Tropical cyclones (TCs) are known for their destructive power, particularly as they make landfall. TCs often are accompanied by extreme winds, storm surges and
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torrential rainfall. The TCs wind fields, area of heavy rain, and rain rate are determined by cloud microphysical processes and accompanying latent heat release. At the same time both experimental and numerical investigations of cloud microphysics in TCs are quite limited. Microphysical observations are usually limited by zones near the melting level (McFarquhar and Black, 2004). Numerical simulations of TCs are often carried out using mesoscale models, when clouds are not resolved. Lightning is one factor, which can shed light on the microphysical cloud structure and TC evolution. For instance, increasing lightning rates indicates invigoration of convection, accompanying increasingly larger volume of graupel or small hail aloft, strengthening updrafts and increased probability of heavier rainfall (e.g. Lhermitte and Krehbiel, 1979; Wiens et al., 2005; Fierro et al., 2007). Appearance and intensification of lightning in the eyewall can be a predictor of TC intensification (Orville and Coyne, 1999, Shao et al., 2005). Rodgers et al. (2000) found that the closer the lightning is to the storm center, the more likely the TC is to intensify. The latter makes lightning and its distribution in TCs to be an important characteristic that can be served as a predictor of change in TCs intensity and precipitation.
Lightning is a much more widespread phenomenon over the land than over the sea (Williams et al., 2005). It is important that lightning over the sea takes place manly nearly the continents or in regions located downwind of the continents (like in the Intertropical convergence zone to the west from Africa). Hurricanes are not an exception: a strong rise of the flash rate takes place in the hours prior to and after the landfall. An example of lightning activity in hurricane Katrina (2005) at several successive time instances is presented in Fig. 1.
One can see that intense and persistent lightning takes place within the 250-300 km-radius ring around the hurricane center, which is a typical feature of landfalling hurricanes (Molinari et al. 1999, Cecil et al., 2002a, b). Besides, the lightning activity within the eye wall takes place during a comparatively short period of the tropical cyclone (TC) intensification. According to Black and Hallet (1999) the updraft velocity Wmax exceeding 10 ms—1 and the existence of supercooled cloud droplets above the — 13°C level are necessary conditions for the lightning in a hurricane eye wall. As soon as the hurricane stops intensify, the lightning in the eye
wall disappears. Figure 1 also shows that the high flash rate over the sea takes place in the regions downwind from the land. Similar features were observed in hurricane Rita as well (Shao et al., 2005). Analysis of TRMM's Tropical Microwave Imager (TMI) images of TC Katrina and Rita indicates that precipitation in clouds at the TC periphery starts at higher levels than in the TC eye wall. The zones of intense reflectivity at higher levels coincide with those of enhanced lightning activity.
The mechanisms responsible for most of the characteristics of lightning in landfalling TCs are not well understood (Molinari et al., 1999). Molinari et al. (1999) assumed that one of the factors fostering the lightning activity at the TC periphery is the higher instability of the atmosphere at the periphery as compared to the TC eyewall. At the same time, the instability at the TC periphery is typical of the entire tropical zone over the oceans during the hurricane seasons (Jordan 1958), but it does not lead to the formation of dense lightning over the oceans. Besides, numerical simulations of evolution of an idealized hurricane-like vortex (Fierro et al., 2007) using a mesoscale 2-km resolution model with a bulk parameterization microphysical scheme describing 12 distinct hydrometeor habits (Straka and Mansell, 2005) and a lightning scheme (Mansell et al., 2002) showed much more intense convection and lightning within a TC central convective zone of —50-km radius rather within outer rain bands.
It is known that the charging of hydrometeors in clouds takes place at temperatures below — 13°C, when collisions of ice crystals and graupel take place in the presence of supercooled drops (e.g., Takahashi, 1978; Saunders, 1993; Cecil et al., 2002b; Sherwood et al., 2006). It means that supercooled drops have to ascend in clouds above —5-6 km to trigger flash formation. One can suppose two main physical reasons retarding lightning formation in tropical maritime clouds. The first one is the low vertical velocity in marine clouds including that in hurricanes, which typically does not exceed 5-6 ms—1 (Jorgensen et al., 1985). The second reason is that the concentration of aerosol particles (AP) over the open oceans is very low. Hence, the concentration of cloud droplets growing on these AP is also small, and cloud droplets grow rapidly by condensation and collisions into raindrops that efficiently collect the remaining cloud droplets. Since the fall velocity of raindrops (about 10 ms—1) exceeds the updraft velocities, raindrops start falling at low levels (3-5 km), and no liquid droplets reach the levels required for lightning formation. A classical illustration is that of Hawaiian clouds, where lightning is extremely rare phenomena.
The lightning onset indicates significant changes in cloud dynamics and/or in the cloud microphysics of maritime clouds. Correspondingly, there are two main plausible mechanisms (dynamical and microphysical) that could lead to the cloud structure suitable for lightning formation. The dynamical mechanism (e.g., increase in CAPE) may increase cloud updrafts decreasing the time for droplet collisions and elevating the level of raindrop formation. If these velocities exceed 10 ms—1 even raindrops will ascend leading to formation of graupel and hail by freezing aloft. The importance of the dynamical mechanism of lightning was stressed in several studies (Sherwood, 2002; Melani et al., 2003; Williams et al., 2002, 2005; Williams and Satori, 2004; Heymsfield et al., 2005; Khain et al., 2008). We suppose, however, that in case of lightning in hurricanes, the changes in the atmospheric instability (CAPE) hardly can be the sufficient factor that is fully responsible for the increase in lightning activity. It is difficult to suggest a mechanism which is able to increase CAPE within a narrow concentric ring of 250-300 km radius around the center of TC located relatively far from the land. Characteristic scales of spatial SST variations in the zone of hurricane Katrina 2005, which could be the reason of variation of the atmospheric instability, are much larger than the scale of a few tens of kilometers representing the width of the ring of the intense TC lightning. Other factors, such as variation of surface properties, the boundary layer moisture and the surface friction take place when TC already penetrates the land (at least, partially), but these inhomogeneities affect usually the convergence in the central zone of TC and TC intensity (see e.g., Khain, 1984), and do not increase the CAPE at TC periphery, at least in the manner that could lead to the lightning structure seen in Fig. 1. Increase in instability of the atmosphere at the TC periphery could be caused by penetration of continental air in case it is colder than that over the sea. This opportunity will be tested in a separate study.
Here we will check other hypothesis, namely that the increase in lightning activity in hurricanes approaching the land is caused by continental aerosols involved into the circulation of the hurricane approaching the land.
To the best of our knowledge, the first attempt to study aerosol effects on TC was made by Khain and Agrenich (1987), who studied a possible effects of the Saharan dust on the TC development related to heating of air mass through the interaction of suspended dust and solar radiation. The relationship between weekly changes of anthropogenic particle pollutants and the TC-induced precipitation was reported first by Cerveny and Balling (1998). Here we discuss possible aerosol effects on lightning in hurricanes (and, on their intensity) in the context of cloud-aerosol interaction.
The "aerosol" hypothesis of lightning formation in deep convective clouds has been discussed by Williams et al. (2002), Sherwood et al. (2006), and Rosenfeld et al. (2007a). Sherwood et al. (2006) concluded that aerosols are a particular contributor to climatological land-ocean lightning contrasts.
How might continental aerosol particles (AP) foster lightning formation in hurricanes approaching the land? The concentration of submicron AP over the land is higher than that over the sea by one to two orders of magnitude. When aerosols penetrate oceanic clouds, they dramatically increase the concentration and decrease the size of cloud droplets. Since collisions of such droplets are inefficient, these droplets remain small for a longer time period and ascend in even moderate cloud updraft to higher levels than in clouds with low aerosol concentration (Khain et al. 2004, 2005, 2008; Wang 2005). As a result, in clouds developing in the dirty air supercooled cloud droplets can coexist with graupel and crystals, which is necessary condition for charge separation. Khain et al. (2005) found an invigoration of maritime tropical convection (an increase in updraft) in case of continental aerosol intrusion into the bases of maritime clouds. This increase also helps to transfer supercooled water to upper levels fostering lightning formation.
The formation of lightning at TC periphery indicates an invigoration of the convection there. The latter can affect structure and intensity of TCs. Significant effects of idealized seeding with small aerosol particles of clouds at TC periphery on the intensity of hurricanes located over open sea was recently found by Rosenfeld et al. (2007b) in simulations using a two nested grid Weather Research and Forecasting (WRF) Model. A decrease in hurricane intensity in dirty air was also reported recently by Cotton et al. (2007) in simulations of an idealized TC using RAMS.
In this study we investigate the effect of aerosols both on individual deep maritime clouds (as an element of TC convection) and on the TC structure as a whole. The main purposes of these simulations are a) to justify that continental aerosols penetrating into TC clouds are able to create conditions favorable for lightning formation, b) to check whether aerosols can lead to the fine "coherent" spatial structure of the lightning rate, and c) to investigate the possible role of continental aerosols on the intensity of TC, structure of cloudiness and precipitation of landfalling TC.
The effects of aerosols on individual convective clouds under conditions typical of TC periphery is investigated using a very high resolution 2-D mixed phase Hebrew University cloud model (HUCM) with spectral bin microphysics (Khain et al. 2004, 2005, 2008). The aerosol effects on the cloudiness structure of TC approaching and penetrating the land is investigated using a two nested grid WRF model (Skamarock et al., 2005).
The combination of the two models for the investigation is natural: the high resolution HUCM can describe realistically vertical velocities, as well as all microphysical processes to evaluate the aerosol effects on the vertical profiles of liquid water content and that of different ice particles in individual clouds. These simulations will represent the main justification to the fact that an increase in the aerosol concentration can create conditions favorable for lightning formation in maritime clouds. At present the 3-D simulations of TCs cannot be carried out with so high resolution and with the microphysical scheme used in the HUCM due to computer limitations. These simulations do not resolve small clouds and underestimate vertical velocities. Correspondingly, the TC simulations provide less exact (but still useful) estimations of aerosol effects on individual clouds. At the same time the 3D model is able to reproduce effects of aerosols on the structure of TC cloudiness and precipitation over large areas.
Taking into account the factors affecting the lightning rate (Fierro et al., 2007), the lightning probability will be characterized by the magnitude of the product of total ice content, supercooled content and the vertical velocity above the 5-km level.
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