The Met Office Unified Model has been adapted to run with a 100 m horizontal grid length for a region of complex terrain whose valleys are too narrow to be represented in the operational forecast (1.5 km grid length) model. Forecasts of temperatures are compared with observations from the COLd air Pooling EXperiment. The model provides a realistic representation of valley cold pools which form during stable nights, although the predictions are shown to be sensitive to vertical resolution. Reducing the vertical grid spacing near the surface results in lower night-time temperatures and larger, more realistic, valley–hill temperature contrasts.

A soft-computing cyclone intensity prediction scheme (SCIPS) is introduced using an artificial neural network (ANN) approach and adding ocean heat content, as an additional predictor to the normally used atmospheric parameters, to predict tropical cyclone intensity change in the western north Pacific Ocean. We used 1997–2004 data to develop and validate this scheme. The ANN-based estimations have been compared with observations and estimations using the multiple linear regression (MLR). SCIPS performance improves upon MLR as the lead hour increases from 12 to 120 h and also for high intensifying cyclones. Copyright © 2013 Royal Meteorological Society

In this study, long-term changes in the summer monsoon rainfall over the west coast of India and adjoining eastern Arabian Sea (WCA) have been examined independently using rain gauge adjusted multisatellite rainfall data available since 1979. Present analysis using the satellite-based rainfall data agrees with the long-term rainfall decrease reported in the earlier studies. However, close examination of the data reveals that the rainfall in the recent years (since 2002) has started increasing over the WCA. Hence, it is stressed that a detailed analysis using the satellite-derived rainfall data of longer period is required to arrive at a firm conclusion. Copyright © 2013 Royal Meteorological Society

Cloud dynamics played a fundamental role in defining Indian summer monsoon (ISM) rainfall during drought of 2009. The anomalously negative precipitation was consistent with cloud properties. Although, aerosols inhibited the growth of cloud effective radius in the background of sparse water vapor, their role is secondary. The primary role, however, is played by the interactive feedback between cloud microphysics and dynamics owing to reduced efficient cloud droplet growth, lesser latent heating release and shortage of water content. Cloud microphysical processes were instrumental for the occurrence of ISM drought 2009. Copyright © 2013 Royal Meteorological Society

We produced 100-member event attribution ensembles during 2009–2012 under all forcing conditions and in two different counterfactual worlds without anthropogenic forcing (mainly greenhouse gases and aerosols) and without aerosol emission changes using the MIROC5 atmospheric general circulation model. It seemed that both human influences and the sea surface temperature (SST) natural variability increased probabilities of the 2010 severe drought in the South Amazon region, and that changes in aerosols emissions had little effect on the drought. It should be noted that our assessments were sensitive to bias corrections according to the relationships between the SST natural variability and precipitation. © 2013 The Authors. Atmospheric Science Letters published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society.

Sea-salt aerosols proposed for injection in marine cloud brightening geoengineering would likely result from evaporation of sea-water droplets. Previous simulations have omitted this mechanism. Using the WRF/Chem model (Weather Research and Forecasting model coupled with Chemistry) in large-eddy simulation mode, we find that droplet evaporation creates cold pools, suppressing initial aerosol plume heights by up to 30% (40 m). This lessens cloud albedo increases from 94.1 to 88.5% in our weakly-precipitating case and from 4.3 to 1.4% for daytime injection into our nonprecipitating case (cloud albedo differences of 0.012 and 0.009, respectively). Inclusion of this effect in future modelling would allow increasingly realistic effectiveness estimates. Copyright © 2013 Royal Meteorological Society

Fronts passing over the Mesosphere–Stratosphere–Troposphere radar near Aberystwyth, Wales, UK, possessed multiple bands in returned power and vertical shear of the horizontal wind. Warm fronts tended to have more bands than cold fronts. Of 82 bands in power with an associated band in the radiosonde data, 45% were associated with a decrease in specific humidity and an increase in stability (drying stable bands) and 21% were associated with an increase in humidity and a decrease in stability (moistening unstable bands). The radar detects airstream boundaries near fronts, some of which are not part of the front.

Trends in the thermal tropopause height observed using radiosondes near landing typhoon tracks in China from 1951 to 2008 were analyzed. The main findings were that the majority of tropopause rises induced by typhoons are in the range of 200–500 m, and the maximum tropopause height is observed between 12 and 24 h after the typhoon had passed at each station. Tropopause height increase induced by a super typhoon is more rapid and prominent than that of a typhoon, although the absolute increases are similar. The magnitude of the increase in the tropopause height induced by typhoons has increased over the last 40 years.

Observed difference in diurnal cycle of precipitation over the eastern Indian Ocean between positive and negative Indian Ocean Dipole (IOD) phases was investigated using a regional model. During the negative IOD phase, prominent precipitation peak was over the land during evening in both observation and model, while precipitation often appeared over the ocean during early morning. In the positive IOD phase, precipitation peak was shifted to the northern area where sea surface temperature was warm. The amount of precipitation in the southern area during the negative phase was more than three times larger than that in the positive phase. Copyright © 2013 Royal Meteorological Society

A major ingredient for Mediterranean Heavy Precipitating Events (HPEs) is the low-level moisture transport towards the coast. This paper proposes a methodology allowing a spatially and temporally continuous study of the water supply to Mediterranean HPEs. It is based on water budgets computed from mesoscale numerical simulation outputs. The method is applied on four typical HPEs over Southeastern France. It allows to identify and assess quantitatively the contributions of local Mediterranean and remote African and Atlantic moisture sources. Copyright © 2013 Royal Meteorological Society

Long-term changes in extreme precipitation were studied using data of daily maximum 10-min and hourly precipitation, as well as daily total precipitation for 60 years (1951–2010) in Japan. It was found that extreme 10-min and hourly precipitations, defined from 5-year averages of annual maximum and 95th percentile values, have a high correlation with temperature with a rate of 9.7 ± 4.1% and 8.8 ± 8.3% K⁻¹, respectively. These values are close to the Clausius–Clapeyron (CC) rate of change in saturation vapor pressure (about 6% per degree), indicating that the CC relation roughly holds for multidecadal changes in extreme short-time precipitation in Japan. Copyright © 2013 Royal Meteorological Society

Twelve coupled model simulations of two multi-model ensemble (MME) systems for boreal winters from 1983 to 2005 are used to improve the climate prediction. From grading the relative capability of each simulation in reproducing the observed link between the tropical El Niño-Southern Oscillation (ENSO)-related Walker circulation and the Pacific rainfall, we find an optimal MME suite with improved prediction skills. This study demonstrates that the climate filter concept, proposed by us in a recent work, is not only useful in improving the MME prediction skills as compared to a single MME system, but also the skills of a grand MME that encompasses two well-performing MMEs. Copyright © 2013 Royal Meteorological Society

Time series analysis of the average rainfall over a target area in southern Amazon Basin showed a spectral peak at 11 day period. An objective method for defining the South Atlantic Convergence Zone (SACZ) is used to identify 28 episodes affecting southern Amazon Basin during the 10 summers in the period 1999–2010. The 28-episode composite precipitation anomalies show significant positive values over the target area. The convergence of moisture over the target area in the SACZ composites is about 35% stronger than the climatological value. Copyright © 2012 Royal Meteorological Society

Ensemble prediction system (EPS) meteorological forecasts are increasingly being used as input to hydrological models in order to extend flood-warning lead-times and to improve forecasts and the knowledge of their uncertainty. Probabilistic skill scores classically used in meteorology are usually also utilized for assessing the quality of hydrological ensemble forecasts. However, the different river discharge magnitudes can make difficult the interpretation and comparisons of these scores, as it is for the continuous rank probability score (CRPS). In this letter, a novel ‘Reduction’ CRPS (RCRPS), which takes into account the different river discharge magnitudes, is proposed, and its usefulness is exhibited. Copyright © 2013 Royal Meteorological Society

Statistical analysis shows that the Indo-Pacific convection oscillation (IPCO) in boreal summer undergoes quasi-biennial variability, and is closely related to the East Asian summer monsoon (EASM). Negative IPCO phases, which exhibit an enhanced convection over the north Indian Ocean and a suppressed convection over the western North Pacific, favor a weakened EASM and an increase of summer rainfall in the Yangtze River valley with the joint actions of the stronger than normal Ural and Okhotsk blocking highs and the subtropical western Pacific high, and vice versa. Copyright © 2013 Royal Meteorological Society

The observed and projected precipitation variability in Athens is assessed for a 2.5 century period. The observed variability is examined using the 150-year historical record of the National Observatory of Athens, while future variability using an ensemble of regional climate models. A good agreement between observations and the model ensemble was found for annual and seasonal precipitation. The model ensemble overestimated the number of rainy days and precipitation amount corresponding to light precipitation and underestimated them corresponding to moderate–heavy precipitation. A 35% decrease in total annual precipitation coupled with an increase in extreme precipitation was projected by the model ensemble up to 2100. Copyright © 2013 Royal Meteorological Society

Residual kriging (RK) is applied to data from measurements performed with an instrumented motorglider in an Alpine Valley, where a coupled lake and valley wind develops. Results provide an insight into a typical mean vertical structure of the valley boundary layer, displaying a rather shallow convective mixed layer, surmounted by a deep stable layer. Mapping with RK allows to detect spatial temperature anomalies associated with the local development of diurnal winds over each explored valley cross section. Local scale features, such as cross-valley thermal asymmetries, amenable to surface inhomogeneities and their effects on heat fluxes, are also identified. Copyright © 2013 Royal Meteorological Society

Many identified effects of atmospheric aerosol particles on climate come from pollutants. The effects of light-absorbing carbon particles (soot) are amongst the most uncertain and they are also considered to cause climate warming on the same order of magnitude as anthropogenic carbon dioxide. This study contributes to the understanding of the potential for transformation of the surface character of soot from hydrophobic to hydrophilic, which in clouds promotes a build-up of water-soluble material. We use molecular dynamics simulations to show how natural surfactants facilitate solubilization of fluoranthene, which we use as a model compound for soot in nanoaerosol water clusters. Copyright © 2013 Royal Meteorological Society

The present study investigates the influence of July to October thermal state of warm pool in the western Pacific on surface sensible heat flux by using observing system experiments with European Centre for Medium-Range Weather Forecasts (ECWMF) operational ocean analysis (ORA-S3) for the period 1959–2009, and multidecade global flux datasets from the Objectively Analyzed Air-sea Fluxes (OAFlux) Project for the period 1958–2009. It is found that the positive (negative) sensible heat flux anomalies over the western Pacific are associated with the warm (cold) state in the warm pool, which are attributed to the air–sea temperature difference anomalies over the western Pacific. Copyright © 2013 Royal Meteorological Society

The decrease in the September sea-ice extent over the Arctic has been accelerating since 1996. This study examined the contributions of atmospheric circulation anomalies and trends in winter and summer to changes in Arctic sea ice during the periods 1979–1996 and 1996–2010. In recent years, winter westerly winds over the Beaufort Sea and summer anticyclonic circulation over the Arctic toward the Fram Strait have contributed to accelerated decreases in sea ice over areas east of Europe and north of Alaska. In particular, recent strong anticyclonic circulation has caused an accelerating decrease in the Arctic sea ice in summer. Copyright © 2013 Royal Meteorological Society

For the tropical Atlantic Ocean two phenomena showing anomalous sea surface temperature (SST) warming have been described: the Atlantic Niño in the equatorial Atlantic and the Benguela Niño off the Angolan coast. In this study, both Niño types are integrated into a new set of SST regions and the definition of a robust standardized index for the long-term period 1870–2011. Comparisons of these regions and indices show a close connection between anomalous warming and cooling in the equatorial Atlantic and the Benguela region. Therefore, instead of considering equatorial Atlantic and Benguela warm and cold events separately, we propose to classify them into three subtypes of one comprehensive Atlantic Niño. Copyright © 2013 Royal Meteorological Society

This article uses an atmospheric general circulation model to show that the cooler North Atlantic sea surface temperature anomalies in summer can produce a Rossby wave-like teleconnection pattern strengthening the Siberian High in winter and next spring. The stronger Siberian High enhances the continent northerlies over East Asia and the associated cyclonic circulation over the western North Pacific, which provides a tropical westerly background for occurrence of the westerly wind bursts. The stronger northerlies over East Asia can also induce frequent cold surges, which tend to produce the equatorial westerly wind anomalies in the western Pacific for initiating an El Niño event. Copyright © 2013 Royal Meteorological Society

To investigate the soil-layer configuration requirement for numerical studying large-eddy scale atmosphere and land surface interactions, we conduct two experiments with an atmosphere and land surface coupled large-eddy model, one with the ‘usual’ soil-layer configuration and the other with a finer one. It is found that the former configuration prevents the soil to respond to atmospheric large eddies and the soil acts solely as a fixed external condition to the atmospheric turbulence, and is thus inadequate for large-eddy atmosphere and land surface coupled modeling. It is shown that soil-layer configuration has a profound impact on the simulated atmospheric boundary layer dynamics. Copyright © 2013 Royal Meteorological Society

In many hydro-meteorological applications, it is not always possible to get access to in situ weather measurements, especially for the ungauged catchments. This study explores the performances of downscaled weather data for Reference Evapotranspiration (ET_o) retrieval using the global European Centre for Medium Range Weather Forecasts (ECMWF) ERA interim and National Centers for Environmental Prediction (NCEP) reanalysis data, simulated through Weather Research and Forecasting (WRF) mesoscale model. The range of the Nash-Sutcliffe efficiency calculated for the ECMWF pooled datasets derived ET_o varies from 0.31 to 0.87, while for NCEP it is found to be 0.11 to 0.38. Bias and Root Mean Square Error (RMSE) are also indicating a very high discrepancy in the NCEP ET_o (Bias = −0.05; RMSE = 0.11) as compared to ECMWF (Bias = 0.00; RMSE = 0.06). The overall findings reveal that ECMWF downscaled products have a much better performance than the NCEP's counterparts. Copyright © 2013 Royal Meteorological Society