Alessandro Dell'Aquila

AMMA-Model Intercomparison Project

The African Monsoon Multidisciplinary Analyses-Model Intercomparison Project (AMMA-MIP) was developed within the framework of the AMMA project. It is a relatively light intercomparison and evaluation exercise of both global and regional atmospheric models, focused on the study of the seasonal and intraseasonal variations of the climate and rainfall over the Sahel. Taking advantage of the relative zonal symmetry of the West African climate, one major target of the exercise is the documentation of a meridional cross section made of zonally averaged (10°W–10°E) outputs. This paper presents the motivations and design of the exercise, and it discusses preliminary results and further extensions of the project.

Southern hemisphere midlatitude atmospheric variability of the NCEP-NCAR and ECMWF reanalyses

[1] We compare the representation of the Southern Hemisphere midlatitude winter variability in the NCEP-NCAR and ERA40 reanalyses by using the Hayashi spectral technique. We find important discrepancies in the description of the atmospheric waves at different spatial and temporal scales. ERA40 is generally characterized by a larger variance, especially in the high-frequency spectral region. Compared to the Northern Hemisphere, the assimilated data are relatively scarce particularly over the oceans, and they provide a weak constraint to the assimilation system even in the period when satellite data are available. In the presatellite period the discrepancies between the two reanalyses are large and randomly distributed; after 1979 the discrepancies are systematic. This study suggests that, as for the winter midlatitude variability in the Southern Hemisphere, a well-defined picture to be used in the evaluation of climate model simulations is still lacking because of the nonconsistency of the reanalyses.

Northern Hemisphere winter midlatitude atmospheric variability in CMIP5 models

The Northern Hemisphere midlatitude winter atmospheric variability simulated by Coupled Model Intercomparison Project phase 5 (CMIP5) models is analyzed at spatial and temporal scales corresponding to the growth of baroclinic eddies and planetary waves. We use a global scalar metric of the wave energy frequency-wave number spectrum to identify potential improvements of the CMIP5 ensemble compared to previous coordinated model simulations (CMIP3). We also evaluate whether CMIP5 models predict future shifts in the global baroclinic eddies and planetary-scale wave activities. With respect to CMIP3, no significant improvements are found, thereby suggesting that no significant breakthrough in the modeling of the climate system has been hit over the last few years. No significant changes are found in RCP4.5 scenarios for the selected metric of the baroclinic and planetary-scale atmospheric flows, thus indicating that localized changes with potential societal impact might not be related to changes in key fundamental properties of the atmospheric circulation.

Benchmarking Northern Hemisphere midlatitude atmospheric synoptic variability in centennial reanalysis and numerical simulations

The representation of midlatitude winter atmospheric synoptic variability in centennial reanalysis products, which assimilate surface observations only, and atmospheric model simulations constrained by observation-based data sets is assessed. Midlatitude waves activity in twentieth century reanalyses (20CR, ERA-20C) and atmospheric model simulations are compared with those estimated from observationally complete reanalysis products. All reanalyses are in good agreement regarding the representation of the synoptic variability during the last decades of the twentieth century. This suggests that the assimilation of surface observations can generate high-quality extratropical upper air fields. In the first decades of the twentieth century a suppression of high-frequency variability is apparent in the centennial reanalysis products. This behavior does not have a counterpart in the atmospheric model integrations. Since the latter differ from one of the reanalysis products considered here (ERA-20C) only in the assimilation of surface observations, it seems reasonable to attribute the high-frequency variability suppression to the poor coverage of the observations assimilated.