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Abstract Satellite observations of tropical maritime convection indicate an afternoon maximum in anvil cloud fraction that cannot be explained by the diurnal cycle of deep convection peaking at night. We use idealized cloud-resolving model simulations of single anvil cloud evolution pathways, initialized at different times of the day, to show that tropical anvil clouds formed during the day are more widespread and longer lasting than those formed at night. This diurnal difference is caused by shortwave radiative heating, which lofts and spreads anvil clouds via a mesoscale circulation that is largely absent at night, when a different, longwave-driven circulation dominates. The nighttime circulation entrains dry environmental air that erodes cloud top and shortens anvil lifetime. Increased ice nucleation in more turbulent nighttime conditions supported by the longwave cloud-top cooling and cloud-base heating dipole cannot compensate for the effect of diurnal shortwave radiative heating. Radiative–convective equilibrium simulations with a realistic diurnal cycle of insolation confirm the crucial role of shortwave heating in lofting and sustaining anvil clouds. The shortwave-driven mesoscale ascent leads to daytime anvils with larger ice crystal size, number concentration, and water content at cloud top than their nighttime counterparts. Significance Statement Deep convective activity and rainfall peak at night over the tropical oceans. However, anvil clouds that originate from the tops of deep convective clouds reach their largest extent in the afternoon hours. We study the underlying physical mechanisms that lead to this discrepancy by simulating the evolution of anvil clouds with a high-resolution model. We find that the absorption of sunlight by ice crystals lofts and spreads the daytime anvil clouds over a larger area, increasing their lifetime, changing their properties, and thus influencing their impact on climate. Our findings show that it is important not only to simulate the correct onset of deep convection but also to correctly represent anvil cloud evolution for skillful simulations of the tropical energy balance.
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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.
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- Award ID(s):
- 1524844
- PAR ID:
- 10460275
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 5
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 2911-2921
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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