Master thesis by Herman F. Fuglestvedt
Title: A Conceptual Model for Self-Organisation of Precipitating Convection
Abstract: Precipitating convective clouds can lead to the formation of spreading patches of evaporatively cooled air caused by the re-evaporation of precipitation in the sub-cloud layer. Studies based on observations and numerical models have found these so-called cold pools to organise convection by modifying the near-surface temperature and moisture fields, as well as dynamically triggering convection by lifting air parcels at the outflow boundaries. This interaction between convective clouds by means of cold pool dynamics has also been linked to precipitation extremes.
Climate models typically operate at horizontal resolutions larger than the scale of convective clouds, requiring their formation to be represented by parametrisation schemes. Including convective self-organisation in such schemes is necessary if sub-grid scale temporal and spatial patterns of convective clouds and precipitation are to be predicted accurately. Cold pools are, however, generally absent from the convection parametrisation schemes implemented in current climate models. In order to rectify this shortcoming, simple representations of the effect of cold pools on convective self-organisation is needed.
In this thesis, a conceptual model of a convectively active atmosphere is constructed and evaluated in comparison with idealised Large-eddy simulations (LES). The dynamics of the model are limited to a simple description of cold pool propagation, formulated as a one-dimensional cellular automaton with rules governed by thermodynamics and the ability to trigger new convection events.
The evaluation shows that cold pools in this conceptual model lead to an organisation of the convective cloud field that to a high degree resembles the organisation exhibited in the LES. The cold pools act to suppress convective activity by cooling the boundary layer, and new events preferentially form along the outflow boundaries of the cold pools. The self-organisation in the conceptual model responds to changing boundary conditions in qualitatively the same way as the LES, in terms of the capability of cold pools to inhibit convection and the number of simultaneous convection events.
These results, though limited to a superficial evaluation of the model, bode well for the future application of a similar approach to representing convective self-organisation in convection parametrisation schemes. The model can also be developed further with the purpose to provide a transparent framework for studying the impact of convective self-organisation on, for example, the diurnal cycle of precipitation or extreme precipitation events.