Results of2D Simulations

The first important result was that the tail of large CCN (within the radii range 0.6 mm to 2 mm) actually does not influence cloud microstructure structure and precipitation in the presence of high concentration of small CCN. For instance, accumulated rain amounts in the simulations M_c and M_c_tail are just similar (Fig. 2). This result can be explained as follows: the rate of diffusion growth is determined by supersaturation. In case when about 800-1000 cm~3 droplets are nucleated (as in the M_c and the M_c_tail cases), the supersaturated water vapor is sheared between a great number of droplets and the supersaturation is small. Note that droplets

Table 1 List of simulations and parameters characterizing aerosol distributions

Type of cloud

Short title

k references

Maritime cloud M

Sum of maritime and M-c continental

Maritime cloud with M_tail increased fraction of large cloud CCN

Sum of maritime and M_c_tail continental but with the increased fraction of large CCN

2500

60 tail: 60 CCN with radii exceeding 0.6 mm 2500 cm"3 60 cm"3 tail: 60 particles with radii exceeding 0.6 mm

0.308 0.921 0.308

0.308

0.92 0.308

Khain et al. 2005 Khain et al. 2001b, 2004, 2005; Khain and Pokrovsky, 2004 Sensitivity study

Sensitivity study

Fig. 2 Time dependence of accumulated precipitation at the surface in simulations with different concentrations of small (continental) and large aerosols (see Table 1). The increase in slope of curves indicates decrease in warm rain precipitation and beginning cold (melted) rain. Horizontal dashed lines denote the approximate amount of warm rain in the simulations. Remaining accumulated rain is mostly cold (melted) precipitation. The warm rain in clouds developing in dirty air is smaller than that in cloud developing in clean air. At the same time, total accumulated rain is larger in clouds developing in dirty air

Fig. 2 Time dependence of accumulated precipitation at the surface in simulations with different concentrations of small (continental) and large aerosols (see Table 1). The increase in slope of curves indicates decrease in warm rain precipitation and beginning cold (melted) rain. Horizontal dashed lines denote the approximate amount of warm rain in the simulations. Remaining accumulated rain is mostly cold (melted) precipitation. The warm rain in clouds developing in dirty air is smaller than that in cloud developing in clean air. At the same time, total accumulated rain is larger in clouds developing in dirty air growing on large CCN also contribute to the decrease in the supersaturation. As a result, the largest nucleated droplets with initial radius of about 10 mm grow relatively slow and reach the size necessary to collect smaller droplets at heights of about 5.5-6 km. It is necessary to recall that smaller droplets increase their size by diffusional growth faster than the largest ones (Rogers and Yau 1989), so that at the 6 km level the contribution of large CCN to the concentration of raindrops is not substantial. Stronger effect of large tail takes place in case of low droplet concentration, when supersaturation is higher than in case of high droplet concentration. Hence, raindrops in the M_tail form at about 3 km instead of 4 km in the M-case. The accumulated rain amount in the M_tail run is larger than in the M during the first 1.5 h. However, the difference in the rain amounts is not substantial in this case too. Low sensitivity of clouds developing under tropical maritime environmental conditions to amount of large CCN allows us to discuss effects of continental aerosols on TC clouds under a large uncertainty in the concentration of large CCN. Since, we expect the existence of the tail of large CCN in the TC clouds, we will discuss below the effects of continental aerosols on the cloud structure in the M_tail and the M_c_tail simulations. Note that no giant CCN with radii above 25-30 mm capable to trigger drop collisions in a short time after their penetration into a cloud, were assumed in the simulations.

Figure 3 shows the fields of cloud water content CWC (droplets with radii below 50 mm) in the M_tail and M_c_tail simulations at t = 25 min. One can see that while CWC in clouds with the low AP concentration decreases dramatically above 3.5-4 km because of rapid raindrop formation, the CWC in clouds with the high AP concentration remains significant to the upper atmosphere. This is a typical feature of clouds developing in dirty air (e.g., Andreae et al. 2004; Ramanathan et al. 2001; Khain et al. 2004, 2005). Specific feature of the present results is that large CWC takes place in dirty air in the presence of a high concentration of large CCN. Figure 4 shows the fields of crystal (upper panels), graupel (middle) and hail content in the M_tail (left) and M_c_tail (right) simulations. One can see that these contents are higher in clouds developing in polluted air, which can be attributed to a larger amount of CWC penetrating above the freezing level in this case. The vertical

HUikt: clmjb tfMP moil, t=15M>s rtJCk Ö.ÖLC MÖP .res* l=l5K)s

HUikt: clmjb tfMP moil, t=15M>s rtJCk Ö.ÖLC MÖP .res* l=l5K)s

Fig. 3 Fields of cloud water content CWC (droplets with radii below 50 mm) in the M_tail and M_c_tail simulations at t = 25 min. CWC is significantly higher and reach higher levels in clouds developing in dirty air
Fig. 4 The fields of crystal (upper panels), graupel (middle) and hail content in the M_tail (left) and M_c_tail (right) simulations. One can see that these contents are higher in clouds developing in the high aerosol concentration

velocities in clouds developing within the high AP concentration air are higher by a few m/s than in clouds developing under the low aerosol concentration cases. This aerosol effect on cloud dynamics was simulated and discussed in detail by Khain et al. (2005) and then simulated in many studies (e.g. Lynn et al., 2005a, b). The increase in the vertical updrafts in tropical clouds developing in dirty air can be attributed to extra latent heat release caused by extra condensational droplet growth (larger CWC) and extra freezing (larger ice contents) (see Figs. 4 and 5). The calculated radar reflectivity fields (not shown) indicate that high values of radar reflectivity in clouds forming in the dirty air reach (better to say, start) at higher levels as compared with the clouds developing in clean air, which agrees with the TRMM's Tropical Microwave images mentioned above. All these results indicate that a) High vertical velocity which is possible in clouds at TC periphery is necessary, but not sufficient condition for the lightning formation.

WFtF Forecast Init: 2005-06-27 00-00 00

Valid: 2005-OB-2B_1BrOO:00 Aerosol Concentration in % of maximum value

WFtF Forecast Init: 2005-06-27 00-00 00

Valid: 2005-OB-2B_1BrOO:00 Aerosol Concentration in % of maximum value

0 20 40 60 80 100

Fig. 5 The field of the aerosol concentration simulated in the lower troposphere in the TC zone on 28 August, 12 Z in % to the maximum value assumed to be over the continent (left). The locations of negative ground flashes observed during hurricane stage in the nine storms in the Western Atlantic, composited with respect to the hourly center position of each hurricane (adopted from Molinari et al. 1999) (right)

0 20 40 60 80 100

Fig. 5 The field of the aerosol concentration simulated in the lower troposphere in the TC zone on 28 August, 12 Z in % to the maximum value assumed to be over the continent (left). The locations of negative ground flashes observed during hurricane stage in the nine storms in the Western Atlantic, composited with respect to the hourly center position of each hurricane (adopted from Molinari et al. 1999) (right)

b) Continental aerosols penetrating into the convective clouds via their cloud base can change dramatically the cloud microphysics and dynamics, making it possible the coexistence of cloud ice and supercooled water at high levels with temperatures below-13°C, i.e. transferring maritime convection into thunderstorms.

Figure 3 shows time dependence of accumulated precipitation at the surface in the simulations with different concentrations of small and large CCN (see Table 1). One can see that a) accumulated precipitation from clouds developing in dirty air is higher than in clean air; b) warm rain amount decreases in clouds developing in dirty air, so that most precipitation in dirty clouds is cold rain formed by melting graupel and hail. On contrary, precipitation from clouds developing in clean air consists mainly of warm rain.

The results agree with observed data (e.g. Shepherd and Burian, 2003) and numerical results by Wang (2005), Lynn et al. (2005a) about aerosol effects on deep tropical convection under wet conditions. As it was discussed by Khain et al. (2004, 2005) and in more details by Khain et al. (2008), the increase in precipitation in dirty air under wet tropical conditions is related to the generation of larger condensate mass due to extra condensational growth of droplets and extra deposi-tional growth of ice and comparatively low loss of precipitating mass in wet air.

The conclusions reached in this section allow us to carry out the simulations using a 3-D mesoscale model.

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