Modelling for Science, for a better future - some recent outcomes
Impact of Assimilation on Heavy Rainfall Simulations Using WRF Model: Sensitivity of Assimilation Results to Background Error Statistics
by V. Rakesh and B. Kantharao
Data assimilation is considered as one of the effective tools for improving forecast skill of mesoscale models. However, for optimum utilization and effective assimilation of observations, many factors need to be taken into account while designing data assimilation methodology. One of the critical components that determines the amount and propagation observation information into the analysis, is model background error statistics (BES). The objective of this study is to quantify how BES in data assimilation impacts on simulation of heavy rainfall events over a southern state in India, Karnataka. Simulations of 40 heavy rainfall events were carried out using Weather Research and Forecasting Model with and without data assimilation. The assimilation experiments were conducted using global and regional BES while the experiment with no assimilation was used as the baseline for assessing the impact of data assimilation. The simulated rainfall is verified against high-resolution rain-gage observations over Karnataka. Statistical evaluation using several accuracy and skill measures shows that data assimilation has improved the heavy rainfall simulation. Our results showed that the experiment using regional BES outperformed the one which used global BES. Critical thermo-dynamic variables conducive for heavy rainfall like convective available potential energy simulated using regional BES is more realistic compared to global BES. It is pointed out that these results have important practical implications in design of forecast platforms while decision-making during extreme weather events.
source: http://link.springer.com/article/10.1007/s00024-017-1471-8
An evaluation strategy of skill of high-resolution rainfall forecast for specific agricultural applications
by V. Rakesh and Prashant Goswami
A strategy for validation of rainfall forecasts for specific agricultural applications is presented. The focus is mainly on the design of specific forecast advisories that are risk-free and useful in spite of their inherent errors. The strategy works for these specific applications because the forecast advisories are based on when NOT to irrigate or apply fertilizer/pesticide because rain is predicted (risk-free because wrong forecast only delays irrigation/application of fertilizer/pesticide within tolerance). Thus, unlike in conventional forecast evaluation, a forecast is considered as valid if the forecasted rain (or no rain) is correct for the day of the forecast (D0C) or the next day or the day after (designated D1C and D2C, respectively), as the farmer can afford to postpone the field application for a couple of days beyond the scheduled date. The methodology has been evaluated for rainfall forecasts over Karnataka (a state in southwest India with nearly 56% of the workforce engaged in agriculture). Here, forecast validation against rain gauge observations is presented at comparable resolutions for the southwest (June to September) and the northeast (October to December) monsoon seasons during 2011–2014. Analyses demonstrate that forecasts over several areas which may appear to be less reliable based on conventional evaluation (D0C) are found to have useful skill for the specific agro-applications as evident from evaluation based on D1C and D2C criteria. Our analysis shows that the evaluation strategy presented is effective during the non-rainy (January–May) season also. It is pointed out that such an approach can help to meet the challenges in designing and implementing best practices in agriculture by combining immediate gains for the end users with long-term sustainability.
source: http://onlinelibrary.wiley.com/doi/10.1002/met.1576/abstract
Reflection and Refraction of Attenuated Waves at the Interface Between Cracked Poroelastic Medium and Porous Solid Saturated with Two Immiscible Fluids
by Sushant Shekhar and Imtiyaz A. Parvez
The present study is mainly focused on understanding the effect of cracks on the energy share of an incident wave at the interface between a cracked poroelastic solid containing single fluid and a porous solid containing two immiscible viscous fluids. We assume that both the media are dissipative due to presence of viscosity in pore fluids. The analysis is based on Snell’s law for the reflection and refraction of an incident wave from the common boundary of these two media. The proposed model is solved for the propagation of harmonic plane waves with the help of the Helmholtz technique. After solving the elastodynamical equations, we find three attenuated reflected waves and four attenuated refracted waves. The propagation of an attenuated wave in the respective dissipative medium defined as an inhomogeneous propagation of a wave, is represented through the propagation and the attenuation directions. These inhomogeneous waves propagate through the cracked poroelastic solid and are incident at a point on the interface. The phase velocities and attenuation coefficients are calculated for the propagation of each inhomogeneous wave. The expressions of reflection and refraction coefficients and energy share of each of the reflected and refracted waves for a given incident wave are obtained in closed form and computed numerically in the present study. Numerical examples are considered for the partition of incident energy in which we have studied the presence and absence of cracks in poroelastic solid with single fluid, effect of crack density, saturation parameter and Poisson’s ratio for the cracked poroelastic solid.
Source: http://link.springer.com/article/10.1007/s11242-016-0704-0
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