INVESTIGATION OF SCALAR FORCING IN DIRECT NUMERICAL SIMULATIONS OF CLOUD TURBULENCE

Direct numerical simulation (DNS) has become an indispensable tool to study turbulence-cloud-aerosol interactions. The DNS approach attempts to resolve the smallest scales of the flow, assuming homogeneous and isotropic turbulence. The governing equations for the flow of air, and two scalars-temperature, and water vapor mixing ratio, are solved in an Eulerian fashion while aerosol particles and cloud droplets are tracked using the Lagrangian method. To sustain turbulence in the scalar fields, forcing is necessary. This paper presents a DNS investigation of two scalar forcing mechanisms, in the spectral and physical space, respectively. We compare the forced and unforced scalar fields in terms of the standard deviation, the spectra, and the probability density functions. Our results show forcing leads to more fluctuations in the scalar fields and broadening of the probability distributions. Also, the rate of condensation decreases with scalar forcing. The scalar spectra for forced and unforced scalar fields are found to be different at small scales.