Seasonal to yearly assessment of temperature and precipitation trends in the North Western Mediterranean Basin by dynamical downscaling of climate scenarios at high resolution (1971–2050)

The complex topography and high climatic variability of the North Western Mediterranean Basin (NWMB) require a detailed assessment of climate change projections at high resolution. ECHAM5/MPIOM global climate projections for mid-21st century and three different emission scenarios are downscaled at 10 km resolution over the NWMB, using the WRF-ARW regional model. High resolution improves the spatial distribution of temperature and precipitation climatologies, with Pearson's correlation against observation being higher for WRF-ARW (0.98 for temperature and 0.81 for precipitation) when compared to the ERA40 reanalysis (0.69 and 0.53, respectively). However, downscaled results slightly underestimate mean temperature (≈1.3 K) and overestimate the precipitation field (≈400 mm/year). Temperature is expected to raise in the NWMB in all considered scenarios (up to 1.4 K for the annual mean), and particularly during summertime and at high altitude areas. Annual mean precipitation is likely to decrease (around ?5 % to ?13 % for the most extreme scenarios). The climate signal for seasonal precipitation is not so clear, as it is highly influenced by the driving GCM simulation. All scenarios suggest statistically significant decreases of precipitation for mountain ranges in winter and autumn. High resolution simulations of regional climate are potentially useful to decision makers. Nevertheless, uncertainties related to seasonal precipitation projections still persist and have to be addressed.     

Projections of temperature and precipitation extremes in the North Western Mediterranean Basin by dynamical downscaling of climate scenarios at high resolution (1971–2050)

The North Western Mediterranean basin (NWMB) is characterised by a highly complex topography and an important variability of temperature and precipitation patterns. Downscaling techniques are required to capture these features, identify the most vulnerable areas to extreme changes and help decision makers to design strategies of mitigation and adaptation to climate change. A Regional Climate Model, WRF-ARW, is used to downscale the IPCC-AR4 ECHAM5/MPI-OM General Circulation Model results with high resolution (10 km), considering three different emissions scenarios (B1, A1B and A2) for 2001–2050. Model skills to reproduce observed extremes are assessed for a control period, 1971–2000, using the ERA40 reanalysis to drive the WRF-ARW simulations. A representative set of indices for temperature and precipitation extremes is projected. The modelling system correctly reproduces amplitude and frequency of extremes and provides a high degree of detail on variability over neighbouring areas. However, it tends to overestimate the persistence of wet events and consequently slightly underestimate the length of dry periods. Drier and hotter conditions are generally projected for the NWMB, with significant increases in the duration of droughts and the occurrence of heavy precipitation events. The projected increase in the number of tropical nights and extreme temperatures could have a negative effect on human health and comfort conditions. Simulations allow defining specifically vulnerable areas, such as the Ebro Valley or the Pyrenees, and foreseeing impacts on socio-economic activities in the region.     

Precipitation and temperature space–time variability and extremes in the Mediterranean region: evaluation of dynamical and statistical downscaling methods

This study evaluates how statistical and dynamical downscaling models as well as combined approach perform in retrieving the space–time variability of near-surface temperature and rainfall, as well as their extremes, over the whole Mediterranean region. The dynamical downscaling model used in this study is the Weather Research and Forecasting (WRF) model with varying land-surface models and resolutions (20 and 50 km) and the statistical tool is the Cumulative Distribution Function-transform (CDF-t). To achieve a spatially resolved downscaling over the Mediterranean basin, the European Climate Assessment and Dataset (ECA&D) gridded dataset is used for calibration and evaluation of the downscaling models. In the frame of HyMeX and MED-CORDEX international programs, the downscaling is performed on ERA-I reanalysis over the 1989–2008 period. The results show that despite local calibration, CDF-t produces more accurate spatial variability of near-surface temperature and rainfall with respect to ECA&D than WRF which solves the three-dimensional equation of conservation. This first suggests that at 20–50 km resolutions, these three-dimensional processes only weakly contribute to the local value of temperature and precipitation with respect to local one-dimensional processes. Calibration of CDF-t at each individual grid point is thus sufficient to reproduce accurately the spatial pattern. A second explanation is the use of gridded data such as ECA&D which smoothes in part the horizontal variability after data interpolation and damps the added value of dynamical downscaling. This explains partly the absence of added-value of the 2-stage downscaling approach which combines statistical and dynamical downscaling models. The temporal variability of statistically downscaled temperature and rainfall is finally strongly driven by the temporal variability of its forcing (here ERA-Interim or WRF simulations). CDF-t is thus efficient as a bias correction tool but does not show any added-value regarding the time variability of the downscaled field. Finally, the quality of the reference observation dataset is a key issue. Comparison of CDF-t calibrated with ECA&D dataset and WRF simulations to local measurements from weather stations not assimilated in ECA&D, shows that the temporal variability of the downscaled data with respect to the local observations is closer to the local measurements than to ECA&D data. This highlights the strong added-value of dynamical downscaling which improves the temporal variability of the atmospheric dynamics with regard to the driving model. This article highlights the benefits and inconveniences emerging from the use of both downscaling techniques for climate research. Our goal is to contribute to the discussion on the use of downscaling tools to assess the impact of climate change on regional scales.     

Rossby wave dynamics of the North Pacific extra-tropical response to El Niño: importance of the basic state in coupled GCMs

The extra-tropical response to El Niño in a “low” horizontal resolution coupled climate model, typical of the Intergovernmental Panel on Climate Change fourth assessment report simulations, is shown to have serious systematic errors. A high resolution configuration of the same model has a much improved response that is similar to observations. The errors in the low resolution model are traced to an incorrect representation of the atmospheric teleconnection mechanism that controls the extra-tropical sea surface temperatures (SSTs) during El Niño. This is due to an unrealistic atmospheric mean state, which changes the propagation characteristics of Rossby waves. These erroneous upper tropospheric circulation anomalies then induce erroneous surface circulation features over the North Pacific. The associated surface wind speed and direction errors create erroneous surface flux and upwelling anomalies which finally lead to the incorrect extra-tropical SST response to El Niño in the low resolution model. This highlights the sensitivity of the climate response to a single link in a chain of complex climatic processes. The correct representation of these processes in the high resolution model indicates the importance of horizontal resolution in resolving such processes.     

Application of the delta‐Eddington method to the absorption of solar radiation in the atmosphere

Abstract

A model to compute rapidly the absorption of solar radiation in the atmosphere is described. The model is based partially on the parameterization of Lacis and Hansen and also makes use of the delta‐Eddington method. In addition to absorption by ozone and water vapour, and scattering by air molecules and clouds, the mode1 includes absorption and scattering by aerosols. Good agreement is found in comparison with the Lacis and Hansen parameterization in the absence of clouds and aerosol. The present model represents an improvement in the treatment of scattering by clouds. Its main advantage though, is in its flexibility in allowing for interactions with the atmospheric aerosol.     

Circulation effect: response of precipitation δ18O to the ENSO cycle in monsoon regions of China

Inter-annual variation in the ratio of ¹⁸O to ¹⁶O of precipitation (δ¹⁸O_p) in the monsoon regions of China (MRC, area approximately east of 100°E) has not yet been fully analyzed. Based on an analysis of the relationships between the time series of amount-weighted mean annual δ¹⁸O in precipitation (δ¹⁸O_w) and meteorological variables such as temperature, precipitation as well as atmospheric/oceanic circulation indices, it is recognized that the El Niño-Southern Oscillation (ENSO) cycle appears to be the dominant control on the inter-annual variation in δ¹⁸O_p in the MRC. Further analysis shows that the trade wind plays a role in governing δ¹⁸O_w through affecting the intensity of the different summer monsoon circulations which are closely linked to the weakening (weaker than normal) and strengthening (stronger than normal) of the trade wind and gives the δ¹⁸O_w different values at or over inter-annual timescales. The southwest monsoon (SWM) drives long-distance transport of water vapor from Indian Ocean to the MRC, and along this pathway increasing rainout leads to more negative δ¹⁸O_w via Rayleigh distillation processes. In contrast, the southeast monsoon (SEM), which is consistent with the changes in the strength of the West Pacific subtropical high, drives short-distance water vapor transport from the West Pacific Ocean to the MRC and leads to less negative δ¹⁸O_w. Therefore, the δ¹⁸O_w value directly reflects the differences in influence between the SWM, which is strong when the SE trade wind is strong, and the SEM, which is strong when the SE trade wind is weak. In addition, the South China Sea Monsoon also transports local water vapor as well as plays a role in achieving the synchronization between the δ¹⁸O_w and ENSO. The author thus terms the δ¹⁸O_p rhythm in the MRC the “circulation effect”. In turn, the δ¹⁸O_p variation in the MRC has the potential to provide information on atmospheric circulation and the signal of δ¹⁸O_p recorded in natural archives can then be used to deduce a long-term behavior of the tropical climate system.     

Application of Linear Thermodynamics to the Atmospheric System.Part Ⅱ: Exemplification of Linear Phenomenological Relations in the Atmospheric System

将边界层相似性理论同线性热力学理论结合，间接地以观测实验事实证明大气边界层内线性唯象关系是成立的，而且线性湍流输送系数同相应的线性唯象系数成正比关系。但实验事实表明，大涡对流的混合层线性唯象系数成正比关系。但实验事实表明，大涡对流的混合层线性唯象关系是不成立的，混合层内湍流输送过程是一种强的非线性过程。所以，对流边界层是一种远离平衡态非线性区的热力学状态。地转风和热成风是一种大气系统特有的动力过程和热力过程和交叉耦合现象，这是大气系统交叉耦合现象的一个实际例证。     

Climatological responses of winter precipitation in Turkey to variability of the North Atlantic Oscillation during the period 1930–2001

Summary Climatological responses of winter (DJFM) precipitation at 78 stations of Turkey to variability of the North Atlantic Oscillation (NAO) were investigated for the period 1930–2001. The analysis was performed with respect to relationships between precipitation and three different NAO indices (NAOIs) and composite precipitation changes corresponding to the extreme phases of the NAOIs, and individual wet conditions and drought events linked to the extreme NAOI events. Main conclusions of the study can be evaluated as follows:(a) The Ponta Delgada–Reykjavik (PD–R) NAOI is superior among the three NAOIs compared, followed by the Lisbon–Stykkisholmur/Reykjavik NAOI, with regards to its ability to control year-to-year variability in winter precipitation series and composite precipitation conditions corresponding to the extreme NAOI phases in Turkey. (b) Variability of winter precipitation at most stations in Turkey is significantly correlated with variability of the three NAOIs. Negative relationships are stronger over the Marmara, the Mediterranean Transition and the Continental Central Anatolia regions, and the Aegean part of the Mediterranean region. (c) Composite precipitation analysis exhibited an apparent opposite anomaly pattern at the majority of stations between the weak and strong phases of the NAOIs. Composite precipitation means corresponding to the weak NAOI phase are explained mostly by wetter than long-term average conditions, whereas composite precipitation responses to the strong NAOI phase mostly produce drier than long-term average conditions. (d) Composite wet (dry) conditions during the weak (strong) phase of the NAOI are significant at about 32% (69%) of 78 stations for the PD–R NAOI, and about 38% (55%) for the L–S(R) NAOI. The dry signals from the strong NAO phases are stronger and show a larger spatial coherence over Turkey. The significant signals are evident in the west, centre and south of the country. (g) Widespread severe droughts in 1943, 1957, 1973, 1974, 1983, 1989, 1990, 1992, 1993 and 1994, and widespread strong wet conditions in 1940–1942, 1956, 1963, 1966, 1969 and 1970 were linked to the extreme high- and low-index events of at least two NAOIs, respectively.