A sensitivity study of diffusional mass transfer of gases in tropical storm hydrometeors

Abstract

This paper quantifies mass transfer and diffusional uptake rates of gases in liquid and solid hydrometeors within a cyclonic system. The non-availability of transfer rates for trace gases diffusing into storm hydrometeors, particularly over polluted urban conurbations, often constrain modellers the world over; however, this is an essential requirement to quantify the scavenging rates over the region concerned. The present paper seeks to provide modellers with such rates. Further, all of the earlier studies apply only to temperate regimes, and surprisingly identical formulations are assumed even for tropical conditions. The present analysis fills this research gap and couples cloud morphology with the associated thermodynamics through Weather Research and Forecasting (WRF) runs for cyclone Chapala (27 October 2015–04 November 2015) which battered the coasts of Yemen (Skamarock et al. 2008). It was a good example for undertaking this sensitivity study because the vertical extent spanned from around 0.75 to 16 km—enabling uptake rate calculations over both droplet and ice phases. Many of the diffusing gases were polar; the dipole moment of sulphur dioxide (SO₂) and water vapour (H₂O) was also included using a full Lennard–Jones model to compute the binary diffusivities of these gases as they diffused into the droplets mixed with water vapour. The first-order uptake rate constants ranged from 2.08 × 10⁻⁰⁷ to 3.44 × 10⁻⁰⁶ (s⁻¹) and 1.97 × 10⁻⁰⁷ to 7.81 × 10⁻⁰⁷ (s⁻¹) for H₂O and SO₂ respectively. The rates are of the order of 10⁻⁰⁹ (s⁻¹) for diffusion of water vapour into ice crystals further aloft. Closely linked with the gas uptake rates is another crucial parameter—the mass accommodation coefficient, α. The most widely used values are 1 and 0.036 (Pruppacher and Klett 1998)—the chosen values are restrictive and warrants a closer look. In storm systems, the vertical extents are in the kilometre range. Chapala with a large vertical extent warrants a full profile calculation. This study shows that for H₂O vapour, α values range from a low of 0.004 reaching up to 0.046, and for SO₂ impacting the liquid droplets, they are 0.004 to 0.077. Using these values in cloud droplet growth equations showed large changes in the positioning of the cloud base height up to about a maximum of 30%—a classic example illustrating the coupling of microphysics with dynamics suggesting that even large-scale models should cautiously use standard un-corrected accommodation and diffusion coefficients. Over polluted environments, aerosol number concentrations are very high—several hundreds of particles in a cubic centimetre—the cumulative effect involving such large-scale scavenging ends up in causing substantive changes in the actual scavenging rates. This is likely to affect overall radiative transfer calculations and must be corrected.