• Vinay Kumar

Articles written in Journal of Earth System Science

• Lead time for medium range prediction of the dry spell of monsoon using multi-models

The purpose of this study is to address prediction of the start date and the duration of breaks in the summer monsoon rains using multi-model superensemble. The availability of datasets from the ‘observing system research and predictability experiment (THORPEX)’ initiated a forecast data archive, called THORPEX interactive grand global ensemble (TIGGE), makes it possible to use forecasts from a suite of individual ensemble prediction systems (member models) and to construct multi-model superensemble forecasts that are designed to remove the collective bias errors of the suite of models. Precipitation datasets are important for this study, we have used high resolution daily gridded rainfall dataset of India Meteorological Department (IMD), in addition to rainfall estimates from tropical rainfall microwave mission (TRMM) satellite and the CPC morphing technique (CMORPH). The scientific approach of this study entails the use of a multi-model superensemble for forecast and to verify against the rainfall information during a training phase, as well as during a forecast phase. We examine the results of forecasts out to day-10 and ask how well do forecast strings of day-1 through day-10 handle the prediction of the onset and duration of the breaks in the summer monsoon rains. Our results confirm that it is possible to predict the onset of a dry spell, around week in advance from the use of the multi-model superensemble and a suite of TIGGE models.We also examine trajectories of the parcels arriving in India in such forecasts from member models and from the multi-model superensemble to validate the arrival of descending dry desert air from the Arabian region during the dry spells and its mode of transition from wet spell. Some phenological features such as a shift in the latitude of the tropical easterly jet and changes in its intensity during break periods are additional observed features that are validated from the history of multi-model superensemble forecasts. Invariably this multi-model superensemble performs better than any single model in proving the better forecasts during our experiment period.

• Analysis of carbon dioxide, water vapour and energy fluxes over an Indian teak mixed deciduous forest for winter and summer months using eddy covariance technique

In the present study, we report initial results on analysis of carbon dioxide (CO2), water vapour (H2O), and energy fluxes (sensible and latent heat flux) over teak mixed deciduous forests of Madhya Pradesh, central India, during winter (November 2011 and January 2012) and summer (February–May 2012) seasons using eddy covariance flux tower datasets. During the study period, continuous fast response measurements of CO2, H2O and heat fluxes above the canopy were carried out at 10 Hz and averaged for 30 minutes. Concurrently, slow response measurements of meteorological parameters are also being carried out. Diurnal and seasonal variations of CO2, H2O and heat fluxes were analysed and correlated with the meteorological variables. The study showed strong influence of leaf off and on scenario on the CO2, H2O and energy fluxes due to prevalence of deciduous vegetation type in the study area. Maximum amount of CO2 was sequestered for photosynthesis during winter (monthly mean of −25 𝜇 mol/m2/s) compared to summer (monthly mean of −2 𝜇 mol/m2/s). Energy flux analysis (weekly mean) showed more energy being portioned into latent heat during winter (668 W/m2) and sensible heat during summer (718 W/m2).

• Reconstruction of specific mass balance for glaciers in Western Himalaya using seasonal sensitivity characteristic(s)

Seasonal sensitivity characteristics (SSCs) were developed for Naradu, Shaune Garang, Gor Garang and Gara glaciers, Western Himalaya to quantify the changes in mean specific mass balance using monthly temperature and precipitation perturbations. The temperature sensitivities were observed high during summer (April–October) and precipitation sensitivities during winter months (November–March), respectively. The reconstructed mass balance correlates well with the field and remote sensing measurements, available between 1980 and 2014. Further, SSCs were used with the monthly mean temperatures and precipitation estimates of ERA 20CM ensemble climate reanalysis datasets to reconstruct the specific mass balance for a period of 110 years, between 1900 and 2010. Mass balance estimates suggest that the Shaune Garang, Gor-Garang and Gara glaciers have experienced both positive and negative mass balance, whereas the Naradu glacier has experienced only negative mass balance since 1900 AD. Further, a cumulative loss of −133±21.5 m.w.e was estimated for four glaciers during the observation period. This study is the first record from Indian Himalaya in evaluating the mass balance characteristics over a century scale.

• Semi-empirical model for retrieval of soil moisture using RISAT-1 C-Band SAR data over a sub-tropical semi-arid area of Rewari district, Haryana (India)

We have estimated soil moisture (SM) by using circular horizontal polarization backscattering coefficient (σoRH), differences of circular vertical and horizontal σooRV− σo RH) from FRS-1 data of Radar Imaging Satellite (RISAT-1) and surface roughness in terms of RMS height (RMSheight). We examined the performance of FRS-1 in retrieving SM under wheat crop at tillering stage. Results revealed that it is possible to develop a good semi-empirical model (SEM) to estimate SM of the upper soil layer using RISAT-1 SAR data rather than using existing empirical model based on only single parameter, i.e., σo. Near surface SM measurements were related to σoRH, σoRV−σoRH derived using 5.35 GHz (C-band) image of RISAT-1 and RMSheight. The roughness component derived in terms of RMSheight showed a good positive correlation with σoRH−σoRH (R2 = 0.65). By considering all the major influencing factors (σoRH, σoRV− σoRH, and RMSheight), an SEM was developed where SM (volumetric) predicted values depend on σoRH, σoRV− σoRH, and RMSheight. This SEM showed R2 of 0.87 and adjusted R2 of 0.85, multiple R=0.94 and with standard error of 0.05 at 95% confidence level. Validation of the SM derived from semi-empirical model with observed measurement (SMObserved) showed root mean square error (RMSE) = 0.06, relative- RMSE (R-RMSE) = 0.18, mean absolute error (MAE) = 0.04, normalized RMSE (NRMSE) = 0.17, Nash–Sutcliffe efficiency (NSE) = 0.91 (≈1), index of agreement (d) = 1, coefficient of determination (R2) = 0.87, mean bias error (MBE) = 0.04, standard error of estimate (SEE) = 0.10, volume error (VE) = 0.15, variance of the distribution of differences (S2d) = 0.004. The developed SEM showed better performance in estimating SM than Topp empirical model which is based only on σo. By using the developed SEM, top soil SM can be estimated with low mean absolute percent error (MAPE) = 1.39 and can be used for operational applications.

• Elucidating intra-seasonal characteristics of Indian summer monsoon. Part-I: Viewed from remote sensing observations, reanalysis and model datasets

In this study, we examine the transitions in the monsoon phases (onset, active, break and the withdrawal) during an entire monsoon season. This makes use of a host of observational tools that come from GPM (Global Precipitation Measurement) and TRMM (Tropical Rainfall Measuring Mission) satellites for precipitation estimates, the vertical structure of rain, hydrometeors and cloud types from TRMM and CloudSat datasets. During onset, the mean moisture convergence, especially over west and south-west coast of India is $2 \times 10^{-4} \rm{kg m^{-1} s^{-1}}$; however, it carries much higher value of >$4 \times 10^{-4 }\rm{kg m^{-1} s^{-1}}$ during the active phase over central eastern India. Much lesser moisture convergence (<$1 \times 10^{-4} \rm{kg m^{-1} s^{-1}}$) is noted over Western Ghats area during the break phase. However, there are northeasterly moisture fluxes present over southern part of India during withdrawal phase. The tall cumulonimbus clouds that extend out to 16 km are illustrate during onset, the active phase is dominated by alto stratus and nimbostratus type clouds that are somewhat shallower. In general, we noted an absence of such clouds during the break and the withdrawal phases. Those structures were consistent in a number of derived fields such as the moisture convergence, moisture fluxes, the energy conversions between the rotational and the divergent kinetic energy and the corresponding phases of the intra-seasonal oscillations.

• Understanding the interactions between meteorological and soil moisture drought over Indian region

The present study aims at analysing the relationship between meteorological and soil moisture droughts over different meteorological regions of India based on change-point, trends and probability analysis. The long term gridded standardized-precipitation-evapotranspiration-index (SPEI) and standardized-soil-moisture-index (SSMI) data during 1980–2015 were used to address the meteorological and soil-moisture drought, respectively. The significant changes in the time-series SPEI and SSMI data at different time-scales were estimated using Pettitt’s test. The variations in change-points among different time-scales of SSMI were much lower than SPEI, specifically in parts of NW and central India. The inter-annual variability of drought parameters, i.e., duration and magnitude, showed good agreement between meteorological and soil moisture droughts for mild drought conditions. However, under moderate drought conditions, the behaviour of meteorological and soil moisture drought was different, specifically over peninsular India. The trend analysis revealed that SSMI based drought occurrences were significantly decreasing over all the four meteorological regions, whereas in case of SPEI, significant trends were observed only over peninsular India for mild drought condition. The probability of occurrences of both meteorological and soil moisture droughts were higher over major parts of India in case of mild droughts, whereas the values were lower for moderate drought conditions.

$\bf{Highlights}$

$\bullet$The Pettitt’s change point analysis revealed that variations of the period of significant changes among different time scale SSMI was much lower comparison to SPEI.

$\bullet$In major parts of the country, good agreement between SPEI and SSMI derived drought parameters, i.e., duration and magnitude, were observed for mild drought conditions. However, it was quite low in moderate drought conditions, more specifically over peninsular and central India.

$\bullet$The trend analysis revealed that the soil moisture drought was decreasing during the study period. Similar observations were recorded in meteorological drought also, but with some increasing drought duration and magnitude in parts of NW and NE India.

$\bullet$The probability of occurrences of both meteorological and soil moisture droughts were higher over major parts of India in case of mild droughts, whereas the values were lower for moderate drought conditions.

• Effects of shape and complexity of ridge topography on the comparative amplification scenario for the SH- and SV-waves

In this paper, the computation of seismic responses of complex ridge topography is documented that can provide a reliable scenario of ridge amplification. This research work is inspired by the topography of the great Himalaya, wherein the increase of height from the south to north is in form of ups (anticlines) and downs (synclines). The viscoelastic SH- and SV-wave responses of triangular and elliptical complex ridge topography models are simulated using fourth-order finite-difference method. The complexity in the model is augmented by adding more number of sub-ridges and sub-valleys along the flanks of the reference mega-ridge. An increase in ridge amplification is obtained with an increase of complexity in both the triangular and elliptical ridge models for both the polarization of S-wave. The increase of amplification with complexity is drastically very high for the horizontal components of the SV-wave in the case of triangular ridges. It is concluded that the horizontal ground motion simulated at the crest of ridge very much depends on the shape and complexity of the topography as well as the polarization of the incident S-wave. It is concluded that the computed topography effects using the spectral ratio of earthquake records at the top and near its base generally overpredicts spectral amplifications and may not be reliable.

$\bf{Highlights}$

$\bullet$ In this paper, the increase of S-wave amplification at the crest of sub-ridges as well as the base of subvalleys with an increase of complexity in the topography models is observed. The increase of amplification of the horizontal components of the SV-wave with complexity was larger to that of the SH-wave in the case of triangular sub-ridges. The predicted ridge amplification using earthquake records at the top and base of a ridge generally over-predicts the ridge amplification.

• # Journal of Earth System Science

Volume 131, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019