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| Titel |
An investigation of how radiation may cause accelerated rates of tropical cyclogenesis and diurnal cycles of convective activity |
| VerfasserIn |
M. E. Nicholls |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 15 ; Nr. 15, no. 15 (2015-08-13), S.9003-9029 |
| Datensatznummer |
250119964
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| Publikation (Nr.) |
 copernicus.org/acp-15-9003-2015.pdf |
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| Zusammenfassung |
| Recent cloud-resolving numerical modeling results suggest that radiative
forcing causes accelerated rates of tropical cyclogenesis and early
intensification. Furthermore, observational studies of tropical cyclones have
found that oscillations of the cloud canopy areal extent often occur that are
clearly related to the solar diurnal cycle. A theory is put forward to
explain these findings. The primary mechanism that seems responsible can be
considered a refinement of the mechanism proposed by Gray and Jacobson (1977)
to explain diurnal variations of oceanic tropical deep cumulus convection. It
is hypothesized that differential radiative cooling or heating between a
relatively cloud-free environment and a developing tropical disturbance
generates circulations that can have very significant influences on
convective activity in the core of the system. It is further suggested that
there are benefits to understanding this mechanism by viewing it in terms of
the lateral propagation of thermally driven gravity wave circulations, also
known as buoyancy bores. Numerical model experiments indicate that mean
environmental radiative cooling outside the cloud system is playing an
important role in causing a significant horizontal differential radiative
forcing and accelerating the rate of tropical cyclogenesis. As an expansive
stratiform cloud layer forms aloft within a developing system the mean
low-level radiative cooling is reduced, while at mid levels small warming
occurs. During the daytime there is not a very large differential radiative
forcing between the environment and the cloud system, but at nighttime when
there is strong radiative clear-sky cooling of the environment it becomes
significant. Thermally driven circulations develop, characterized by
relatively weak subsidence in the environment but much stronger upward motion
in the cloud system. This upward motion leads to a cooling tendency and
increased relative humidity. The increased relative humidity at night appears
to be a major factor in enhancing convective activity, thereby leading in the
mean to an increased rate of genesis. It is postulated that the increased
upward motion and relative humidity that occur throughout a deep layer aid
both in the triggering of convection and in providing a more favorable local
environment at mid levels for maintenance of buoyancy in convective cells due
to a reduction of the detrimental effects of dry air entrainment.
Additionally, the day/night modulations of the environmental radiative
forcing appear to play a major role in the diurnal cycles of convective
activity in the cloud system. It is shown that the upward velocity tendencies
in the system core produced by the radiative forcing are extremely weak when
compared to those produced by latent heat release in convective towers, but
nevertheless over the course of a night they appear capable of significantly
influencing convective activity. |
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