• SRABANTI BALLAV

      Articles written in Journal of Earth System Science

    • Simulation of CO2 concentrations at Tsukuba tall tower using WRF-CO2 tracer transport model

      Srabanti Ballav Prabir K Patra Yousuke Sawa Hidekazu Matsueda Ahoro Adachi Shigeru Onogi Masayuki Takigawa Utpal K De

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      Simulation of carbon dioxide (CO2) at hourly/weekly intervals and fine vertical resolution at the continental or coastal sites is challenging because of coarse horizontal resolution of global transport models. Here the regional Weather Research and Forecasting (WRF) model coupled with atmospheric chemistry is adopted for simulating atmospheric CO2 (hereinafter WRF-CO2) in nonreactive chemical tracer mode. Model results at horizontal resolution of 27 × 27 km and 31 vertical levels are compared with hourly CO2 measurements from Tsukuba, Japan (36.05°N, 140.13°E) at tower heights of 25 and 200 m for the entire year 2002. Using the wind rose analysis, we find that the fossil fuel emission signal from the megacity Tokyo dominates the diurnal, synoptic and seasonal variations observed at Tsukuba. Contribution of terrestrial biosphere fluxes is of secondary importance for CO2 concentration variability. The phase of synoptic scale variability in CO2 at both heights are remarkably well simulated the observed data (correlation coefficient >0.70) for the entire year. The simulations of monthly mean diurnal cycles are in better agreement with the measurements at lower height compared to that at the upper height. The modelled vertical CO2 gradients are generally greater than the observed vertical gradient. Sensitivity studies show that the simulation of observed vertical gradient can be improved by increasing the number of vertical levels from 31 in the model WRF to 37 (4 below 200 m) and using the Mellor–Yamada–Janjic planetary boundary scheme. These results have large implications for improving transport model simulation of CO2 over the continental sites.

    • Assessment of spatio-temporal distribution of $\rm{CO}_{2}$ over greater Asia using the $\rm{WRF–CO}_{2}$ model

      SRABANTI BALLAV MANISH NAJA PRABIR K PATRA TOSHINOBU MACHIDA HITOSHI MUKAI

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      In-depth knowledge of global and regional carbon budget is required for effective policymaking to mitigate the global climate change. However, Asian carbon budget shows large uncertainty due to both lack of sufficient observations and detailed understanding of the existing $\rm{CO}_{2}$ observations. A regional air quality model ($\rm{WRF–CO}_{2}$) is set up for simulating atmospheric $\rm{CO}_{2}$ variations over the greater Asia region ($68–124^{0}\rm{E}$, $2^{0}\rm{S}–45^{0}\rm{N}$) for the period 2010–2012. The $\rm{WRF–CO}_{2}$ simulations are compared with observations from nine sites and a global Atmospheric Chemistry Transport Model (ACTM). The comparisons suggest $\rm{WRF–CO}_{2}$ simulation is able to capture large scale features in the observed variabilities, with varied ability at fine scales depending on representation of surface fluxes and meteorology around the observation sites. Analysis of $\rm{CO}_{2}$ signals from individual flux components suggests that ocean flux has least contribution to the $\rm{CO}_{2}$ variation (<10%). Four sites (Mt.Waliguan, Nainital, Cape Rama and Lulin) show dominance of biospheric flux over fossil flux to the $\rm{CO}_{2}$ variation (>80%). $\rm{CO}_{2}$ mixing ratios are found to be maximum in northern hemisphere (NH) winter over East Asia, while they are maximum in NH spring over Indian subcontinent. Observed peak-to-trough seasonal amplitude is lowest (4.5 ppm) for the site Bukit Koto Tabang, Indonesia and highest (29.5 ppm) for Shangdianzi in China. Statistical analysis from monthly mean $\rm{CO}_{2}$ time series shows that correlation coefficient and normalised standard deviation with observations, are generally equal or better for the $\rm{WRF–CO}_{2}$ than the coarser resolution ACTM. Study of synoptic scale $\rm{CO}_{2}$ variations shows that the $\rm{WRF–CO}_{2}$ is able to better resolve daytime signatures than those in the night. Year-to-year $\rm{CO}_{2}$ variations of seasonal cycle amplitude is highest ($\sim 5 \rm{ppm}$) at Nainital, India compared to all other sites.

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