60 research outputs found
Recommended from our members
Impact of increased horizontal resolution in coupled and atmosphere-only models of the HadGEM1 family upon the climate patterns of South America
This study investigates the impact of increased horizontal resolution in coupled and atmosphere-only global climate models on the simulation of climate patterns in South America (SA). We analyze simulations of the HadGEM1 model family with three different horizontal resolutions in the atmosphereâN96 (~135 km at 50°N), N144 (~90 km) and N216 (~60 km)âand two different resolutions in the oceanâ1° and 1/3°. In general, the coupled simulation with the highest resolution (60 km in the atmosphere and 1/3° in the ocean) has smaller systematic errors in seasonal mean precipitation, temperature and circulation over SA than the atmosphere-only model at all resolutions. The models, both coupled and atmosphere-only, properly simulate spatial patterns of the seasonal shift of the Intertropical Convergence Zone (ITCZ), the formation and positioning of the South Atlantic Convergence Zone (SACZ), and the subtropical Atlantic and Pacific highs. However, the models overestimate rainfall, especially in the ITCZ and over the western border of high-elevation areas such as southern Chile. The coupling, combined with higher resolution, result in a more realistic spatial pattern of rain, particularly over the Atlantic ITCZ and the continental branch of the SACZ. All models correctly simulate the phase and amplitude of the annual cycle of precipitation and air temperature over most of South America. The overall results show that despite some problems, increasing the resolution in the HadGEM1 model family results in a more realistic representation of climate patterns over South America and the adjacent oceans
Recommended from our members
A comprehensive analysis of coherent rainfall patterns in China and potential drivers. Part I: interannual variability
Interannual rainfall variability in China affects agriculture, infrastructure and water resource management. To improve its understanding and prediction, many studies have associated precipitation variability with particular causes for specific seasons and regions. Here, a consistent and objective method, Empirical Orthogonal Teleconnection (EOT) analysis, is applied to 1951â2007 high-resolution precipitation observations over China in all seasons. Instead of maximizing the explained spaceâtime variance, the method identifies regions in China that best explain the temporal variability in domain-averaged rainfall. The EOT method is validated by the reproduction of known relationships to the El Niño Southern Oscillation (ENSO): high positive correlations with ENSO are found in eastern China in winter, along the Yangtze River in summer, and in southeast China during spring. New findings include that wintertime rainfall variability along the southeast coast is associated with anomalous convection over the tropical eastern Atlantic and communicated to China through a zonal wavenumber-three Rossby wave. Furthermore, spring rainfall variability in the Yangtze valley is related to upper-tropospheric midlatitude perturbations that are part of a Rossby wave pattern with its origin in the North Atlantic. A circumglobal wave pattern in the northern hemisphere is also associated with autumn precipitation variability in eastern areas. The analysis is objective, comprehensive, and produces timeseries that are tied to specific locations in China. This facilitates the interpretation of associated dynamical processes, is useful for understanding the regional hydrological cycle, and allows the results to serve as a benchmark for assessing general circulation models
Recommended from our members
Contribution of tropical cyclones to atmospheric moisture transport and rainfall over East Asia
The coastal region of East Asia (EA) is one of the regions with the most frequent impacts from tropical cyclones (TCs). In this study, rainfall and moisture
transports related to TCs are measured over the EA, and the contribution of TCs to the regional water budget is compared with other contributors, especially the mean circulation of the EA summer monsoon (EASM). Based on
ERA-Interim re-analysis (1979â2012), the trajectories of TCs are identified using an objective feature tracking method. Over 60% of TCs occur from July to October (JASO). During JASO, TC rainfall contributes 10-30% the of monthly total rainfall over the coastal region of EA; this contribution is highest over the south/southeast coast of China in September. TCs make a larger contribution to daily extreme rainfall (above the 95th percentile): 50-60% over the EA coast and as high as 70% over Taiwan island. Compared
with the mean EASM, TCs transport less moisture over the EA. However, as the peak of the mean seasonal cycle of TCs lags two months behind that of the EASM, the moisture transported by TCs is an important source for the water
budget over the EA region when the EASM withdraws. This moisture transport is largely performed by westward-moving TCs. These results improve our understanding of the water cycle of EA and provide a useful test bed for evaluating and improving seasonal forecasts and coupled climate models
Recommended from our members
Interannual rainfall variability over China in the MetUM GA6 and GC2 configurations
Six climate simulations of the Met Office Unified Model Global Atmosphere 6.0 and Global Coupled 2.0 configurations are evaluated against observations and reanalysis data for their ability to simulate the mean state and year-to-year variability of precipitation over China. To analyze the sensitivity to air-sea coupling and horizontal resolution, atmosphere-only and coupled integrations at atmospheric horizontal resolutions of N96, N216 and N512 (corresponding to ~200, 90, and 40 km in the zonal direction at the equator, respectively) are analyzed. The mean and interannual variance of seasonal precipitation are too high in all simulations over China, but improve with finer resolution and coupling. Empirical Orthogonal Teleconnection (EOT) analysis is applied to simulated and observed precipitation to identify spatial patterns of temporally coherent interannual variability in seasonal precipitation. To connect these patterns to large-scale atmospheric and coupled air-sea processes, atmospheric and oceanic fields are regressed onto the corresponding seasonal-mean timeseries. All simulations reproduce the observed leading pattern of interannual rainfall variability in winter, spring and autumn; the leading pattern in summer is present in all but one simulation. However, only in two simulations are the four leading patterns associated with the observed physical mechanisms. Coupled simulations capture more observed patterns of variability and associate more of them with the correct physical mechanism, compared to atmosphere-only simulations at the same resolution. However, finer resolution does not improve the fidelity of these patterns or their associated mechanisms. This shows that evaluating climate models by only geographical distribution of mean precipitation and its interannual variance is insufficient. The EOT analysis adds knowledge about coherent variability and associated mechanisms
Recommended from our members
Sahel decadal rainfall variability and the role of model horizontal resolution
Substantial low-frequency rainfall fluctuations occurred in the Sahel throughout the twentieth century, causing devastating drought. Modeling these low-frequency rainfall fluctuations has remained problematic for climate models for many years. Here we show using a combination of state-of-the-art rainfall observations and high-resolution global climate models that changes in organized heavy rainfall events carry most of the rainfall variability in the Sahel at multiannual to decadal time scales. Ability to produce intense, organized convection allows climate models to correctly simulate the magnitude of late-twentieth century rainfall change, underlining the importance of model resolution. Increasing model resolution allows a better coupling between large-scale circulation changes and regional rainfall processes over the Sahel. These results provide a strong basis for developing more reliable and skilful long-term predictions of rainfall (seasons to years) which could benefit many sectors in the region by allowing early adaptation to impending extremes
Recommended from our members
Moisture sources for East Asian precipitation: mean seasonal cycle and interannual variability
This study investigates the moisture sources that supply East Asian (EA)
precipitation and their interannual variability. Moisture sources are tracked
using theWater Accounting Model-2layers (WAM-2layers), based on the Eulerian
framework. WAM-2layers is applied to five subregions over EA, driven
by the ERA-Interim reanalysis from 1979 to 2015. Due to differences in regional
atmospheric circulation and in hydrological and topographic features,
the mean moisture sources vary among EA subregions. The tropical oceanic
source dominates southeastern EA, while the extratropical continental source
dominates other EA subregions. The moisture sources experience large seasonal
variations, due to the seasonal cycle of the EA monsoon, the freeze-thaw
cycle of the Eurasian continent and local moisture recycling over the Tibetan
Plateau. The interannual variability of moisture sources is linked to interannual
modes of the coupled ocean-atmosphere system. The negative phase
of the North Atlantic Oscillation increases moisture transport to northwestern
EA in winter by driving a southward shift in the mid-latitude westerly jet
over theMediterranean Sea, the Black Sea and the Caspian Sea. Atmospheric
moisture lifetime is also reduced due to the enhanced westerlies. In summers
following El Ni Ënos, an anti-cyclonic anomaly over the western North Pacific
increases moisture supplied from the South China Sea to the southeastern EA
and shortens the travelling distance. A stronger Somali Jet in summer increases
moisture to the Tibetan Plateau and therefore increases precipitation
over the eastern Tibetan Plateau. The methods and findings in this study can
be used to evaluate hydrological features in climate simulations
Recommended from our members
Tropical cyclones in the UPSCALE ensemble of high resolution global climate models
The UPSCALE (UK on PRACE: weather-resolving Simulations of Climate for globAL Environmental risk) project, using PRACE (Partnership for Advanced Computing in Europe) resources, constructed and ran an ensemble of atmosphere-only global climate model simulations, using the Met Office Unified Model GA3 configuration. Each simulation is 27 years in length for both the present climate and an end-of-century future climate, at resolutions of N96 (130 km), N216 (60 km) and N512 (25 km), in order to study the impact of model resolution on high impact climate features such as tropical cyclones. Increased model resolution is found to improve the simulated frequency of explicitly tracked tropical cyclones, and correlations of interannual variability in the North Atlantic and North West Pacific lie between 0.6 and 0.75. Improvements in the deficit of genesis in the eastern North Atlantic as resolution increases appear to be related to the representation of African Easterly Waves and the African Easterly Jet. However, the intensity of the modelled tropical cyclones as measured by 10 m wind speed remain weak, and there is no indication of convergence over this range of resolutions. In the future climate ensemble, there is a reduction of 50% in the frequency of Southern Hemisphere tropical cyclones, while in the Northern Hemisphere there is a reduction in the North Atlantic, and a shift in the Pacific with peak intensities becoming more common in the Central Pacific. There is also a change in tropical cyclone intensities, with the future climate having fewer weak storms and proportionally more stronger storm
Recommended from our members
The resolution sensitivity of Northern Hemisphere blocking in four 25-km atmospheric global circulation models
The aim of this study is to investigate if the representation of Northern Hemisphere blocking is sensitive to resolution in current-generation atmospheric global circulation models (AGCMs). An evaluation is conducted of how well atmospheric blocking is represented in four AGCMs whose horizontal resolution is increased from a grid spacing of more than 100 km to about 25 km. It is shown that Euro/Atlantic blocking is simulated overall more credibly at higher resolution, i.e. in better agreement with a 50-year reference blocking climatology created from the ERA-40 and ERA-Interim reanalyses. The improvement seen with resolution depends on the season and to some extent on the model considered. Euro/Atlantic blocking is simulated more realistically at higher resolution in winter, spring and autumn, and robustly so across the model ensemble. The improvement in spring is larger than that in winter and autumn. Summer blocking is found to be better simulated at higher resolution by one model only, with little change seen in the other three models. The representation of Pacific blocking is not found to systematically depend on resolution. Despite the improvements seen with resolution, the 25-km models still exhibit large biases in Euro/Atlantic blocking. For example, three of the four 25-km models underestimate winter northern European blocking frequency by about one third. The resolution sensitivity and biases in the simulated blocking are shown to be in part associated with the mean-state biases in the modelsâ mid-latitude circulation
Recommended from our members
Effects of horizontal resolution and air-sea coupling on simulated moisture source for East Asian precipitation
Precipitation over East Asia in six MetUM simulations are compared with observation and ERA-Interim reanalysis.
These simulations include three different horizontal resolutions, from low, medium to high, and including atmosphere-only version (GA6.0) and air-sea coupling version (GC2.0).
Precipitations in simulations are systematically different from that of observation and reanalysis.
Increasing horizontal resolution and including air-sea coupling improve simulated precipitation but cannot eliminate bias.
%
Moisture sources of East Asian precipitations are identified using the WAM-2layers - a moisture tracking model that traces moisture source using collective information of evaporation, atmospheric moisture and circulation.
Similar to precipitation, moisture sources in simulations are systematically different from that of ERA-Interim.
Major differences in moisture sources include underestimated moisture contribution from tropical Indian Ocean and overestimate contribution from Eurasian continent.
%
By increasing horizontal resolution, precipitation bias over the Tibetan Plateau is improved.
From the moisture source point of view, this is achieved by reducing contribution from remote moisture source and enhancing local contribution over its eastern part.
%
Although including air-sea coupling does not necessarily change East Asian precipitation, moisture sources show differences between coupled and atmospheric-only simulations.
These differences in moisture sources indicate different types of models biases caused by surface flux or/and atmospheric circulation on different locations.
These information can be used to target model biases on specified locations and due to different mechanisms
- âŠ