although significant progress has been made, much more remains...
Long-term trends in precipitable water (PW) are an important component of climate change assessments for the Tibetan Plateau (TP). PW products from Moderate Resolution Imaging Spectroradiometer (MODIS) are able to provide good spatial coverage of PW over the TP but limited in time coverage, while the meteorological stations in the TP can estimate long-term PW but unevenly distributed. To detect the decadal trend in PW over the TP, Bayesian inference theory is used to construct long-term and spatially continuous PW data for the TP based on the station and MODIS observations. The prior information on the monthly-mean PW from MODIS and the 63 stations over the TP for 2000–06 is used to get the posterior probability knowledge that is utilized to build a Bayesian estimation model. This model is then operated to estimate continuous monthly-mean PW for 1970–2011 and its performance is evaluated using the monthly MODIS PW anomalies (2007–11) and annual GPS PW anomalies (1995–2011), with RMSEs below 0.65 mm, to demonstrate that the model estimation can reproduce the PW variability over the TP in both space and time. Annual PW series show a significant increasing trend of 0.19 mm decade−1 for the TP during the 42 years. The most significant PW increase of 0.47 mm decade−1 occurs for 1986–99 and an insignificant decrease occurs for 2000–11. From the comparison of the PW data from JRA-55, ERA-40, ERA-Interim, MERRA, NCEP-2, and ISCCP, it is found that none of them are able to show the actual long-term trends and variability in PW for the TP as the Bayesian estimation.
In the atmosphere, phenomena and events are loosely divided into the realms of “weather” and “climate.” Climate is usually defined to be average weather and thus is thought of as the prevailing weather, which includes not just average conditions but also the range of variations and extremes. Climate inherently involves variations in which the atmosphere is influenced by and interacts with other parts of the climate system, and the external forcings. The large fluctuations in the atmosphere from hour to hour, or day to day, constitute the weather.
The mean annual cycle in surface wind stress over the global oceans from surface wind analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for seven years (1980–86) is presented. The drag coefficient is a function of wind speed and atmospheric stability, and the density is computed for each observation. Annual and seasonal mean climatologies of wind stress, wind stress and Sverdrup transport and the first two annual harmonies of the wind stress are presented. The Northern and Southern hemispheres are contrasted as an the Pacific and Atlantic basins. The representativeness of the climatology is also assessed. The main shortcomings with the current results are in the topics. The wind stress statistics over the southern ocean are believed to be the moon reliable because of the paucity of direct wind observations. Annual mean values exceed 2 dyn cm−2 over the eastern hemisphere near 50°S and locally exceed 3 dyn cm−2 in the southern Indian Ocean; values much larger than in previous climatologies. The 12 month variations dominate the annual cycle over most of the globe and are strongest in the Arabian Sea, North Pacific and North Atlantic. But strong semiannual components occur especially over the Southern Ocean and in the North Pacific. The former are associated with semiannual increases in the strength of the southern westerlies whereas in the North Pacific, the semiannual cycle occurs locally largely because of the annual variations in intensity and meridional movement of the Aleutian low and subtropical high. The wind stress considerably from year to year. Over most of the world's ocean the mean annual cycle explains less then 45% of the monthly variance in each of the wind stress components and the curl of wind stress. In addition, mean values for the climatology differ significantly from those of previous periods. There is good reason to believe that these differences in the Northern Hemisphere an mostly real and represent climate variations on interannual and decadal time scales that have major implications for the circulation of the oceans. A related factor is that this period included two Pacific Warm Events (El Niños), but no Cold (La Niña) Events.
An observable constraint on climate sensitivity, based on variations in mid-tropospheric relative humidity (RH) and their impact on clouds, is proposed. We show that the tropics and subtropics are linked by teleconnections that induce seasonal RH variations that relate strongly to albedo (via clouds), and that this covariability is mimicked in a warming climate. A present-day analog for future trends is thus identified whereby the intensity of subtropical dry zones in models associated with the boreal monsoon is strongly linked to projected cloud trends, reflected solar radiation, and model sensitivity. Many models, particularly those with low climate sensitivity, fail to adequately resolve these teleconnections and hence are identifiably biased. Improving model fidelity in matching observed variations provides a viable path forward for better predicting future climate.
The seasonal, spatial, and latitudinal variability of precipitation (P), evapotranspiration (E), and runoff (R) are examined for large Arctic river basins and for the entire pan‐Arctic domain using a 21‐year off‐line simulation of the Variable Infiltration Capacity (VIC) macroscale hydrology model and the ERA‐40 reanalysis. Observed P used in the VIC model (corrected for gauge catch deficiency) is compared with that from the ERA‐40 reanalysis. Gridded values of evapotranspiration minus precipitation (E‐P) are calculated from the ERA‐40 atmospheric water budget, and estimates of implied E are obtained as the residual of observed P and ERA‐40 E‐P. The ERA‐40 P is surprisingly close to observations on an annual basis over the large river basins (especially accounting for known errors in the observations). Furthermore, ERA‐40 P is quite consistent with observations in terms of interannual, spatial, and latitudinal variations. ERA‐40 E is generally higher than both VIC E and implied E in spring and autumn. However, VIC estimates more E in June and July than either ERA‐40 or the atmospheric budget for the Yenisei, Ob, and Mackenzie River basins. The ERA‐40 bias toward early snowmelt and a double runoff peak (not present in VIC or observations) indicates the need for improvements in the ECMWF land surface scheme. The long‐term means of ERA‐40 vapor convergence P‐E for the Lena, Yenisei, Ob, and Mackenzie are not in balance with observed runoff, mainly due to the uncertainties in computed P‐E and observed streamflow.
Extreme rainfall increases with temperature and this is predicted to continue in a future warmer climate. Whilst a downturn in rainfall scaling is observed at high temperatures in the present day, this does not imply an upper limit on rainfall extremes in the future. Theoretical models predict that, in the absence of moisture limitation, extreme precipitation intensity could exponentially increase with temperatures at a rate determined by the Clausius–Clapeyron (C–C) relationship1,2. Climate models project a continuous increase of precipitation extremes for the twenty-first century over most of the globe3,4,5. However, some station observations suggest a negative scaling of extreme precipitation with very high temperatures6,7,8,9, raising doubts about future increase of precipitation extremes. Here we show for the present-day climate over most of the globe, the curve relating daily precipitation extremes with local temperatures has a peak structure, increasing as expected at the low–medium range of temperature variations but decreasing at high temperatures. However, this peak-shaped relationship does not imply a potential upper limit for future precipitation extremes. Climate models project both the peak of extreme precipitation and the temperature at which it peaks (Tpeak) will increase with warming; the two increases generally conform to the C–C scaling rate in mid- and high-latitudes, and to a super C–C scaling in most of the tropics. Because projected increases of local mean temperature (Tmean) far exceed projected increases of Tpeak over land, the conventional approach of relating extreme precipitation to Tmean produces a misleading sub-C–C scaling rate.
No abstract is provided for this article.
An analysis has been made of the variability of the Australian hemispheric monthly mean analyses at 0000 GMT from May 1972 to January 1978 throughout the troposphere of the Southern Hemisphere. The annual cycle of the variance with respect to time of the zonal mean and the transient eddy components of the monthly geopotential height and geostrophic westerly wind fields are presented as a function of latitude at 1000, 500 and 200 mb. A pronounced quasi-biennial oscillation (QBO) with a systematic pole-ward progression of the anomalies is present throughout the troposphere in the zonal-mean anomaly fields of the height and wind but is absent in the thickness fields. The QBO is clearest at the bottom of the atmosphere and appears to have a barotropic structure.
The distribution of the mean westerly wind over the globe is described for June, July and August, the southern winter, using analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) from 1979 to 1982. The tropospheric momentum budget is analyzed from both the traditional Eulerian and the transformed Eulerian perspectives. Thus an assessment is made of the Eliassen-Palm flux divergence and the diabatically driven residual mean circulation. The vertical mean budget is also analyzed to allow deductions about the mean surface torque by the atmosphere on the earth and the grow surface stress. In the southern winter, transient eddies dominate the poleward momentum transports in both hemispheres, and total transports are somewhat larger than found in previous studies. Values of the deduced mean westerly surface stress are therefore larger than most previous estimates, but seem very reasonable and are probably more reliable in midlatitudes. A strange vertical structure analyzed to be present in the ECMWF zonal mean meridional wind is found to be inconsistent with the momentum budget, and the analyzed Hadley circulation is shown to be much too weak. The latter was expected since diabatic effects were not included in the initialization at ECMWF for this period. The total momentum budget is determined without including vertical eddy fluxes of momentum, but the residual is fairly small outside of the tropics and can probably be accounted for by fairly small errors in the analyzed divergent wind component. The traditional Eulerian view reveals that the midlatitude westerlies are maintained mainly by convergence of westerly momentum by the transient eddies, while the induced Ferrel cell decelerates the westerlies aloft and transports momentum down to the surface to balance losses by surface friction. The transformed Eulerian view shows that the net effect of the eddies in the upper troposphere, above 300 mb, is small, but there is a marked net deceleration by the transient eddies between 700 and 300 mb, and the westerlies there are maintained by the Coriolis torque acting on the diabatically driven residual mean circulation. The observed Ferrel cell is thus revealed to be a fairly small residual of the direct diabatically driven cell and the eddy-induced indirect cell. However, the vertical mean budget clearly shows that it is the meridional transport of westerly momentum by the eddies that is primarily acting to maintain the midlatitude westerlies against losses by surface friction.
The trends of the surface water and energy budget components in the Mississippi River basin from 1948 to 2004 are investigated using a combination of hydrometeorological observations and observation-constrained simulations of the land surface conditions using the latest version of the Community Land Model version 3 (CLM3). The atmospheric forcing data for the CLM3 were constructed by adding the intramonthly variations from the 6-hourly National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis to observation-based analyses of monthly precipitation, surface air temperature, and cloud cover. The model-based analysis suggests that, for the surface water budget, the observed increase in basin-averaged precipitation is compensated by increases in both runoff and evapotranspiration. For the surface energy budget, the decrease of net shortwave radiation associated with observed increases in cloudiness is compensated by decreases in both net longwave radiation and sensible heat flux, while the latent heat flux increases in association with wetter soil conditions. Both the simulated surface water and energy budgets support the view that evapotranspiration has increased in the Mississippi River basin from 1948 to 2004. Sensitivity experiments show that the precipitation change dominates the evapotranspiration trend, while the temperature and solar radiation changes have only small effects. Large spatial variations within the Mississippi River basin and the contiguous United States are also found. However, the increased evapotranspiration is ubiquitous despite spatial variations in hydrometeorology.
The effects of the inclusion of a Helmholtz term in a quasi-geostrophic model, formulated as part of a lower boundary condition, are investigated using a two-layer model. The Helmholtz term has two main effects, both of which appear desirable in a quasi-geostrophic model. As well as reducing the phase speed of all waves, and thereby controlling the otherwise large retrograde motion of ultra-long Rossby waves, it reduces growth rates of unstable waves. The Helmholtz term reduces the westward slope with height of unstable waves, more so for shorter wave lengths.The Helmholtz term is shown to be important for all quasi-geostrophic models. However, its use with an enhanced factor of about 10, although desirable for “best” forecasts, has little physical basis and merely serves to offset shortcomings of the model.
