An investigation of how radiation may cause accelerated rates of tropical cyclogenesis and diurnal cycles of convective activity

Abstract

Abstract. 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.

The Two Diurnal Modes of Tropical Upward Motion

“…This change causes tropospheric static stability to increase in convective regions during daytime and decrease during nighttime, which explains the nocturnal invigoration of convection and rainfall to first order (Liu & Moncrieff, ; Randall et al, ; Ruppert & Hohenegger, ). The low‐level diurnal radiation‐circulation adjustment mechanism of Gray and Jacobson () augments this nocturnal peak (Nicholls, ; Ruppert & Hohenegger, ). Latent heating from this nocturnally enhanced convection increases upward motion in the lower‐midtroposphere (Ruppert & Hohenegger, ).…”

“…Over the open ocean, radiative heating and its interaction with clouds plays a dominant role in diurnally modulating deep convection, causing rainfall to peak at night‐early morning (Gray & Jacobson, ; Kraus, ; Randall et al, ). Fully understanding the interaction between radiative heating, circulation, and clouds on this time scale is a challenge, since the transient impacts of gravity waves can regularly manifest within it (Nicholls, ; O'Neill et al, ; Ruppert & Hohenegger, ). Yet understanding this interplay is critical to both resolving deficiencies of the diurnal cycle in climate models (Dirmeyer et al, ; Jiang et al, ) and understanding how it impacts the radiation budget at longer time scales (Bergman & Salby, ; Cronin et al, ; Ruppert, ).…”

On the cumulus diurnal cycle over the tropical warm pool

“…Observations reveal that regimes of organized deep moist convection over the tropical oceans exhibit a pronounced diurnal cycle (i.e., distinct from land influences), with a prominent predawn rainfall peak [ Gray and Jacobson , ]. The exact mechanism that accounts for this diurnal rainfall phasing is still debated [ Randall et al ., ; Mapes and Houze , ; Xu and Randall , ; Chen and Houze , ; Liu and Moncrieff , ; Dai , ; Yang and Slingo , ; Woolnough et al ., ; Yang and Smith , ; Nicholls , ]. Convective systems in such regimes tend to first initiate during the day, and build through the afternoon and night [ Mapes and Houze , ; Chen and Houze , ; Sui et al ., ; Bellenger et al ., ].…”

“…When cloud systems are more stationary and organized on larger scales, however, the differential radiative cooling/warming mechanism may dominate. In tropical cyclones, for instance, Nicholls [] argues that enhanced nocturnal radiative cooling adjacent to the cloud shield augments ascent and moistening within the cyclone core overnight, leading to nocturnal intensification. This argument is consistent with the findings from case studies [ Dunion et al ., ; Melhauser and Zhang , ].…”

The Rainfall Responses of Typhoon Soudelor (2015) to Radiative Processes of Cloud Species

“…In the past decades, the physical processes responsible for heavy rainfall during the landfall of typhoons have been studied (e.g., Bender, Tuleya, & Kurihara, 1985;Bu et al, 2014;Cecelski & Zhang, 2016;Colle, 2003;Elsberry, 2002;Elsberry & Marks, 1999;Lou & Li, 2016;Tuleya & Kurihara, 1978;Wang, Li, & Carey, 2007;Wang, Li, & Tao, 2010;Xu & Li, 2017;Yue & Shou, 2011). One of major physical processes is cloud radiative effects on the rainfall systems (Bu et al, 2014;Cecelski & Zhang, 2016;Lou & Li, 2016;Nicholls, 2015;Nicholls & Montgomery, 2013).…”