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- Title
On the sensitivity of deep‐convection initiation to horizontal grid resolution.
- Authors
Tang, S. L.; Kirshbaum, D. J.
- Abstract
Idealized numerical simulations are used to study the sensitivity of deep‐convection initiation to horizontal grid spacing (Δh). In a conditionally unstable but strongly inhibited environment, a localized surface heating function gives rise to low‐level convergence and a vigorous subcloud updraught that breaches the level of free convection (LFC). The vertical cloud development is sensitive to Δh: the clouds reach 8–9 km for Δh=250 m (CTRL) and Δh=500 m (DX500) but only 6–7 km for Δh=125 m (DX125). This trend is not associated with major differences in midlevel cloud vigour (e.g., buoyancy and vertical velocity), but can be explained on the basis of cloud‐core mass flux (Mc). The Mc profile is regulated by its value at the LFC, largely set by subcloud processes, and its core‐layer gradient, set by entrainment and detrainment. The former is the largest in CTRL and weakens as the grid is coarsened (DX500) or refined (DX125), both due to a widening and weakening of the subcloud updraught. Whereas in DX500 this widening stems from a failure to adequately resolve the 1‐km‐wide updraught on the model grid, in DX125 it results from stronger total (subgrid plus resolved) turbulent mixing, which increases monotonically as Δh is decreased. The wider and more diffuse updraughts in DX125 and DX500 are also more hydrostatic and generate weaker buoyancy‐driven accelerations. Within the cloud layer, the entrainment is very similar in the three cases, as is the detrainment for the higher‐resolution cases (DX125 and CTRL). However, the midlevel detrainment is relatively weak in DX500, which facilitates slightly deeper ascent than in CTRL. By contrast, the small core‐base Mc and strong midlevel detrainment yields the shallowest clouds in DX125.
- Subjects
NATURAL heat convection; ENTHALPY; COMPUTER simulation; FLUX (Energy); VELOCITY; ENTRAINMENT (Physics)
- Publication
Quarterly Journal of the Royal Meteorological Society, 2020, Vol 146, Issue 728, p1085
- ISSN
0035-9009
- Publication type
Article
- DOI
10.1002/qj.3726