Europe has experienced many extreme heat waves over the past few decades. In this study, the physical processes underlying these long-lasting and wide-ranging heat wave events are investigated based on a case study in Europe in June 2021. Heat waves are associated with barotropic anticyclonic anomalies accompanied by positive geopotential height anomalies locally. These anomalies persist under the conditions of increased meridional air temperature gradients of the mid-upper troposphere in the high latitudes of Eurasia and the formation of the Arctic front jet. The shrinking high-latitude snow cover in April-May favors higher surface temperatures and larger meridional temperature gradients in June in the mid-upper troposphere due to the soil moisture-evaporation-temperature positive feedback process. The summer Arctic front jet is then strengthened, and the mid-latitude westerly winds are weakened. This atmospheric circulation background favors waveguide formation and wave resonance that produces high-amplitude atmospheric waves and the stagnation of ridges in the midlatitudes. Numerical experiments using the Community Atmosphere Model version 5 verify the proposed physical mechanisms, with the climatic responses in sensitivity experiments to anomalous snowfall rates closely resembling the observational results. Therefore, in June 2021, under the identified atmospheric circulation background and the perturbation of the upstream positive phase of the North Atlantic Oscillation, the large-scale barotropic high pressure and barotropic anticyclonic circulation in the study region tended to be stable and persistent, which is favorable for the production of long-lasting and wide-ranging heat wave events.

Heat waves in India during the pre-monsoon months have significant impacts on human health, productivity and mortality. While greenhouse gas-induced global warming is believed to accentuate high temperature extremes, anthropogenic aerosols predominantly constituted by radiation-scattering sulfate are believed to cause an overall cooling in most world regions. However, the Indian region is marked by an abundance of absorbing aerosols, such as black carbon (BC) and dust. The goal of this work was to understand the association between aerosols, particularly those that are absorbing in nature, and high-temperature extremes in north-central India during the pre-monsoon season. We use 30-year simulations from a chemistry-coupled atmosphere-only general circulation model (GCM), ECHAM6-HAM2, forced with evolving aerosol emissions in an interactive aerosol module, along with observed evolving SSTs. A composite of high-temperature extremes in the model simulations, compared to climatology, shows large-scale conditions conducive to heat waves. Importantly, it reveals concurrent positive anomalies of BC and dust aerosol optical depths. Changes in near-surface properties include a reduction in single scattering albedo (implying greater absorption) and enhancement in short-wave heating rate, compared to climatological conditions. Alterations in surface energy balance include reduced latent heat flux, but increased sensible heat flux, consistent with enhanced temperatures. Thus, chemistry-coupled GCM simulations capture an association of absorbing aerosols with high-temperature extremes in north India, arising from radiative heating in the surface layer.

The Eurasian continent has experienced significant year-to-year variations of summer heat waves during the past decades. Several possible factors, such as ocean temperature, soil moisture, and changes in land use and greenhouse gases, have been identified in previous studies, but the mechanisms are still unclear. In this study, it is found that the Tibetan Plateau snow cover (TPSC) is closely linked to the interannual variations of summer heat waves over Eurasia. The TPSC variability explains more than 30 % of the total variances of heat wave variability in the southern Europe and northeastern Asia (SENA) region. A set of numerical experiments reveal that the reduced TPSC may induce a distinct teleconnection pattern across the Eurasian continent, with two anomalous high pressure centers in the upper troposphere over the SENA region, which may lead to a reduction of the cloud formation near the surface. The less cloud cover tends to increase the net shortwave radiation and favor a stronger surface sensible heat flux in the dry surface condition over the SENA region, resulting in a deeper, warmer and drier atmospheric boundary layer that would further inhibit the local cloud formation. Such a positive land-atmosphere feedback may dry the surface even further, heat the near-surface atmosphere and thereby intensify the local heat waves. The above dynamical processes also operate on interdecadal time scales. Given the reduction of the TPSC could become more pronounced with increasing levels of greenhouse gases in a warming climate, we infer that the TPSC may play an increasingly important role in shaping the summer heat waves over the SENA region in next decades.

Uncertainties in the climate response to a doubling of atmospheric CO2 concentrations are quantified in a perturbed land surface parameter experiment. The ensemble of 108 members is constructed by systematically perturbing five poorly constrained land surface parameters of global climate model individually and in all possible combinations. The land surface parameters induce small uncertainties at global scale, substantial uncertainties at regional and seasonal scale and very large uncertainties in the tails of the distribution, the climate extremes. Climate sensitivity varies across the ensemble mainly due to the perturbation of the snow albedo parameterization, which controls the snow albedo feedback strength. The uncertainty range in the global response is small relative to perturbed physics experiments focusing on atmospheric parameters. However, land surface parameters are revealed to control the response not only of the mean but also of the variability of temperature. Major uncertainties are identified in the response of climate extremes to a doubling of CO2. During winter the response both of temperature mean and daily variability relates to fractional snow cover. Cold extremes over high latitudes warm disproportionately in ensemble members with strong snow albedo feedback and large snow cover reduction. Reduced snow cover leads to more winter warming and stronger variability decrease. As a result uncertainties in mean and variability response line up, with some members showing weak and others very strong warming of the cold tail of the distribution, depending on the snow albedo parametrization. The uncertainty across the ensemble regionally exceeds the CMIP3 multi-model range. Regarding summer hot extremes, the uncertainties are larger than for mean summer warming but smaller than in multi-model experiments. The summer precipitation response to a doubling of CO2 is not robust over many regions. Land surface parameter perturbations and natural variability alter the sign of the response even over subtropical regions.

Estimation of Particulate Matter (PM) concentration and aerosol absorption is very important in air quality and climate studies. To date, smoke, mineral dust and anthropogenic pollutants are the most uncertain aerosol components in their optical and microphysical properties. In this study, we retrieve the PM2.5 and Absorbing Aerosol Optical Depth (AAOD) from the Total Ozone Mapping Spectrometer (TOMS), the Moderate Resolution Imaging SpectroRadiometer (MODIS) and the Multiangle Imaging SpectroRadiameter (MISR) measurements. A global chemical transport model (GEOS-CHEM) is used to simulate the vertical profiles of PM2.5 and AAOD. We find that the 2003 heat wave has strong impact on PM2.5 across Europe and increased the average PM2.5 concentration by 18%. The aerosol species with the largest concentration increase are ammonium nitrate, black carbon and mineral dust. The Aerosol Robotic Network (AERONET) measurements have been used to validate our retrieval of AAOD. We find that there is a significant agreement between AERONET measurements and our retrievals with the correlation coefficient, slope and intercept of 0.91, 0.99 and 0.001, respectively. The absorbing aerosols can exert negative health effect, increase positive aerosol radiative forcing and contribute positive aerosol-climate feedbacks. (C) 2009 Elsevier Ltd. All rights reserved.

During June, July and August 2003, an exceptional heat wave affected western and central Europe. In Piedmont, a region located in northwestern Italy at the foot of the Alps, many stations recorded the highest mean summer temperatures since the beginning of their instrumental record. Some consequences of this extraordinary hot summer in Piedmont and in many European countries include severe drought conditions, with strong effects on agriculture and electric production, an acceleration of glacier ablation, and an increase in the frequency of forest fires. This heat wave has been analyzed by running a Soil-Vegetation Atmosphere Transfer scheme for 5 years (1999-2003): the LSPM (Land Surface Process Model). The attention was focused on energy and hydrologic budget components by performing two simulations in climatically different sub-areas of Piedmont. The increment in the observed solar radiation during summer 2003 produced an increment in the net radiation, which in turn generated an increase of sensible (more) and latent (less) heat flux, and soil-vegetation heat flux. The latter caused a consistent warming of soil and vegetation surfaces, which acted partially as a negative feedback increasing the longwave radiation emitted by the terrestrial surface. Latent heat flux showed a small increment in summer 2003, because the evapotranspiration was limited by the soil moisture unavailability, particularly during July and August, due to the scarcity of precipitations during the previous spring. The drought conditions, acting as a positive feedback, caused the effects of the heat wave to be more severe, favored its persistence and enhanced the further reduction of soil moisture. The comparison among the results of the two simulations allowed to highlight the role of two phenomena that concurred to exacerbate the heat wave: the enhancement of the drought conditions and the increment of the adiabatic compression connected with the anticyclonic conditions. A rough estimate allowed us to quantify in about 2 C the contribution of the former.