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The Yangtze River basin (YRB) experienced a record-breaking mei-yu season in June–July 2020. This unique long-lasting extreme event and its origin have attracted considerable attention. Previous studies have suggested that the Indian Ocean (IO) SST forcing and soil moisture anomaly over the Indochina Peninsula (ICP) were responsible for this unexpected event. However, the relative contributions of IO SST and ICP soil moisture to the 2020 mei-yu rainfall event, especially their linkage with atmospheric circulation changes, remain unclear. By using observations and numerical simulations, this study examines the synergistic impacts of IO SST and ICP soil moisture on the extreme mei-yu in 2020. Results show that the prolonged dry soil moisture led to a warmer surface over the ICP in May under strong IO SST backgrounds. The intensification of the warm condition further magnified the land thermal effects, which in turn facilitated the westward extension of the western North Pacific subtropical high (WNPSH) in June–July. The intensified WNPSH amplified the water vapor convergence and ascending motion over the YRB, thereby contributing to the 2020 mei-yu. In contrast, the land thermal anomalies diminish during normal IO SST backgrounds due to the limited persistence of soil moisture. The roles of IO SST and ICP soil moisture are verified and quantified using the Community Earth System Model. Their synergistic impacts yield a notable 32% increase in YRB precipitation. Our findings provide evidence for the combined influences of IO SST forcing and ICP soil moisture variability on the occurrence of the 2020 super mei-yu. [ABSTRACT FROM AUTHOR]

Flash droughts, defined as events with unusually rapid onset and intensification, are emerging into the spotlight as dangerous subseasonal climatic phenomena capable of causing substantial socioenvironmental impacts. However, research on their spatiotemporal variability and major drivers in Central Europe has been limited thus far. This study used gridded soil moisture data from the SoilClim model for the region consisting of the Czech Republic, Slovakia and northern part of Austria in the 1961–2021 period. Established methods of flash drought detection were implemented and adapted to conduct their comprehensive spatiotemporal analysis. The gridded flash drought results were divided into four clusters using the Ward's hierarchical agglomerative method. Individual flash drought episodes were delimited for each cluster, divided into three phases (onset, course, end) and investigated separately in terms of drivers, represented by three meteorological variables (precipitation, actual evapotranspiration, maximum temperature) and atmospheric circulation types based on the objective classification (derived from flow strength, direction and vorticity). The frequency of flash droughts slightly decreased in the winter half-year and slightly increased in the summer half-year, with substantial amplification in the April–June season. The increase was slower than in the case of seasonal droughts, being driven by the longer-term accumulation of water deficit. Circulation drivers exhibited much stronger and more direct influence in the summer half-year, particularly causing the onset of flash drought episodes during the predominance of anticyclonic types and absence of cyclonic types, while the course of flash drought episodes was also connected to increased temperatures and often connected to warm airflow. [ABSTRACT FROM AUTHOR]

The temperature rise during the growing season on the Tibetan Plateau (TP) is crucial for the local vegetation phenology and atmospheric circulation. In this study, we investigated the possible effects of soil moisture (SM) anomalies associated with the freeze–thaw process on the surface air temperature (SAT) rise during the growing season using observations and reanalysis data from 1979 to 2014. Results demonstrate that winter SM anomalies associated with the freeze–thaw process can persist until July, significantly modulating SAT rise intensity from May to July on the TP. Positive winter SM anomalies can lead to decreased in SAT in May but increased in SAT in July, thus strengthening the SAT rise from May to July. Further analysis of physical mechanisms suggest the positive winter SM anomaly leads to the increase of surface latent heat flux but the decrease of surface sensible heat flux in May, thus cooling the SAT. As July approaches, significantly increased atmospheric water vapor caused by the positive winter SM anomaly leads to the enhancement of surface downward longwave radiation, which exceeds the cooling effect of decreased sensible heat, resulting in a warmer SAT. Previous studies mainly revealed the "drier SM—higher SAT" feedback. Here we highlight that the thermal variations caused by SM anomalies related with the freeze–thaw process are complicated in the TP, with completely opposite climatic effects during different months. [ABSTRACT FROM AUTHOR]

Understanding the contributions of large-scale atmospheric circulation and local land surface processes to precipitation is essentially important for the climate prediction. This study adopts a dynamic adjustment (DA) approach based on constructed circulation analogs to quantitatively isolate the contribution of atmospheric circulation to summer land precipitation (Pr) over Asian mid-low latitudes during 1980–2019. The atmospheric circulation factor is represented by the 500 hPa geopotential height (Z500), and the residual component after DA is regarded as the contribution from land surface processes via evaporation mainly resulting from soil moisture (SM). The results indicate that the key SM-Pr feedback areas are mainly located in northeast China and the northern Indian Peninsula. The key influencing area of Z500 on the land Pr anomaly shows a "- + -" tripole pattern in the mid-latitude region. Atmospheric circulation determines the magnitude of summer land Pr, while the residual components reflect the land–atmosphere coupling effect and dominate Pr trend. This conclusion is helpful for better understanding the evolution mechanism of summer climate over Asia mid-low latitudes and may also have application value for climate prediction. [ABSTRACT FROM AUTHOR]

During summer 2021, Western North America (WNA) experienced an unprecedented heatwave with record-breaking high temperatures associated with a strong anomalous high-pressure system, i.e., a heat dome. Here, we use a flow analog method and find that the heat dome over the WNA can explain half of the magnitude of the anomalous temperature. The intensities of hot extremes associated with similar heat dome-like atmospheric circulations increase faster than background global warming in both historical change and future projection. Such relationship between hot extremes and mean temperature can be partly explained by soil moisture-atmosphere feedback. The probability of 2021-like heat extremes is projected to increase due to the background warming, the enhanced soil moisture-atmosphere feedback and the weak but still significantly increased probability of the heat dome-like circulation. The population exposure to such heat extremes will also increase. Limiting global warming to 1.5 °C instead of 2 °C (3 °C) would lead to an avoided impact of 53% (89%) of the increase in population exposure to 2021-like heat extremes under the RCP8.5-SSP5 scenario. The heat dome explains about 55% of the 2021 Western North American high temperatures. The intensity of heat extremes associated with such circulations are increasing faster than background global warming due to soil moisture feedbacks. [ABSTRACT FROM AUTHOR]

Soil moisture is one of the essential climate variables with a potential impact on local climate variability. Despite the importance of soil moisture, studies on soil moisture characteristics in Ethiopia are less documented. In this study, the spatiotemporal variability of Ethiopian soil moisture (SM) has been characterized, and its local and remote influential driving factors are investigated. An empirical orthogonal function (EOF) and KMeans clustering algorithm have been employed to classify the large domain into homogeneous zones. Complex maximum covariance analysis (CMCA) is applied to evaluate the covariability between SM and selected local and remote variables such as rainfall (RF), evapotranspiration (ET), and sea surface temperature (SST). Inter-comparison among SM datasets highlight that the FLDAS dataset better depicts the country's SM spatial and temporal distribution (i.e., a correlation coefficient r = 0.95 , r m s d = 0.04 m 3 m - 3 with observations). Results also indicate that regions located in northeastern Ethiopia are drier irrespective of the season (JJAS, MAM, and OND) considered. In contrast, the western part of the country consistently depicted a wetter condition in all seasons. During summer (JJAS), the soil moisture variability is characterized by a strong east–west spatial contrast. The highest and lowest soil moisture values were observed across the country's central western and eastern parts, respectively. Furthermore, analyses indicate that interannual variability of SM is dictated substantially by RF, though the impact on some regions is weaker. It is also found that ET likely drives the SM in the eastern part of Ethiopia due to a higher atmospheric moisture demand that ultimately invokes changes in surface humidity and rainfall. A composite analysis based on the extreme five wettest and driest SM years revealed a similar spatial distribution of wet SM with positive anomalies of RF across the country and ET over the southern regions. Remote SSTs are also found to have a significant influence on SM distribution. In particular, equatorial central Pacific and western Indian oceans SST anomalies are predominant factors for spatiotemporal SM variations over the country. Major global oceanic indices: Oceanic Nino Index (ONI), Indian Ocean Dipole (IOD), Pacific warm pool (PACWARMPOOL), and Pacific Decadal Oscillations (PDO) are found to be closely associated with the SM anomalies in various parts of the country. The associationship between these remote SST anomalies and local soil moisture is via large-scale atmospheric circulations that are linked to regional factors such as precipitation and temperature anomalies. [ABSTRACT FROM AUTHOR]

Terrestrial ecosystems are fully coupled with the climate. The planet has been greening owing to the increased vegetation growth in response to the changing atmosphere, which in turn has feedback on the climate. Greening has slowed down the rise in global land-surface air temperature mainly through a coincident increase of evapotranspiration and precipitation in wet regions. In dry regions, greening intensifies the decrease in soil moisture induced by greening-enhanced transpiration. Uncertain, however, is how the climate effects of greening in semi-arid lands might differ for variable wet and dry conditions. Here, we focus on the biosphere–atmosphere interactions in Australia by modeling the perturbation of vegetation changes under various states of sea surface temperature (SST), including the climatology mean, El Niño, and La Niña conditions. For the dry conditions of El Niño, greening exacerbates water stress and largely depletes the soil moisture, while for the wet conditions of La Niña, greening-enhanced evapotranspiration and precipitation resupply the soil moisture. For the normal conditions using the climatology mean SST, a small decrease in soil moisture occurs but with large spatial contrast because of heterogeneous changes of evapotranspiration and precipitation induced by greening. We emphasize that the alternating dry and wet conditions modulated by the large-scale climate variability are vital to understanding the response of climate to greening. Furthermore, vegetation-based warming mitigation policies need to be cautious when inferring distinct climate effects associated with greening. [ABSTRACT FROM AUTHOR]

This paper reports findings from a diagnostic and modeling analysis that investigates the impacts of the boreal late-spring soil moisture anomalies over North Eurasia on the summer rainfall over northern East Asia (NEA). The soil moisture in May from the Kara-Laptev Sea coasts to Central Siberian Plateau is found to be negatively correlated with the summer rainfall from Mongolia to Northeast China. The atmospheric circulation anomalies associated with the anomalously dry soil are characterized by a pressure dipole with the high-pressure center located over North Eurasia and low-pressure center over NEA, where an anomalous water vapor convergence occurs, favoring rainfall formation. Diagnoses and modeling experiments demonstrate that the effects of the spring lower soil moisture over North Eurasia may persist into the following summer, then increase the low-level air temperature at higher latitudes through modulating local surface latent and sensible heat fluxes, and effectively reduce the meridional temperature gradient north of NEA. The weakened temperature gradient could induce the decreased vertical shear of zonal wind and generate an anomalously cyclonic center over NEA by affecting the baroclinicity around 60° N, associated with a favorable condition of local synoptic activity to increase rainfall. The above relationships and mechanisms are vice versa for the prior wetter North Eurasian soil and decreased NEA rainfall. These findings suggest that the soil moisture anomalies at higher latitudes may act as a new precursor providing an additional predictability source for better predicting the summer rainfall in NEA. [ABSTRACT FROM AUTHOR]

Projected changes in near-surface relative humidity (RH) remain highly model-dependent over land and may have been underestimated by the former generation global climate models. Here the focus in on the recent CNRM-CM6-1 model, which shows an enhanced land surface drying in response to quadrupled atmospheric CO2 compared to its CNRM-CM5 predecessor. Atmosphere-only experiments with prescribed sea surface temperature (SST) are used to decompose the simulated RH changes into separate responses to uniform SST warming, pattern of SST anomalies, changes in sea-ice concentration, as well as direct radiative and physiological CO2 effects. Results show that the strong drying simulated by CNRM-CM6-1 is due to both fast CO2 effects and a SST-mediated response. The enhanced drying compared to CNRM-CM5 is partly due to the introduction of the physiological CO2 effect that was not accounted for in CNRM-CM5. The global ocean warming also contributes to the RH decline over land, in reasonable agreement with the moisture advection mechanism proposed by earlier studies which however does not fully capture the contrasted RH response between the two CNRM models. The SST anomaly pattern is a significant driver of changes in RH humidity at the regional scale, which are partly explained by changes in atmospheric circulation. The improved land surface model may also contribute to a stronger soil moisture feedback in CNRM-CM6-1, which can amplify the surface aridity induced by global warming and, thereby, lead to a non-linear response of RH. [ABSTRACT FROM AUTHOR]

In the period 1960–2010, the land surface air temperature (SAT) warmed more rapidly over some regions relative to the global mean. Using a set of time-slice experiments, we highlight how different physical processes shape the regional pattern of SAT warming. The results indicate an essential role of anthropogenic forcing in regional SAT changes from the 1970s to 2000s, and show that both surface–atmosphere interactions and large-scale atmospheric circulation changes can shape regional responses to forcing. Single forcing experiments show that an increase in greenhouse gases can lead to regional changes in land surface warming in winter (DJF) due to snow-albedo feedbacks, and in summer (JJA) due to soil-moisture and cloud feedbacks. Changes in anthropogenic aerosol and precursor (AA) emissions induce large spatial variations in SAT, characterized by warming over western Europe, Eurasia, and Alaska. In western Europe, SAT warming is stronger in JJA than in DJF due to substantial increases in clear sky shortwave radiation over Europe, associated with decreases in local AA emissions since the 1980s. In Alaska, the amplified SAT warming in DJF is due to increased downward longwave radiation, which is related to increased water vapor and cloud cover. In this case, although the model was able to capture the regional pattern of SAT change, and the associated local processes, it did not simulate all processes and anomalies correctly. For the Alaskan warming, the model is seen to achieve the correct regional response in the context of a wider North Pacific anomaly that is not consistent with observations. This demonstrates the importance of model evaluation that goes beyond the target variable in detection and attribution studies. [ABSTRACT FROM AUTHOR]

The current study investigates the observed land surface feedbacks on the Australian monsoon system by applying a multivariate statistical method, Stepwise Generalized Equilibrium Feedback Assessment (SGEFA), to a spectrum of observational, remote sensing, and reanalysis datasets. In particular, the influence of Madden Julian Oscillation (MJO) on the Australian monsoon system is accounted for in the assessment of land surface feedbacks. The approach reveals the quantitative impacts and relative contribution of key individual oceanic, vegetation, and soil moisture forcings, along with the MJO, on the Australian monsoon's observed sub-seasonal to interannual variability. During both the flanks and core of the Australian summer monsoon season, terrestrial and oceanic drivers impose comparable contributions, both exceeding MJO's contribution, to the regional climate variability. Vegetation and soil moisture generally exert opposing feedbacks on the monsoon system, thereby weakening the net impact of terrestrial drivers. Vegetation feedbacks peak in the mid- to post-monsoon period, with positive anomalies in leaf area index generating a modified atmospheric circulation pattern that reduces moisture advection and rainfall across northern Australia. Soil moisture feedbacks are most pronounced during the pre- and post-monsoon seasons, with positive soil moisture anomalies favoring more rainfall through enhanced moisture recycling and modified atmospheric circulation. The current observational evidence of a negative vegetation-rainfall feedback across northern Australia and its underlying dynamical feedback mechanism deviates from most previous modeling studies that concluded a positive vegetation-rainfall feedback via a moisture recycling mechanism. [ABSTRACT FROM AUTHOR]

The ERA-Interim (1979–2016) and National Center for Environmental Prediction (NCEP) (1958–2016) reanalysis data are applied in an empirical orthogonal function (EOF) decomposition of the summer zonal wind anomaly in the upper troposphere (at 200 hPa) over Asia. Consistent spatio-temporal modes are derived from the two datasets on different time scales, with the first three dominant modes representing the principal features of the summer Asian subtropical westerly jet (ASWJ). The first and third modes respectively describe the meridional position deviation and the overall strength variations of ASWJ, and both exhibit zonally symmetric distributions, which have been extensively investigated. The second mode describes a zonally asymmetric distribution of the jet over Asia, which reflects coordinated variations in the upstream and downstream systems and has not yet been systematically studied. In this study, we focus on the analysis of the second mode of ASWJ. The observation results show that the second mode is closely related to external forcing factors as well as the large-scale circulation at mid-high latitudes. Through the interaction of the preceding North Atlantic sea surface temperature (SST), the simultaneous soil moisture, and the atmospheric circulation that propagates zonally via the waveguide effect of the jet, the ocean–land–atmosphere coupled system can produce coordinating impacts on the variability of ASWJ. These results aid in a more complete and comprehensive understanding of the physical characteristics of the summer westerly jet and provide a new perspective for the study of the variability of ASWJ. [ABSTRACT FROM AUTHOR]

In this study we assess impacts of land-surface initialization on sub-seasonal and seasonal forecast skills from a coupled model named ACCESS-S1 (a seasonal prediction version 1 of the Australian Community Climate and Earth-System Simulator). A series of sensitivity experiments is conducted to explore to what extent the model skill and mean bias can be affected by different land-surface initialisations. By focusing the analysis on the Australian continent, our study tries to address three questions: (1) how strong is soil moisture memory in the model and how realistic is that compared with some observational evidence; (2) how does the soil moisture memory affect surface fluxes; and (3) how these impacts on surface fluxes are translated into the impacts on forecasting rainfall, temperature and atmospheric circulation. Firstly, we run an offline experiment with the ACCESS-S1 land surface model JULES for the period of 1990–2012 using ERA-interim forcing data, with precipitation further adjusted by monthly observations. This produces 23-year soil moisture time series corresponding to "observed" meteorological forcing. Lagged correlations between soil moisture at different levels and at different months are compared with some in-situ observations over Murrumbidgee catchment. We show good agreement between modelled and observed soil moisture coupling between its top-layer and sub-surface root-zone in the southeast part of the continent, and notable soil moisture memory over these regions. We then use the JULES offline soil moisture data to initialize ACCESS-S1 in its 3-month hindcasts with start date of 1st May for the 23-year period. In contrast to the default ACCESS-S1 setup which uses climatological soil moisture in its land-surface initialisation, our results show significant improvements to ACCESS-S1 forecast skill of surface maximum temperature (Tmax) and evapotranspiration by initialising the model with JULES offline data. It also has moderate improvements of surface minimum temperature (Tmin) and precipitation forecasts. The skill gain is particularly evident over the eastern part of the continent where the modelled and observed soil moisture memory is strong. Our study demonstrates that in future forecast system development, we not only need to initialise the model with updated soil moisture anomalous conditions, but also need to make sure these anomalies are combined with correct and consistent soil moisture climatology for both hindcast and real-time forecast. [ABSTRACT FROM AUTHOR]

The differences in planetary boundary layer characteristics, in particular atmospheric boundary layer height (ABLH), humidity, and local circulations in pre-monsoon and monsoon period over the Erhai Lake, were simulated by the lake-atmosphere coupled model WRF v3.7.1. No lake simulations were also conducted to investigate lake effects over complex topography. During pre-monsoon period, local circulation was fully developed under weak synoptic system. The ABLH ran up to 2300 m or so. During monsoon period, temperature difference between land and lake became smaller, resulting in weaker local circulations. The height of circulation reduced by 500 m, and ABLH ran up to 1100 m during the day. Enhanced soil moisture and low surface temperature due to monsoon rainfalls in July could be the main reason for the slightly lower ABLH over the Erhai Lake area. Specific humidity of the boundary layer increased 8.8 g kg−1 or so during monsoon period. The Erhai Lake enlarged thermal contrast between valley and mountain slope in the Dali Basin. The lake reduced air temperature by 2~3 °C during daytime and increased air temperature by nearly 2 °C in the evening. Due to its small roughness length and large thermal capacity, the Erhai Lake enlarged lake-land temperature difference and local wind speed. A cyclonic circulation was maintained by the combination of mountain breeze and land breeze in the south of the lake. The lake decreased air temperature, increased specific humidity, and reduced ABLH during daytime, whereas the opposite effect is presented at night. [ABSTRACT FROM AUTHOR]

The local hydrological and climatic impacts of Eurasian snowmelt are studied using advanced land surface and atmospheric data. It is found that intense melting of snow is located at mid-high latitudes in April and May. Snowmelt plays an important role in determining the seasonal cycles of surface runoff and soil moisture (SM). Specifically, melting is accompanied by sharp responses in surface runoff and surface SM while the impacts are delayed for deeper-layer of soil. This is particularly significant in the western sector of Eurasia. On interannual timescales, the responses of various surface parameters to snowmelt in the same month are rather significant. However, the persistence of surface SM anomalies is weak due to the strong soil evaporation anomalies and surplus of surface energy for evaporation. Strong impacts on the sensible heat flux, planetary boundary layer height and precipitation in the next month following the melting of snow are identified in west Russia and Siberia. Downward propagation of surface SM anomalies is observed and a positive evaporation-convection feedback is identified in west Russia. However, the subsequent impacts on the local convective precipitation in late spring-summer and its contribution to the total precipitation are seemingly weak. The atmospheric water vapor convergence has strong control over the total precipitation anomalies. Overall, snowmelt-produced SM anomalies are not found to significantly impact the late spring-summer local climate anomalies in Northern Eurasia. Therefore, the delayed remote-responses of atmospheric circulation and climate to the melting of Eurasian snow may be only possible near the melting period. [ABSTRACT FROM AUTHOR]

Atmospheric variability is driven not only by internal dynamics, but also by external forcing, such as soil states, SST, snow, sea-ice cover, and so on. To investigate the forecast uncertainties and effects of land surface processes on numerical weather prediction, we added modules to perturb soil moisture and soil temperature into NCEP's Global Ensemble Forecast System (GEFS), and compared the results of a set of experiments involving different configurations of land surface and atmospheric perturbation. It was found that uncertainties in different soil layers varied due to the multiple timescales of interactions between land surface and atmospheric processes. Perturbations of the soil moisture and soil temperature at the land surface changed sensible and latent heat flux obviously, as compared to the less or indirect land surface perturbation experiment from the day-to-day forecasts. Soil state perturbations led to greater variation in surface heat fluxes that transferred to the upper troposphere, thus reflecting interactions and the response to atmospheric external forcing. Various verification scores were calculated in this study. The results indicated that taking the uncertainties of land surface processes into account in GEFS could contribute a slight improvement in forecast skill in terms of resolution and reliability, a noticeable reduction in forecast error, as well as an increase in ensemble spread in an under-dispersive system. This paper provides a preliminary evaluation of the effects of land surface processes on predictability. Further research using more complex and suitable methods is needed to fully explore our understanding in this area. [ABSTRACT FROM AUTHOR]

This study uses experiments with an atmospheric general circulation model (AGCM) to address the role of the oceans and the effect of land-atmosphere coupling on the predictability of summertime rainfall over northern Argentina focusing on interdecadal time scales during 1901-2006. Ensembles of experiments where the AGCM is forced with historical sea surface temperature (SST) in the global, Pacific and tropical-North Atlantic domains are used. The role of land-atmosphere interaction is assessed comparing the output of simulations with active and climatological soil moisture. A maximum covariance analysis between precipitation and SST reveals the impact of the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation and the equatorial-tropical South Atlantic on rainfall over northern Argentina. Model simulations further show that while the dominant influence comes from the Pacific basin, the Atlantic influence can explain a large transition from dry to wet decades over northern Argentina during the beginning of the 1970s. Analysis of anomalies before and after the transition reveals an upper level anticyclonic circulation off the Patagonian coast with barotropic structure. This circulation enhances the moisture transport and convergence in northern Argentina and, together with enhanced evaporation, increased the rainfall after 1970. The SST pattern is dominated by cold conditions in the equatorial Atlantic and warm eastern Pacific and South Atlantic. We also found that land-atmosphere interaction leads to a representation of the long term rainfall evolution over northern Argentina that is closer to the observed one. Moreover, it leads to a smaller dispersion among ensemble members, thus resulting in a larger signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The annual cycle of solar radiation, together with the resulting land-ocean differential heating, is traditionally considered the dominant forcing controlling the northward progression of the Indian monsoon. This study makes use of a state-of-the-art atmospheric general circulation model in a realistic configuration to conduct 'perpetual' experiments aimed at providing new insights into the role of land-atmosphere processes in modulating the annual cycle of precipitation over India. The simulations are carried out at three important stages of the monsoon cycle: March, May, and July. Insolation and SSTs are held fixed at their respective monthly mean values, thus eliminating any external seasonal forcing. In the perpetual May experiment both precipitation and circulation are able to considerably evolve only by regional internal land-atmosphere processes and the mediation of soil hydrology. A large-scale equilibrium state is reached after approximately 270 days, closely resembling mid-summer climatological conditions. As a result, despite the absence of external forcing, intense and widespread rains over India are able to develop in the May-like state. The interaction between soil moisture and circulation, modulated by surface heating over the northwestern semi-arid areas, determines a slow northwestward migration of the monsoon, a crucial feature for the existence of desert regions to the west. This also implies that the land-atmosphere system in May is far from being in equilibrium with the external forcing. The inland migration of the precipitation front comprises a succession of large-scale 35-50 day coupled oscillations between soil moisture, precipitation, and circulation. The oscillatory regime is self-sustained and entirely due to the internal dynamics of the system. In contrast to the May case, minor changes in the land-atmosphere system are found when the model is initialized in March and, more surprisingly, in July, the latter case further emphasizing the role of northwestern surface heating. [ABSTRACT FROM AUTHOR]

An ensemble of six 22-year numerical experiments was conducted to evaluate the ability of Regional Climate Model version 3 (RegCM3) to simulate the energy and water budgets of the midwestern United States. RegCM3 was run using two surface physics schemes: Integrated Biosphere Simulator (IBIS) and Biosphere-Atmosphere Transfer Scheme 1e (BATS1e), and two convective closure assumptions: Fritsch & Chappell (FC80) and Arakawa & Schubert (AS74). Boundary conditions were provided by the National Centers for Environmental Prediction-Department of Energy Reanalysis 2 dataset and the ECHAM5 general circulation model. A companion paper examines the American Midwest under future climate scenarios. Overall, the model that reproduces the observed seasonal cycles of the midwestern United States climate system best is RegCM3 using IBIS and the AS74 convective closure assumption. IBIS simulates shortwave radiation more accurately, while BATS1e simulates longwave radiation more accurately. Summer two-meter air temperature is overestimated by the combination of IBIS and the FC80 convective closure assumption. All models contain a wet bias and overestimate evapotranspiration during the spring. Total runoff, surface runoff, groundwater runoff, and root zone soil moisture are best simulated by RegCM3 using IBIS and the AS74 convective closure assumption. While BATS1e does capture the seasonal cycle of total runoff, gross errors in the partitioning of total runoff between surface runoff and groundwater runoff exist. The seasonal cycle of root zone soil moisture simulated by RegCM3 using IBIS and the AS74 convective closure assumption is dry, but agrees with observations during the summer. The rest of the models underestimate root zone soil moisture. [ABSTRACT FROM AUTHOR]

Simulations with the Consortium for Small Scale Modelling model were performed to investigate the impact of land surface inhomogeneities on the initiation of convection. A case from the African Monsoon Multidisciplinary Analysis campaign, 11 June 2006, was selected. On this day, a mesoscale convective system was observed and simulated. The simulation scenarios included a realistic and an increased initial soil moisture distribution as well as a homogeneous soil moisture and texture field. Land use and orography were the same in all runs. Heat and moisture budget calculations were applied to analyse the processes responsible for the evolution of pre-convective atmospheric conditions and convection-triggering thermally induced circulation systems. Convective cells were initiated in all experiments. However, the amount of cells, their origin, evolution, and precipitation amount differed. First shallow clouds were initiated over areas with higher sensible heat fluxes. The evolution of subsequent deep convection was triggered by secondary circulation systems caused by baroclinic conditions generated by clouded and unclouded regions. The further evolution of the precipitation cells strongly depended on convective inhibition in the areas the cells moved into. [ABSTRACT FROM AUTHOR]

An analysis of Italian seasonal temperatures from 1961 to 2006 was carried out, using homogenized data from 49 synoptic stations well distributed throughout Italy. The results show remarkable differences among seasons. Stationarity characterizes winter series, except for Northern Italy (where a warming trend from 1961 is identified); a positive trend over the entire period is recognized for spring series. Summer series are marked by a negative trend until 1981 and by a positive trend afterwards; finally, autumn series show a warming starting from 1970. The relationship between seasonal temperatures and four teleconnection patterns (North Atlantic Oscillation, East Atlantic Pattern, Scandinavian Pattern and Arctic Oscillation) influencing European climate was investigated through Spearman rank correlation and composites. Among the results, the strong linear correlation with the East Atlantic Pattern in all seasons but autumn is remarkable; moreover, the explained variance varies between 31.9% and 50.4% (leaving out autumn). Besides these four atmospheric patterns the role of other factors (e.g. soil moisture) is not dealt with, but their importance and the need for more investigation is pointed out. [ABSTRACT FROM AUTHOR]

During the last two decades, several land surface schemes for use in climate, regional and/or mesoscale, hydrological and ecological models have been designed. Incorrect parametrization of land-surface processes and prescription of the surface parameters in atmospheric modeling, can result in artificial changes of the horizontal gradient of the sensible heat flux. Thus, an error in horizontal temperature gradient within the lower atmosphere may be introduced. The reliability of the model depends on the quality of boundary layer scheme implemented and its sensitivity to the bare soil and vegetation parameters. In this study, a series of sensitivity experiments has been conducted over broad time scales, using a version of the ARPEGE Climate Model coupled to the ISBA land surface scheme in order to investigate model sensitivity to separate changes in land surface parameters over Africa. Effects of perturbing vegetation cover, distribution of soil depth, albedo of vegetation, roughness length, leaf area index and minimum stomatal resistance were explored by using a simple statistical analysis. Identifying which parameters are important in controlling turbulent energy fluxes, temperature and soil moisture is dependent on which variables are used to determine sensibility, which type of vegetation and climate regime is being simulated and the magnitude and sign of the parameter change. This study does not argue that a particular parameter is important in ISBA, rather it shows that no general ranking of parameters is possible. So, it is essential to specify all land surface parameters with greater precision when attempting to determine the climate response to modification of the land surface. The implication of ISBA being sensitive to parameters that cannot be validated suggests that there will always be considerable doubt over the predictive quality of land-surface schemes. [ABSTRACT FROM AUTHOR]

Our study found that local mesoscale circulations associated with highland areas, and transient evapotranspiration discontinuities, are likely to influence the timing and location of the initiation of deep convection across the Canadian prairie provinces when synoptic-scale forcing is weak (e.g., weak low level winds and no frontal boundaries). The cumulus congestus and cumulonimbus clouds that formed over the highland areas were initiated by anabatic wind induced mesoscale circulations. These deep convective clouds generally formed relatively early in the day (about 1030 Central Standard Time (CST)). In the relatively flat cropped grassland region, transient evapotranspiration gradients influenced the location of deep convection initiation, and the average soil moisture in the root zone had a direct impact on their timing. As root zone soil moisture declined from greater than 70% to less than 30%, convection initiation was delayed from about 0930 to 1630 CST. Cumulus congestus and cumulonimbus clouds that formed over the ephemeral evapotranspiration gradients were initiated by land-land circulations. The study has improved the understanding of the influence of local surface forcing on the development of deep convective cloud on the Canadian prairie provinces. The identification of areas where deep convection is likely to be initiated with weak synoptic forcing will also aid in the forecasting of thunderstorms in this region. [ABSTRACT FROM AUTHOR]

Presents a study which examined the relationship between long-term changes in North Atlantic atmospheric circulation, sea surface temperatures and soil water tables in Great Britain. Analysis of temperature changes in northwestern Europe; Methodology used; Results and discussion.