Diurnal Circulation Adjustment and Organized Deep Convection

Abstract

This study investigates the diurnal cycle of tropical organized deep convection and the feedback in large-scale circulation. By considering gravity wave phase speeds, we find that the circulation adjustment into weak temperature gradient (WTG) balance occurs rapidly (<6 h) relative to diurnal diabatic forcing on the spatial scales typical of organized convection (≤500 km). Convection-permitting numerical simulations of self-aggregation in diurnal radiative–convective equilibrium (RCE) are conducted to explore this further. These simulations depict a pronounced diurnal cycle of circulation linked to organized convection, which indeed maintains WTG balance to first order. A set of sensitivity experiments is conducted to assess what governs the diurnal cycle of organized convection. We find that the “direct radiation–convection interaction” (or lapse-rate) mechanism is of primary importance for diurnal precipitation range, while the “dynamic cloudy–clear differential radiation” mechanism amplifies the range by approximately 30%, and delays the nocturnal precipitation peak by around 5 h. The differential radiation mechanism therefore explains the tendency for tropical heavy rainfall to peak in the early morning, while the lapse-rate mechanism primarily governs diurnal amplitude. The diurnal evolution of circulation can be understood as follows. While nocturnal deep convection invigorated by cloud-top cooling (i.e., the lapse-rate mechanism) leads to strong bottom-heavy circulation at nighttime, the localized (i.e., differential) top-heavy shortwave warming in the convective region invigorates circulation at upper levels in daytime. A diurnal evolution of the circulation therefore arises, from bottom heavy at nighttime to top heavy in daytime, in a qualitatively consistent manner with the observed diurnal pulsing of the Hadley cell driven by the ITCZ.

Diurnal Cloud and Circulation Changes in Simulated Tropical Cyclones

“…The important exception is the high, thick anvil clouds of the TC inner core, where direct shortwave absorption causes net Q R warming of ∼4 K/day slightly beneath the level of maximum nocturnal cooling (the decomposition of Q R into longwave and shortwave at 12 L is provided as Figure S1). The resulting change in static stability or lapse rate is the primary driver of the nocturnal invigoration of deep convection and rainfall (Liu & Moncrieff, 1998;Ruppert & Hohenegger, 2018;Xu & Randall, 1995). These Q R patterns are consistent with the signatures of thick anvils previously reported (Powell et al, 2012;Webster & Stephens, 1980).…”

“…This radial range captures nearly the entire radial region characterized by mean upward motion and is henceforth referred to simply as the TC inner core (Figure 2a; results are qualitatively insensitive to modest changes in the bounds of this range). Two distinct diurnal peaks are apparent, which are of the same order of those described by Navarro and Hakim (2016) and Ruppert and Hohenegger (2018): a nocturnal peak centered at ∼5 km vertically and an early-afternoon peak centered at ∼13 km. These peaks correspond to the bottom-heavy nighttime and top-heavy daytime regimes in Figure 2a.…”

“…During daytime, warming due to shortwave absorption nearly perfectly offsets longwave cooling in most places. The top-heavy diurnal radiative heating-cooling cycle described above induces a top-heavy variation in temperature of O(1 K) in the TC inner core (e.g., Figure 7 of Navarro & Hakim, 2016 and Figure 4 of Ruppert & Hohenegger, 2018). The diurnal-anomaly signal Q ′ R averaged within the TC inner core further emphasizes the very top-heavy nature of this diurnal heating/cooling cycle (Figure 2b).…”

“…The diurnal-anomaly signal Q ′ R averaged within the TC inner core further emphasizes the very top-heavy nature of this diurnal heating/cooling cycle (Figure 2b). An additional process that augments this nocturnal invigoration is the differential radiative cooling between the TC inner core and surrounding environment, which promotes low-level convergence into the inner core at night (Gray & Jacobson, 1977;Nicholls, 2015;Ruppert & Hohenegger, 2018). The top-heavy diurnal radiative heating-cooling cycle described above induces a top-heavy variation in temperature of O(1 K) in the TC inner core (e.g., Figure 7 of Navarro & Hakim, 2016 and Figure 4 of Ruppert & Hohenegger, 2018).…”

“…While longwave emission perpetually acts to cool cloud tops, direct shortwave absorption warms cloud tops during daytime (Webster & Stephens, 1980). This effect is augmented by radiatively driven circulation changes: as low-level radiative cooling nocturnally increases in the cloud-free surroundings of a large convective system, low-level convergence into the system is enhanced (Gray & Jacobson, 1977;Nicholls, 2015;Ruppert & Hohenegger, 2018). This effect is augmented by radiatively driven circulation changes: as low-level radiative cooling nocturnally increases in the cloud-free surroundings of a large convective system, low-level convergence into the system is enhanced (Gray & Jacobson, 1977;Nicholls, 2015;Ruppert & Hohenegger, 2018).…”