Study of the L-Mode tokamak plasma ``shortfall'' with local and global nonlinear gyrokinetic $\delta f$ simulation

L-Mode plasmas in DIII-D and Alcator C-Mod tokamaks have been analyzed using the nonlinear gyrokinetic simulation GEM based on particle-in-cell method. It is observed that the simulation results for ion heat flux are close to the experimental values at the core, but substantially lower than the experimental results at the outer radial location in the DIII-D case. On the contrary, the simulations show good agreement with the experimental values of heat flux for ions in Alcator C-Mod. Global simulations are carried out for DIII-D L-Mode plasmas to study the effect of turbulence spreading from the edge into the outer core where the ion heat transport shortfall is observed. It is found that edge turbulence enhances the outer core ion heat transport significantly through turbulence spreading. However, ion heat flux in the shortfall region even in the presence of the edge drive is still much lower than the experimentally observed value.