La Integración de Lagunas como Aportación de Conocimiento Creativo

dm dt = 4πC(Si− 1) [( Ls RvT − 1) Ls KT + RvT ei(T )Dv] (2.10) Here, as before, ventilation and kinetic eects have been neglected. In reality, all ice crystals fall relative to air and hence it is necessary to consider the eect of the falling motion to obtain more accurate estimate of the diusion crystal growth rate (Wang, 2013). Ventilation eects tend to intensify growth speed and can be assimilated into equation 2.10 modifying the denominator with appropriate correction terms. Nevertheless, equation 2.10 remains a good approximation to calculate the growth rate in the cases of large ice crystals.

2.1.6 Habits of ice crystals

As said earlier, the electrostatic analogy is only an approximation of the real growth process, since vapor density and temperature are assumed to be con- stant on the particle surface. Indeed, while the assumption on temperature could be somehow valid, the constant of vapor density, because the irregular- ity of crystal surface, is far from being true. The surface of growing crystals is made up of at terraces of dierent heights, terminating at ledges and separated by steps (Liou and Yang, 2016), and steps themselves are irregular and erratically distributed. These are the reasons why vapor density varies from place to place on the particle surface.

Environmental conditions, specically temperature and supersaturation, determine not only the growth rate but also shape and aspect of ice crystals modifying their preferential growth direction. Several studies have been car- ried out in the last 70 year, producing a variety of habit diagrams describing ice crystal shapes as a function of temperature and ice supersaturation. The diagrams were drawn from laboratory, in situ observations or combinations of the two. Pruppacher and Klett (1997) list dierent laboratory results (e.g. Kobayashi 1961). Magono and Lee (1966) put together diagrams com- ing from observation of snowfall obtained at the ground where snow crystals were classied into 80 dierent types, and each of these types may exist over a wide range of sizes and various degrees of aggregation (Locatelli and Hobbs, 1974).

Bailey and Hallett (2009) made a review of existing diagrams in the lit- erature, and noticed that almost all habit diagrams agree on the behavior of ice crystals at temperatures from 0◦_{C to -18}◦_{C, whereas they dier in a}

fundamental way at lower temperatures. Hence they proposed a new habit diagram as result of the previous studies as well as of their laboratory works and also adding eld observations utilizing a nonimpacting cloud particle imager. The comprehensive diagram (Figure 2.1), in which the deep com- plexity and the great variety of the dierent forms as well as the intriguing links with supersaturation and temperature are well-represented, covers a temperature range from from 0◦_{C to -70}◦_{C and an ice supersaturation val-}

ues from 0.1 to 0.6. It retains the habits description of older diagram for temperatures above -18◦_{C. It should be noted, for instance, the ice crystal}

appearance evolution at a xed supersaturation of 0.1, that varies from plates to columns, again to plates and, as temperature becomes progressively lower, again to column. Shape dependence from supersaturation is clear from the

4. Modification of columnar and rosette habits precipitating from cirrus clouds

While studying the CPI images provided by Korolev and Isaac, another pattern emerged with respect to bullet

rosettes and columns. Rosette ‘‘bullets’’ and columns grown in the laboratory and observed at in situ at tem- peratures lower than 2408C typically have aspect ratios of 3–4 or greater, except in the case of single columns growing at low to moderate ice supersaturations (below

FIG. 5. Habit diagram in text and pictorial format for atmospheric ice crystals derived from laboratory results (BH04) and CPI images gathered during AIRS II and other field studies. Diagonal bars near the middle of the upper diagram are drawn to suggest the possibility of the extension of the bullet rosette habit to temperatures slightly higher than 2408C.

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Fig. 2.1: Diagram of habits in text and pictorial format for ice crystals drawn form laboratory results and from observations. Source: Bailey and Hallett (2009).

diagram, for example, xing temperature at -15◦_{C crystal habit switch form}

plates to dendrites, whereas at -30◦_{C from plates to columns. In addition,}

it must be noted that the complexity of forms increases with increasing su- persaturation. The study of crystal characteristics at temperatures between -20◦_{C and -70}◦_{C was carried out in the laboratory using a static diusion}

chamber and it constitutes the new element of the research made by Bai- ley and Hallett (2004). From -20◦_{C and -40}◦_{C they found that plates are}

the dominant shape whereas columns mainly appears at lower temperature. In both regimes complexity depends on supersaturation as said previously. Also, it should be pointed out that temperatures of about -20◦_{C and -40}◦_C

act virtually as borders dividing single crystal and polycristalline (plates and columnar) regimes. Single crystals (also known as pristine) represent the var- ious shapes of ice particles in the simplest way, but are not fully indicative of all natural snow crystals, which are often dominated by polycristalline forms (Libbrecht, 2005). The latter instead reveal nucleation processes that lead to the formation of irregular polycrystals and crystal twins namely crystals with two or more components growing from a common grain boundary dis- location (Bailey and Hallett, 2004). These complicated shapes should not be confused with aggregate particles (resulting from accretion processes, see section 2.12), as often happens in the literature (Bailey and Hallett, 2009).