The Two Diurnal Modes of Tropical Upward Motion

Abstract

This study describes a new mechanism governing the diurnal variation of vertical motion in tropical oceanic heavy rainfall zones, such as the intertropical convergence zone. In such regions, the diurnal heating of widespread anvil clouds due to shortwave radiative absorption enhances upward motion in these upper layers in the afternoon. This radiatively driven ascent promotes an afternoon maximum of anvil clouds, indicating a diurnal cloud‐radiative feedback. The opposite occurs at nighttime: While rainfall exhibits a dominant peak at night‐early morning, the boundary layer rooted upward motion and latent heating tied to this peak are forced to be more bottom heavy by the nighttime anomalous radiative cooling at upper levels. This mechanism therefore favors the stratiform top‐heavy heating mode during daytime and suppresses it nocturnally. These diurnal circulation signatures arise from microphysical‐radiative feedbacks that manifest on the scales of organized deep convection, which may ultimately impact the daily mean radiation budget.

What Drives the Life Cycle of Tropical Anvil Clouds?

“…Convective strength, anvil cloud extent, and their properties are also modulated by the daily cycle of insolation (Chen & Houze, ; Dai, ). Ruppert and Klocke () show that the diurnal peak in the cloud radiative heating increases the vertical velocities within anvils and leads to an anvil cloud peak in the afternoon. Similarly, the anvil life cycle was found to be suppressed during nighttime.…”

Diurnal Forcing and Phase Locking of Gravity Waves in the Maritime Continent

“…The inability of many general circulation models (GCMs) to properly represent the MJO may indeed stem from their inability to represent the basic moisture state over the MC (Gonzalez and Jiang 2017;Jiang 2017). Such model issues have been remarkably enduring (Jiang et al 2009), which emphasizes our insufficient understanding of how local diurnal processes-the prevailing driver of moist convection in the MC-rectify and interact with largerscale circulation (Peatman et al 2014;Cronin et al 2015;Ruppert 2016;Yamanaka et al 2018). This study seeks a better understanding of this link.…”

Coupling of Clouds and Tropospheric Relative Humidity in the Tropical Western Atlantic: Insights From Multisatellite Observations

“…The increased temperature gradient yields mixing moisture vertically. We hypothesize that this behavior is remotely connected to the ITCZ diurnal pulses and its afternoon peak, as suggested by Ruppert and Klocke (2019). (iv) The lifting of moist air in the upper troposphere triggers the formation of high-altitude cirriform clouds at night (18:00-24:00 LT).…”

“…These studies suggest that large anvil clouds dampen the heating of the atmospheric column (via their shade). It feedbacks on precipitation via locally enhanced upward motion within the cloudy column as compared to neighboring cloud-free columns (Chen & Houze, 1997;Ruppert & Klocke, 2019).…”

“…The impact of clouds on solar radiation and related heating has also been addressed in numerous works. The direct absorption of shortwave radiation by optically opaque and elevated clouds (anvil-type clouds) is thought to cause a strong cloud-top heating (Ackerman et al, 1988;Hartmann et al, 1984;Mather et al, 2007;Powell et al, 2012) that increases buoyancy below, and a lifting toward the upper levels, thereby causing a daytime peak in upper-level cloudiness to prevail (Ruppert & Klocke, 2019). Atmospheric tides, driven by solar heating and ozone absorption, have also an effect in the diurnal convection cycle and subsequent precipitation (Woolnough et al, 2004).…”