Design of Simulations with the Cloud Model

Simulations of development of individual maritime deep convective clouds under conditions typical of tropical oceans during hurricane season (Jordan 1958) have been performed. The sounding used indicates high humidity near the surface of about 90%, and relatively stable maritime conditions. The zero temperature level is at 4.2 km.

We assume that the AP size distributions over the sea at the periphery of landfalling TC can be represented as a sum of two distributions: one typical of maritime and, another, typical of continental conditions. Aerosol particles giving rise droplet formation in all simulations were assumed soluble. Under high winds, aerosol size distribution can contain a significant amount of large cloud condensational nuclei (CCN) arising because of the sea spray formation. Correspondingly, penetration of continental aerosol should create aerosol size distributions containing a significant concentration of both small continental aerosols and tails of large aerosols. To investigate the effects of a large concentration of small continental aerosols on cloud microphysics and dynamics under the existence of a significant amount of large maritime CCN, the following simulations have been performed (see Table 1): a) the "M-case" corresponding to typical maritime distribution outside of the area of strong winds. In this case the maximum radius of AP was set equal to 2-p.m. The CCN number (at S = 1%) was set equal to 60 cm—3. Activation of the largest APs leads to formation of 10 mm-radius droplets. b) the "M_c case", in which the AP distribution represents sum of a continental AP distribution (with maximum of dry AP radius of 0.6 mm) and a maritime distribution as in the previously described M-case. We suppose that this case represents the AP size distribution over the sea in case of continental aerosol intrusion under weak and moderate winds. c) The "M_tail-case", which has the same AP distribution as in the M-case until a dry CCN radius of 0.6 mm, but with 100 times higher AP concentration with radii exceeding 0.6 mm. As a result, the concentration of dry CCN (at S = 1%) with radii exceeding 0.6 mm is 60 cm—3, which includes a concentration of 2 pm-radius CCN of 3.5 cm—3. We suppose that this case may represent the AP distribution under hurricane winds in the central TC zone. d) In the ''M_c_tail case'' the AP is the same as in M_c, but with the tail of large CCN as in M-tail. We suppose that this case represents AP size distribution under intrusion of continental aerosols and strong wind conditions.

Parameters No and k were assumed in these simulations and corresponding references are presented in Table 1. We do not include CCN with dry radius above 2 mm into the simulations. In all simulations clouds were triggered by the initial heating within the zone centered at x = 54 km, to allow the cloud hydro-meteors to be located longer in the computational zone. The maximum value of the dynamical time step was 5 s. Most simulations were conducted for time periods of 3 to 4 hours.

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