Cloud‐Radiation Interactions and Their Contributions to Convective Self‐Aggregation
This study investigates the direct radiative‐convective processes that drive and maintain aggregation within convection‐permitting elongated channel (and smaller square) simulations of the UK Met Office Unified Model. Our simulations are configured using three fixed sea surface temperatures (SSTs) f...
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| Published in: | Journal of advances in modeling earth systems Vol. 13; no. 9 |
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| Main Authors: | , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Washington
John Wiley & Sons, Inc
01-09-2021
American Geophysical Union (AGU) |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | This study investigates the direct radiative‐convective processes that drive and maintain aggregation within convection‐permitting elongated channel (and smaller square) simulations of the UK Met Office Unified Model. Our simulations are configured using three fixed sea surface temperatures (SSTs) following the Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) protocol. By defining cloud types based on the profile of condensed water, we study the importance of radiative interactions with each cloud type to aggregation. We eliminate the SST dependence of the vertically integrated frozen moist static energy (FMSE) variance budget framework by normalizing FMSE between hypothetical upper and lower limits based on SST. The elongated channel simulations reach similar degrees of aggregation across SSTs, despite shortwave and longwave interactions with FMSE contributing less to aggregation as SST increases. High‐cloud longwave interactions are the main drivers and maintainers of aggregation. Their influence decreases with SST as high clouds become less abundant. This SST dependence is consistent with changes in grid spacing and the critical humidity threshold for condensation (RHcrit). However, the domain‐mean longwave‐FMSE feedback would likely decrease as grid spacing and RHcrit are reduced by lowering the condensed water path and cloud top height of high‐cloud, and altering the distribution of different cloud types. Shortwave interactions with water vapor are key maintainers of aggregation and are dependent on SST and the degree of aggregation itself. The analysis method used provides a new framework to compare the effects of radiative‐convective processes on self‐aggregation across different SSTs and model configurations to help improve our understanding of self‐aggregation.
Plain Language Summary
The spontaneous clustering of rainstorms (termed convective self‐aggregation) is a common feature in weather and climate models. The amount of aggregation has a large influence on both weather and climate, so being able to understand how aggregation develops and how it is affected by a warming climate is important in both weather and climate modeling. Previous studies have shown that interactions between convection and radiation (both solar radiation and thermal radiation) are crucial for driving and maintaining aggregation. This study provides a detailed analysis of the key radiative‐convective interactions that influence aggregation within simulations of the Met Office Unified Model. We assess their sensitivities to the model's sea surface temperature (SST), grid spacing, and critical humidity for cloud formation. We find that the contribution of radiative‐convective interactions to aggregation decreases as the SST is increased because the amount of high cloud decreases, and because the difference in absorption of solar radiation between humid and dry regions becomes less significant for aggregation. Decreasing both the model grid spacing, and the model's critical humidity for cloud formation has the effect of decreasing the magnitude of the cloud interactions with thermal radiation, leading to a hypothesized slowing of the rate of aggregation.
Key Points
The normalized FMSE variance budget is a consistent framework to study aggregation at all SSTs
Longwave interactions with high clouds and shortwave interactions with moisture affect FMSE variance
Longwave feedbacks contribute less to aggregation as SST increases as high cloud fraction decreases |
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| ISSN: | 1942-2466 1942-2466 |
| DOI: | 10.1029/2021MS002535 |