Impact of soil moisture variability on convective rainfall activity over the Indian sub-continent

Soil moisture is an important geophysical parameter affecting land atmosphere processes, and hence free convection, by controlling the partitioning of the surface heat flux into latent and sensible heat flux. Interaction between these fluxes and the atmosphere gives rise to different types of soil moisture-precipitation feedback, namely “wet advantage” where rain is favoured over a wet (high latent heat flux) surface and “dry advantage” where rain is favoured over a dry (high sensible heat flux) surface. Previous studies over different parts of the world have shown that these feedback processes can take different pathways, according to one-dimensional and three-dimensional models. According to the one-dimensional model there is probability of rain initiation when the boundary layer top meets the level of free convection either by heating (increase in sensible heat flux over a dry surface) or by moistening (increase of latent heat flux over wet soil) of the boundary layer. On the other hand three-dimensional models explain convective triggering due to wind convergence near gradients in soil moisture. This is a first study to compare and evaluate the existing soil moisture-precipitation feedback theories presented in the literature, over the Indian sub-continent under a single environment, by using high resolution convection-permitting (non-parameterized, or “explicit” convection) EMBRACE model simulation. Initially, a brief synoptic observational study shows evidence of surface-atmosphere coupling. More detailed case studies from the model output show further evidence for the land-atmosphere interaction in this region. The model indicates that all the processes defined by different theoretical models do exist under different surface, and atmospheric conditions. The relative contribution of different processes under different soil moisture conditions prevailing over different climatic zones of the Indian sub-continent during the 20-day wet monsoon period from mid-July to early August is statistically studied. Dry-to-wet downwind soil moisture gradient is found to be the statistically significant pattern for initiation of the majority of afternoon convective initiation in the East, Centre and South study domains of India. It is also found that the so-called “CTP-HIlow” predictive framework is not sufficient to address the observed behaviour of convective initiation under the full three-dimensional modelling environment. The use of the parameter HIlow, which is defined as the sum of humidity within and just above the inversion, as a predictive parameter is not physically understandable. This framework also lacks generality and solutions are empirically derived based on one-dimensional modelling and observations, which vary from place to place. To offer a solution to these theoretical difficulties, this study provides a new quantitative model, using the basic idea behind the CTP-HIlow framework to find new predictive parameters depending on sound physical relationships instead of empirical solutions. The system is governed by two non-dimensional parameters, namely inversion Bowen ratio and a “stiffness ratio”, and a third, dimensional parameter ΔR. Analysis of the EMBRACE simulations shows occurrence of both the dry and wet advantage, but the majority of the morning profiles favour prediction of dry advantage. Thus, the equations derived from the new quantitative model offer a quantitative prediction of wet and dry advantage occurring systematically, which is a question of great importance to weather and climate prediction, especially over moisture-limited areas

Impact of soil moisture variability on convective rainfall activity over the Indian sub-continent

Soil moisture is an important geophysical parameter affecting land atmosphere processes, and hence free convection, by controlling the partitioning of the surface heat flux into latent and sensible heat flux. Interaction between these fluxes and the atmosphere gives rise to different types of soil moisture-precipitation feedback, namely “wet advantage” where rain is favoured over a wet (high latent heat flux) surface and “dry advantage” where rain is favoured over a dry (high sensible heat flux) surface. Previous studies over different parts of the world have shown that these feedback processes can take different pathways, according to one-dimensional and three-dimensional models. According to the one-dimensional model there is probability of rain initiation when the boundary layer top meets the level of free convection either by heating (increase in sensible heat flux over a dry surface) or by moistening (increase of latent heat flux over wet soil) of the boundary layer. On the other hand three-dimensional models explain convective triggering due to wind convergence near gradients in soil moisture. This is a first study to compare and evaluate the existing soil moisture-precipitation feedback theories presented in the literature, over the Indian sub-continent under a single environment, by using high resolution convection-permitting (non-parameterized, or “explicit” convection) EMBRACE model simulation. Initially, a brief synoptic observational study shows evidence of surface-atmosphere coupling. More detailed case studies from the model output show further evidence for the land-atmosphere interaction in this region. The model indicates that all the processes defined by different theoretical models do exist under different surface, and atmospheric conditions. The relative contribution of different processes under different soil moisture conditions prevailing over different climatic zones of the Indian sub-continent during the 20-day wet monsoon period from mid-July to early August is statistically studied. Dry-to-wet downwind soil moisture gradient is found to be the statistically significant pattern for initiation of the majority of afternoon convective initiation in the East, Centre and South study domains of India. It is also found that the so-called “CTP-HIlow” predictive framework is not sufficient to address the observed behaviour of convective initiation under the full three-dimensional modelling environment. The use of the parameter HIlow, which is defined as the sum of humidity within and just above the inversion, as a predictive parameter is not physically understandable. This framework also lacks generality and solutions are empirically derived based on one-dimensional modelling and observations, which vary from place to place. To offer a solution to these theoretical difficulties, this study provides a new quantitative model, using the basic idea behind the CTP-HIlow framework to find new predictive parameters depending on sound physical relationships instead of empirical solutions. The system is governed by two non-dimensional parameters, namely inversion Bowen ratio and a “stiffness ratio”, and a third, dimensional parameter ΔR. Analysis of the EMBRACE simulations shows occurrence of both the dry and wet advantage, but the majority of the morning profiles favour prediction of dry advantage. Thus, the equations derived from the new quantitative model offer a quantitative prediction of wet and dry advantage occurring systematically, which is a question of great importance to weather and climate prediction, especially over moisture-limited areas

Vefrifikacija prognoza oborine WRF modelom nad Indijom tijekom monsuna 2010.: CRA metoda

The WRF model forecast during monsoon season 2010 has been verified with daily observed gridded rainfall analysis with 0.5° spatial resolution. First- ly, the conventional neighborhood technique has been deployed to calculate common scores like mean error and root mean square error. Along with, widely used two categorical skill scores have been computed for seven different rainfall thresholds. The scores only found the general nature of the model performance and depicted the degradation of forecast accuracy exceeding moderate rainfall category of 7.5 mm. The object oriented Contiguous Rain Area method also has been considered for the verification of rainfall forecasts to gather more informa- tion about model performance. The method similarly has endorsed that the performance of the model degrades along with the increase in rainfall amount. But at the same time, the decomposition of mean square error has pointed out that the maximum error occurred due the shifting of rain object or event in the forecast compared to observation. The volume error contributes less as compared to pattern error in 24 hour forecasts irrespective of rainfall thresholds. But in 48 hour forecasts, their values are comparable and change along with rainfall threshold. During whole monsoon season, all contiguous rain areas in model forecasts have been searched over observed rainfall analyses applying best-fit criteria. For contiguous rain areas below 50 mm more than 70 percent match was found.Prognoza oborine dobivena modelom WRF za monsunsku sezonu 2010. verificirana je korištenjem analize dnevne opažene oborine u mreži prostorne rezolucije od 0,5°. Određeni su jednostavni, standardni pokazatelji poput srednje pogreške i srednje kvadratne pogreške, a također i dva uobičajena kategorička pokazatelja uspješnosti koji su izračunati za sedam različitih pragova oborine. Ti pokazatelji su omogućili općenitu procjenu uspješnosti modela te su ukazali na smanjenu pouzdanost za kategorije oborine veće od 7,5 mm. kako bi se detaljnije procijenila uspješnost modela, verifikacija prognoze oborine je također napravljena pomoću objektno orijentirane metode bliskih oborinskih područja CRA (Contiguous Rain Area). Ova metoda je također ukazala na smanjenje uspješnosti modela s povećanjem količine oborine. Međutim, dekompozicija srednje kvadratne pogreške je ukazala da najveću pogrešku uzrokuje pomak prognoziranog oborinskog područja ili događaja u odnosu na izmjerene vrijednosti. Za 24-satne prognoze volumna pogreška doprinosi manje u usporedbi s prostornom pogreškom, neovisno o pragovima oborine. Međutim, za 48-satne prognoze iznosi volumne i prostorne pogreške su usporedivi te rastu s pragom oborine. Susjedna oborinska područja za prognoziranu oborinu su određena obzirom na izmjerenu oborinu primjenom kriterija nabolje podudarnosti. Postupak je proveden za cijelu monsunsku sezonu. Za područja s količinom oborine manjom od 50 mm podudaranje je veće od 70%

Recent changes in the climatological characteristics of daily contiguous rain areas over India

Abstract This study documents the climatological feature (1951–1980) and recent changes (1981–2020) in rainfall characteristics considering the observed nearly full spectrum of rain event sizes (daily contiguous rain area (CRA) events) in all seasons over India. It is found that the low frequency very large CRA (~synoptic scale) from monsoon season contributes ~50% of annual rainfall. However, the small-sized CRA (isolated thunderstorms) are the most frequent daily rain events (~70% of annual frequency) and hence are important for rain-fed agricultural practices. The well-documented widespread drying trend in the monsoon season has manifested in the annual rainfall trend but with reduced magnitude illustrating the compensatory effect from other seasons. Spatial aggregated annual statistics show that there is no significant change in rainfall amount and frequency of occurrence of rain events in the recent past compared to the base period. However, seasonally the pre-monsoon rainfall amount has increased significantly. Annually, the number of extremely heavy CRA (EHR) events have significantly increased by ~55% owing to a significant increase in pre-monsoon and monsoon rainfall. In all seasons, small-sized extremely heavy CRA has intensified substantially by 50–200% as compared to the base period. Additionally, the rain events from areal category large (~Mesoscale Convective Complexes (MCC)) have intensified in all seasons except winter. Thus, to decrease the uncertainty in rain-fed agricultural practices and better prediction of EHR to develop effective climate change mitigation strategies; process studies beyond monsoon season and processes other than synoptic scales are also required

Projected precipitation changes over the south Asian region for every 0.5 °C increase in global warming

Using all ensemble members of NCAR CCSM4 for historical natural, RCP4.5 and RCP8.5 scenarios from CMIP5, we analyse changes in mean and extreme precipitation over the south Asian region for every 0.5 ^o C increase in global warming. An increase in mean annual precipitation is projected over majority of the south Asian region with increased levels of warming. Over Indian land, the spatially-averaged annual mean precipitation shows an increase in the range of ~2-14 % based on the RCP scenario and level of warming. However, a decrease in mean annual precipitation is projected over northwest parts of the Indian sub-continent and the equatorial Indian Ocean with increased levels of warming. In general, we find multifold increase in the frequency of occurrence of daily precipitation extremes over the Indian subcontinent and surrounding oceans. Over Indian land, frequency of occurrence of daily precipitation extremes show up to three-fold increase under both RCP scenarios for global warming levels in the range of 1.5 ^o C–2.5 ^o C. With further increase in warming we find that the frequency of occurrence of daily precipitation extremes could show a massive four- to six-fold increase over majority of Indian land. Notably, unlike the projected increase in the frequency of occurrence of daily precipitation extremes, the projected change in annual mean precipitation is found to be insignificant in a 1.5 ^o C warmer world, over majority of the south Asian region, under both RCP scenarios. Given the projected large increase in frequency of daily precipitation extremes with increased levels of warming, our study provides scientific support to the recommendations of the Paris Agreement of 2015

Analytical solution to a thermodynamic model for the sensitivity of afternoon deep convective initiation to surface Bowen ratio

The tendency of convective rainfall to initiate over a wetter or drier land surface is a critical feedback process in the climate system, influencing the hydrological cycle on a variety of spatial scales, especially in parts of the world where water is limited. A simple algebraic solution is derived from fundamental physical equations, to predict the sign of this convective rainfall feedback with the surface. The tendency for convection to occur is evaluated by the rate at which the convective boundary layer top approaches the level of free convection. Well-known integral models predict the rate of ascent of the boundary layer top, which tends to be faster over a dry surface. The associated changes in equivalent potential temperature in the boundary layer determine the rate at which the level of free convection descends, typically faster over a wet surface, as a function of the ambient profile, the thermodynamic forcing, and the surface Bowen ratio. The resulting system is controlled by three parameters. Two non-dimensional parameters determine whether there is wet or dry `advantage'; the Bowen ratio at the boundary layer top and a `convective instability parameter', defined as the ratio of the vertical gradient of saturated equivalent potential temperature at the level of free convection to the profile stability just above the boundary layer. A dimensional function, dependent on the surface fluxes, the boundary layer depth and the profile stability, provides the magnitude of the response. In comparison with previous work, the solution is both rigorously derived from physical principles, and encapsulated in a simple algebraic form. A first evaluation of the theoretical framework has been made using data from a convection-permitting numerical model simulation over India, and indicates that the equations successfully determine the conditions under which convection is triggered over dry surfaces