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Abstract We describe internal, low‐frequency variability in a 21‐year simulation with a cloud‐resolving model. The model domain is the length of the equatorial Pacific and includes a slab ocean, which permits coherent cycles of sea surface temperature (SST), atmospheric convection, and the convectively coupled circulation. The warming phase of the cycle is associated with near‐uniform SST, less organized convection, and sparse low cloud cover, while the cooling phase exhibits strong SST gradients, highly organized convection, and enhanced low cloudiness. Both phases are quasi‐stable but, on long timescales, are ultimately susceptible to instabilities resulting in rapid phase transitions. The internal cycle is leveraged to understand the factors controlling the strength and structure of the tropical overturning circulation and the stratification of the tropical troposphere. The overturning circulation is strongly modulated by convective organization, with SST playing a lesser role. When convection is highly organized, the circulation is weaker and more bottom‐heavy. Alternatively, tropospheric stratification depends on both convective organization and SST, depending on the vertical level. SST‐driven variability dominates aloft while organization‐driven variability dominates at lower levels. A similar pattern is found in ERA5 reanalysis of the equatorial Pacific. The relationship between convective organization and stratification is explicated using a simple entraining plume model. The results highlight the importance of convective organization for tropical variability and lay a foundation for future work using coupled, idealized models that explicitly resolve convection.
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Quasi‐2‐Day and Diurnal Cloud Variation Timescales Over Convectively Active Regions
Abstract Climatological features of the cloud variability on quasi‐2‐day (Q2D) and diurnal cycle (DC) timescales are investigated by utilizing the high‐resolution satellite infrared brightness temperature (IRBT) observations from January 1998 to December 2019. A distinct land‐sea contrast between the distributions of Q2D and DC signals is evident. Diurnally driven cloud activity mainly occurs over land and mountainous regions, and the Q2D timescale is more prominent over tropical ocean basins and land where organized convection is usually observed, for example, Congo and Amazon Rainforests, the United States and subtropical South America during warm seasons. The long‐term relationship between the Q2D variability and sea surface temperature (SST) shows that the clouds are more active on Q2D timescales over higher SST environments. The Q2D variability correlates well with both the Indian Ocean Dipole (IOD) and El Niño/Southern Oscillation (ENSO) from 1998 to 2019. The cloud variability associated with a range of convective available potential energy (CAPE) values is analyzed. The result over land shows that increased Q2D cloud variability emerges with higher CAPE, suggesting the coincidence of Q2D and organized convection, particularly given that this effect is strongest over regions with frequent mesoscale convective systems (MCSs) around the world. The cloud variability and the Q2D timescale analyses provide an alternative perspective to understand the global features of mesoscale convective systems. Overall, this study objectively examines the global variability of convective timescales related to the diurnal cycle and longer‐lived convective systems to provide a greater understanding of how the global convection population varies in space and time.
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- PAR ID:
- 10444040
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 126
- Issue:
- 21
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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