| 1. |
- Fitch, Anna C.
(författare)
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An Improved Double-Gaussian Closure for the Subgrid Vertical Velocity Probability Distribution Function
- 2019
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 76, s. 285-304
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Tidskriftsartikel (refereegranskat)abstract
- The vertical velocity probability distribution function (PDF) is analyzed throughout the depth of the lower atmosphere, including the subcloud and cloud layers, in four large-eddy simulation (LES) cases of shallow cumulus and stratocumulus. Double-Gaussian PDF closures are examined to test their ability to represent a wide range of turbulence statistics, from stratocumulus cloud layers characterized by Gaussian turbulence to shallow cumulus cloud layers displaying strongly non-Gaussian turbulence statistics. While the majority of the model closures are found to perform well in the former case, the latter presents a considerable challenge. A new model closure is suggested that accounts for high skewness and kurtosis seen in shallow cumulus cloud layers. The well-established parabolic relationship between skewness and kurtosis is examined, with results in agreement with previous studies for the subcloud layer. In cumulus cloud layers, however, a modified relationship is necessary to improve performance. The new closure significantly improves the estimation of the vertical velocity PDF for shallow cumulus cloud layers, in addition to performing well for stratocumulus. In particular, the long updraft tail representing the bulk of cloudy points is much better represented and higher-order moments diagnosed from the PDF are also greatly improved. However, some deficiencies remain owing to fundamental limitations of representing highly non-Gaussian turbulence statistics with a double-Gaussian PDF.
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| 2. |
- Fitch, Anna C.
(författare)
-
Improving stratocumulus cloud turbulence and entrainment parametrizations in OpenIFS
- 2022
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 148:745, s. 1782-1804
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Tidskriftsartikel (refereegranskat)abstract
- A series of modifications to the stratocumulus cloud turbulence and entrainment parametrizations in the Open Integrated Forecasting System (OpenIFS) model are performed in a Lagrangian single-column model framework to determine whether they are able to improve the representation of the stratocumulus-topped boundary layer and its transition to cloud break-up. Two very different regimes are studied and compared with large-eddy simulations: a subtropical marine stratocumulus-to-cumulus transition case driven by a warming sea-surface temperature, and an Arctic air-mass transformation case where warm and moist air is advected over sea ice during wintertime. The addition of nonlocal mixing driven by stratocumulus cloud-top cooling for stable and dry convective boundary-layer types is found to sustain the cloud deck for longer in the Arctic case, while it dissipates around one day earlier in the original OpenIFS. The assumption that cloud-top-driven mixing reaches the surface, forming a coupled boundary layer, is relaxed. The resulting eddy diffusivity profile is altered such that mixing at cloud top may be enhanced relative to cloud base, promoting decoupling. Decoupling in the subtropical boundary layer is further promoted when the cloud-top turbulent velocity scale is based on the integral of the in-cloud buoyancy flux, accounting for the turbulence generated by latent heat release within the cloud, which increases cloud-top entrainment further relative to cloud base. Some long-standing model issues persist, such as underestimation of the subtropical boundary-layer deepening rate, which leads to a cold and moist bias in the boundary layer. In the Arctic, the boundary layer is generally too well mixed and warm at the surface. The degree of decoupling is also underestimated in both cases. However, the modifications discussed here lead to some improvement in OpenIFS.
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