Discussion and Conclusions

The potential effects of continental aerosol penetrated into the circulation of TC approaching the land on the cloud structure and the lightning rate in hurricane clouds has been investigated using a 2-D cloud model with spectral bin microphysics and a 3D mesoscale model WRF with bulk microphysics. Numerical experiments with the 2D cloud model with the resolution of 250 m and 125 m in the horizontal and vertical directions, respectively, show that the continental aerosols with the CCN concentrations of about 1000 cm-3 significantly increase the amount of supercooled cloud water, as well as ice (mainly ice crystals, graupel and hail) even under the high concentration of large CCN, which may have maritime nature.

Averaged Precipitation (mm) Vs. Time

Averaged Precipitation (mm) Vs. Time

Graupel Charge Separation
Fig. 11 Time dependences of precipitation averaged over 3 h periods within the TC zone (the internal grid) in "warm rain'' and "no warm rain'' (NWRP-30) simulations

In addition, the vertical updrafts were stronger in the polluted clouds. All these factors taken together lead to coexistence of ice and cloud water within a supper-cooled cloud zone, which is considered to be favorable for charge separation and lightning formation.

The purpose of simulations using a mesoscale 3-km resolution model was to investigate the possible effects of aerosols on cloud structure and lightning of hurricanes, as well on precipitation and TC intensity. The utilization of 3-km resolution and crude vertical resolution, as well as the bulk-parameterization scheme used does not allow one to reproduce aerosol effects related to a fine balance between the fall velocity of growing droplets and the vertical velocity. For instance the results of supplemental simulation with droplet concentration Nd of 1000 cm~3 indicate some increase in supercooled CWC, etc., but, in general, the results were quite similar to those obtained in case when droplet concentration was set equal to 30 cm~3.

As a result, the aerosol effects were parameterized by shutting of droplet collisions, preventing warm rain at the TC periphery, where the surface wind speed was weaker than 35 m/s. This simple parameterization of the aerosol effects turned out to lead to more realistic results, which better agrees with those, obtained using a spectral microphysical model with the resolution of 100-250 m.

The product of the updraft velocity, ice content and supercooled cloud water content was chosen as the measure of lightning activity (lightning probability LP).

It was shown that the LP field calculated in the model resembles very well the structure of observed lightning: a) the maximum lightning takes place within a comparatively narrow rings with radius 250-300 km; b) lightning in the TC central zone is, as a rule, weaker than that in the rain bands at the TC periphery. In the simulation, where no aerosol effects were taken into account, the magnitude of the LP parameter was smaller and concentrated in the eye wall, which does not agree with the observations.

The analysis of the intensity variations of simulated TCs, as well as the observed variation of the intensity of hurricanes Katrina (2005) and Rita (2005) (Fierro et al., 2007), shows that the disappearance of lightning in the TC central zone and its intensification (convection invigoration) at the TC periphery can be a good indicator of the TC decaying. Such behavior of TC lightning may be useful for a short range TC intensity forecast. Note that simulations of TC lightning in an idealized TC performed by Fierro et al. (2007) showed negligible lightning activity at the TC periphery as compared to that in the TC eye wall. We attribute this result to the fact that no aerosol effects were taken into account in the simulations performed by Fierro et al. (2007). Their results indicate that the changes of the instability of the atmosphere can lead to a variability of the lightning in the TC eye wall, but not within a narrow ring at the TC periphery. Correspondingly, the comparison of the results obtained by Fierro et al. (2007) and those in this study supports the assumption that lightning at the TC periphery is caused by continental aerosols.

The results indicate also that aerosols affect cloud structure, intensity and spatial distribution of precipitation of TCs approaching and penetrating the land. Precipitation in the TC zone of 350-400 km radius decreases in dirty air mainly due to weakening of convection in the central zone of the TCs. These results can serve as the first numerical justification of the observed weekly cycle of intensity and precipitation of landfalling TCs found by Cerveny and Balling (1998), who attributed these changes to the weekly variation of anthropogenic aerosol concentration.

According to the results obtained using the 2-D cloud model, aerosols lead to larger transport of CWC upward than it was simulated using the WRF mesoscale model. We suppose, therefore, that aerosol effect on cloudiness, precipitation and intensity of TCs may be even more pronounced than that was demonstrated in the study. The utilization of high resolution models with spectral bin microphysics is desirable to make the results quantitative. More observational studies are required for investigation of microphysical structure (supercooled water, cloud ice) of clouds in TCs. Observational and numerical studies are needed also for determination of aerosol fluxes from the land to landfalling tropical cyclones.

The monitoring of lightning in hurricanes both over open sea and in the vicinity to the land can provide useful information about the climatic changes of structure, intensity and precipitation in tropical cyclones.

The decrease in intensity of TC under the influence of small aerosols penetrating cloud base of deep convective clouds found in the simulation allows one to consider a possibility to decrease of hurricane intensity by seeding of small aerosols near cloud base at TC periphery. This seeding can be done during the time period before penetration of natural continental aerosols to distances 250-300 km from the TC

center. The idea to decrease TC intensity by cloud seeding is not new. Hurricane mitigation was first attempted between 1962 and 1983 in the framework of project STORMFURY by the US government (Willoughby et al., 1985). The envisioned modification technique involved artificial stimulation of convection at the outer periphery of the eyewall through seeding of strong convective cloud towers with silver iodide for the purpose of freezing super-cooled water (water in liquid state but colder than 0°C). It was hypothesized that the release of the latent heat of freezing would invigorate convection (Simpson and Malkus, 1964) that would compete with the original eyewall, leading to its reformation at a larger radius, and thus, through partial conservation of angular momentum, produce a decrease in the strongest winds. Modification was attempted in four hurricanes. The analysis of the microphysical structure of tropical convective (TC) clouds performed later (Willoughby et al., 1985) showed that at the levels where seeding was applied there was too little supercooled water in agreement with the results of the present study (see, e.g., Fig. 3 left panel). Consequently, the glaciogenic seeding was not likely to affect cloud dynamics, at least in the way assumed in the STORMFURY conceptual model. As a result, changes in intensity of seeded hurricanes were attributed to natural fluctuations of TC intensity. Seeding with small aerosols at cloud base leads, as it was shown in the study, to convection invigoration at the TC periphery and to the TC weakening. Note that the seeding of cloud bases with small aerosols increases amount of the supercooled water in the upper atmosphere. Thus, the seeding with small aerosols at cloud base may be combined with glaciogenic seeding with silver iodide to reach stronger invigoration of convection on the TC periphery. In more detail the possibility to decrease TC intensity by cloud seeding is discussed by Rosenfeld et al. (2007).

Acknowledgements The study was supported by the Israel Science Foundation, grant N 140/07.

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    What is the conclusion of the tropical cyclone?
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