発表要旨
1. プロジェクトの目的
The aim of this collaboration is to produce ground-breaking climate simulations on the Earth Simulator which could not be achieved elsewhere. A key scientific goal is to evaluate uncertainly due to structural limitations on current climate models, either due to limited resolution or omitted physical or dynamical processes, with particular focus on improving the representation of regional climate and its variability. This will be achieved through a partnership between the Earth Simulator Center, the NERC Centre for Global Atmospheric Modelling (NCAS-Climate, Reading, UK) and the Hadley Centre, Met Office (Exeter, UK).
2. 今年度当初の計画
- Systematic investigation of the role of model resolution, based on a model matrix featuring 150km, 100km and 60km resolution atmosphere models, and 100km and 30km ocean models. This required the completion of:
- Longer simulations of climate integrations, more than 30 years for each of the 4 coupled models, and more than 10 years for the 60km high resolution atmosphere model.
- Analysis of critical processes and regional variability using high resolution simulations:
- India and monsoon
- Tropical Pacific and El Nino-Southern Oscillation
- Maritime Continent and Madden-Julian Oscillation
- Tropical cyclones and mid-latitude storms
- Representation of convection, precipitation, diurnal cycle
- Land surface processes, including the generation of high-resolution model initial and boundary conditions
3. 今年度得られた成果
Model integrations:
Completed 50 years of simulation with all 4 models in the coupled model resolution matrix, using 150km and 100km atmosphere and 100km and 30km ocean models. This has allowed us to start to systematically investigate the role of model resolution on global and regional scales on climate timescales.
Completed 20 years of 100km and 60km atmosphere model integrations forced by AMIP2 SSTs and SSIs, to investigate the role of atmosphere resolution in processes such as monsoon, tropical cyclones, local rainfall and the diurnal cycle.
8 x 5 year ensemble integrations of 60km model to test model sensitivity to model formulation, parameter choices and initial and boundary conditions.
Technical
The efficiency of high resolution 60km model has been increased by 100% to enable the use of L system, and these improvements have been propagated into the lower resolution models.
Scientific research:
Monsoon and soil moisture dynamics
The monsoon simulation in the 60km atmosphere model (NUGAM) is generally better than at lower resolution, and in some years reproduces the observed regime quite well. The influence of local orography seems to be important, as is the representation of soil properties in the model. Some hypotheses have been suggested in order to understand what would initiate and maintain the monsoon in that particular model configuration. Factors such as the inclusion of mesoscale mountains, which affect the local circulation and enable a more realistic representation of Himalayan and Eurasian snow cover, seem to be important for the onset of the monsoon. Another difference between the low and high-resolution models is due to a modification in the treatment of snowmelt infiltration in the land surface parametrisation. This had a large impact on the surface energy balance and consequently on the Asian continent-Indian ocean temperature gradient, which drives the monsoon. The land surface also seems to have an impact on the maintenance of the monsoon process, via evapo-transpiration limitations; in this context, a better representation of soil physical properties is important. A new set of soil physical parameters, using Earth Observatory-generation data, has been constructed for the high-resolution model. Further hypotheses about the importance of high-resolution for land surface processes (e.g. precipitation frequency and distribution) are currently being formulated and will allow us to test and improve the low-resolution models. Predictors of monsoon, including snow cover and storm track position, are also being studied.
Tropical Pacific and ENSO
The simulation of the tropical Pacific climatology is much improved when using the higher resolution ocean model. Tropical instability waves act to converge heat at the equator and warm the sea surface temperatures, a process not well represented at low resolution. A paper is nearing completion on this work, to be submitted to Journal of Climate.
The El Nino/Southern Oscillation variability is poorly represented in the low resolution coupled model, but with higher atmosphere and ocean resolution the characteristics of ENSO are reasonably well simulated, both in terms of the local process, and the impact of ENSO on global climate. Further analysis will reveal whether this is due to better representation of local processes such as westerly wind bursts or improved coupling of mesoscale weather in atmosphere and ocean. It is essential to model ENSO well, in order to be able to say anything about regional climate variability and change. A paper on this work will be started shortly.
Tropical cyclones and mid-latitude storms
Initial analysis of tropical cyclones (TCs) in the range of atmosphere models suggest that their intensities are better represented as model resolution is increased to 60km. Mid-latitude storm tracks show some improvement at higher resolution, in particular over the North-Atlantic sector. A systematic storm-track intercomparison, covering all our model resolutions and including similar integrations from some of our Japanese collaborators are underway. Comparison with other data sets, including Japanese very high resolution 5-10km models and re-analysis data (e.g. ERA-40), may suggest how well such storms are simulated. Multi-decadal integrations will allow an examination of how TCs impact on the large-scale mean climate at mid-latitudes.
Regional climate and its variability also depend much on storms and blocking, so these statistics will be examined in the AMIP and coupled integrations.
Madden-Julian Oscillation and the Maritime Continent
The MJO is the most important intra-seasonal mode of variability in the atmosphere, and may play a role in ENSO initiation. The MJO variability in the model is present in the coupled models but does not seem to propagate well in atmosphere-only integrations. Results suggest that the MJO signal has better properties at higher resolution, and a paper on this work is being started.
Due to the high proportion of atmospheric energy generated over the Maritime Continent, and its impact on the global circulation, this is a vital area to simulate accurately. Recent work suggests that the representation of islands, and local processes such as sea breezes, are an important component of the circulation, able to feed energy upscale, which require high resolution.
Land surface
The development of a higher-resolution AGCM has made it necessary to produce appropriate initial conditions, due to significant changes in elevation, snow cover distribution and soil physical characteristics (e.g. soil moisture storage capacity). These initial conditions have not only cured many of the model biases present in early integrations, but have also been behind some of the successes in the simulation of the summer Indian monsoon.
Convection and precipitation
The simulation of convection and precipitation, the effect of different parameterizations, the role of local orography, and the diurnal cycle are all being investigated. The tendency of convection to display high-frequency intermittency means that precipitation intensity and timescales are poorly simulated, including their role in the control of climate sensitivity, so that their improved representation will make future predictions
more realistic, and allow better regional representation (which is vital for climate change studies). Higher resolution also means that resolved and parameterized processes may be changed and require careful examination.
Atlantic variability
The coupled models can simulate the primary modes of variability in the Atlantic, a zonal and a meridional mode. The zonal mode is similar to the El Nino phenomenon in the Pacific Ocean, but can not sustain itself. Both modes are associated with variability in precipitation over the continents to either side of the Atlantic basin. The higher atmosphere resolution is needed to simulate the decrease in precipitation over northeast Brazil due to warmer SSTs in the North Atlantic. It also improves the local distribution of precipitation over the ITCZ and over the land to both sides of the Atlantic. The underlying causes for these changes are being investigated.