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This study demonstrated the influence of downscaling using the regional climate model (RCM) driven by Era-Interim reanalysis (EIN) in simulating different aspects of the Indian summer monsoon (ISM). It is also examined, whether increasing the horizontal resolution of RCM will inevitably be capable of adding more information to ISM characteristics and its spatio-temporal variability. In this regard, two RCM (at 50 km: Reg50 and 25 km: Reg25) simulations were conducted for six years from 2000 to 2005 for the South Asia Coordinated Regional Downscaling Experiment (CORDEX) domain. The added value (AV) is found to be strongly dependent on region and considered metrics. A slight improvement towards increasing spatial resolution is observed in the simulation of the mean ISM characteristics, while considerable improvements are noticed for the frequency distribution of extremes. The notable improvement in the daily climatology of precipitation is observed over the region of northeast India (~ 35%) and the Hilly region (~ 32%) and the lowest improvement over north-central India (~ 8%). The reduction of anomalously strong northeasterly flow over the southeastern Arabian Sea and strengthening of the moisture leaden southeasterly wind flow from the Bay of Bengal in Reg25 compared to Reg50 is consistent with the reduction of dry bias over India in Reg25. The robust improvements are noticed for the heavy precipitation events (probability density function: PDF tails) and mean precipitation due to extreme precipitation events, particularly over the areas characterized by complex topographical features (e.g., the Western Ghats, Indo-Gangetic plains, and northeast India and Hilly regions) as well as over the areas having substantial bias (e.g., central India), indicating its strong sensitivity towards model resolution. The increasing latent heat flux in Reg25 contributes to increasing the moisture and hence rainfall over India. Both simulations apparently simulate many of the ISM characteristics better than the EIN, thereby emphasizing the usefulness of finer resolutions in the better simulation of the Indian monsoon, especially for heavy rainfall. However, the RegCM bias is comparable to or even greater in some places than the EIN bias. This suggests that high-resolution models are important for improving performance; however, it does not necessarily mean that they can have AV for every aspect and all places. Apart from this, the substantial difference in the AV over different regions or aspects highlights the importance of carefully selecting AV matrices for the different areas and characteristics being investigated. RegCM exhibits some systematic biases in precipitation despite substantial improvement due to misrepresentation of dynamical and thermodynamical processes, including northward and eastward propagating convective bands. [ABSTRACT FROM AUTHOR]

A new high-resolution Regional Earth System Model, namely ROM, has been implemented over CORDEX-SA towards examining the impact of air–sea coupling on the Indian summer monsoon characteristics. ROM's simulated mean ISM rainfall and associated dynamical and thermodynamical processes, including the representation of northward and eastward propagating convention bands, are closer to observation than its standalone atmospheric model component (REMO), highlighting the advantage of air–sea coupling. However, the value addition of air–sea coupling varies spatially with more significant improvements over regions with large biases. Bay of Bengal and the eastern equatorial Indian Ocean are the most prominent region where the highest added value is observed with a significant reduction up to 50–500% precipitation bias. Most of the changes in precipitation over the ocean are associated with convective precipitation (CP) due to the suppression of convective activity caused by the negative feedback due to the inclusion of air–sea coupling. However, CP and large-scale precipitation (LP) improvements show east–west asymmetry over the Indian land region. The substantial LP bias reduction is noticed over the wet bias region of western central India due to its suppression, while enhanced CP over eastern central India contributed to the reduction of dry bias. An insignificant change is noticed over Tibetan Plateau, northern India, and Indo Gangetic plains. The weakening of moisture-laden low-level Somalia Jets causes the diminishing of moisture supply from the Arabian Sea (AS) towards Indian land regions resulting in suppressed precipitation, reducing wet bias, especially over western central India. The anomalous high kinetic energy over AS, wind shear, and tropospheric temperature gradient in REMO compared to observation is substantially reduced in the ROM, facilitating the favourable condition for suppressing moisture feeding and hence the wet bias over west-central India in ROM. The warmer midlatitude in ROM than REMO over eastern central India strengthens the convection, enhancing precipitation results in reducing the dry bias. Despite substantially improved ROM'performance, it still exhibits some systematic biases (wet/dry) partially associated with the persistent warm/cold SST bias and land–atmosphere interaction. [ABSTRACT FROM AUTHOR]

An effort is made to implement a regional earth system model (RESM); ROM, over CORDEX-South Asia (SA). The added value of RESM is assessed for mean precipitation, its variability (intraseasonal to interannual), extremes, and associated processes. In this regard, ROM's fields are compared with the respective fields of its standalone version (REMO), the models belonging coupled model intercomparison project (CMIP5 and CMIP6), and regional climate models of CORDEX-CORE simulations. RESM shows substantial improvement for most of the Indian monsoon's aspects; however, the magnitude of the value addition varies spatiotemporally and also with different aspects.. The improved representation of intraseasonal variability (active-break spell's duration and intensity) and Interannual variability attributed to improved mean seasonal precipitation. Additionally, correct representation of sea surface temperature, Indian Ocean Dipole, and its underlying dynamics also contribute to improving the mean precipitation. The notable improvement is seen especially over the south-eastern regions of the Bay of Bengal (BoB) and South-Central India, where increasing (decreasing) low-pressure systems over Central India (BoB) are noticed as a consequence of air-sea coupling, leading to enhanced (reduced) precipitation over Central India (BoB), reducing dry (wet) bias found in REMO and the other models. Despite substantial improvements, RESM has a systematic wet bias in the mean precipitation associated with a warm bias over the western coast of the Arabian Sea. An overestimation of very high extreme precipitation due to the enhanced contribution of low-pressure systems indicates the model's limitations, suggesting the need for further tuning of the RESM. [ABSTRACT FROM AUTHOR]

An effort is made to identify a suitable convective parameterization scheme (CPS) in a regional climate model (RegCM4.5) for simulating the spatiotemporal variability of precipitation extremes over India. In this regard, a set of seven sensitivity experiments are conducted using different CPS. The performance of the mixed scheme, MIT over land and Grell over ocean (MLGO), bears a considerable resemblance to observations in the spatial distribution of mean extreme precipitation compared to other schemes. All schemes underestimate the mean precipitation due to heavy and extreme events, particularly over central India. However, this underestimation is considerably reduced in MLGO. The overestimation of the rainy days and underestimation of the dry days is one of the possible regions of higher mean rainfall in MLGO. Despite having considerable performance in simulating mean rainfall, MLGO fails to correctly simulate rainy days, dry days, and the frequency of events for very low intensities. However, Grell simulates the intensity and frequency of events closer to observation. In general, the Grell has the highest performance in simulating extreme events, dry/wet days, and daily precipitation intensity. The better performance of Grell and MLGO in simulating the precipitation extremes is due to the stronger south-westerly wind from the Arabian Sea (AS), and convergent flow from the northern Bay of Bengal (BoB) to central India (CI) facilitates the moisture pulling mechanisms enhancing moisture availability and hence precipitation. In addition, the higher atmospheric instability in MLGO enhances the moist convection, strengthening convective activity, leading to enhanced precipitation. [ABSTRACT FROM AUTHOR]

This study investigated the impact of the Arabian Sea (AS) warming in modulating the winter precipitation over the Indian subcontinent and associated dynamical response such as subtropical jet variability using regional climate model, RegCM4.7 of the International Centre for Theoretical Physics (ICTP). In this regard, a control run (Reg CTL) is forced by real sea surface temperature (SST), and two sensitivity experiments with uniformly increased SST over AS by 0.45 °C (Reg SST45) and 0.90 °C (Reg SST90) are performed. The results obtained from a control experiment (Reg CTL) are compared with corresponding values from RegSST45 and RegSST90 to assess the warming impact. The model (Reg CTL) realistically simulates the circulation features, precipitation, and temperature over the Indian subcontinents in the winter season (November–December-January-February; NDJF) during 2006–2015. It is found that AS warming increases the precipitation over most parts of India during winter. The precipitation response varies with the month (NDJF) and the magnitude of AS warming. In general, precipitation is found to intensify with increasing AS warming over northern India and adjoining south Asian nations like Afghanistan, Pakistan, and some parts of Burma and Nepal. The strengthening of STJ is observed due to AS warming (Reg SST45). The strengthening of the subtropical jet (STJ) persists with an intensification of AS warming (Reg SST90); however, the magnitude of strengthening is less than Reg SST45, which is consistent with the corresponding increase of precipitation over India. Cold wave duration is found to be significantly sensitive to the intensity of AS warming. It is noticed that the AS warming increases the cold wave duration over the northern part of India with major increases over the western Himalayan region. • RegCM shows resemblance to observation in simulating winter monsoon characteristics and associated mechanisms. • The Arabian Sea warming impact in modulating winter precipitation over the Indian subcontinent is investigated • The precipitation increases over most parts of India during winter that varies with months and levels of warming. • The strengthening of the subtropical jet (STJ) is consistent with the precipitation increase. [ABSTRACT FROM AUTHOR]

For the first time for CORDEX-South Asia, a high-resolution regional earth system model (ROM) is adopted to assess the impact of horizontal resolution (0.22° and 0.11°) in simulating the Indian summer monsoon rainfall (ISMR) and the underlying spatiotemporal variability. ROM at both resolutions bears a close resemblance to observations in simulating the mean precipitation climatology compared to other regional climate models (RCMs) participated in CORDEX- South Asia. ROM shows substantial improvement relative to the ensemble mean of the RCMs included in CORDEX-South Asia. While comparing both simulations with observations, some systematic wet and dry bias over Central India (CI) and Northern Western Ghats is noticed. In general, the wet/dry bias over India is mainly associated with the overestimation/underestimation of the large-scale/convective component. Increasing horizontal resolution from 0.22° to 0.11° significantly adds value in simulating the JJAS mean precipitation by reducing the wet bias over western central India (WCI) and southern peninsular India and dry bias over eastern CI. The reduction in wet/dry bias is mainly associated with suppression/enhancement of the large scale/convective precipitation. This improvement in mean precipitation is partially due to the improved representation of the propagation of mesoscale systems such as boreal summer intraseasonal oscillation (eastward and northward). Despite the above improvements, the wet precipitation bias, particularly over WCI, persists. The weaker Findlater Jet associated with weaker land-ocean thermal contrast caused by the warm sea surface temperature (SST) bias over the western Arabian Sea (AS) suggests that AS moisture transport does not contribute to the wet bias over India. The wet bias is possibly associated with favourable atmospheric conditions (atmospheric instability). • A high-resolution regional earth system model namely ROM is implemented over CORDEX-South Asia. • The impact of horizontal resolution (0.22° and 0.11°) in simulating the summer monsoon characteristics is investigated. • The improved representation of the dynamical and thermodynamical processes yield better monsoon simulation. [ABSTRACT FROM AUTHOR]