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Discussion papers
https://doi.org/10.5194/nhess-2018-288
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2018-288
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Nov 2018

Research article | 13 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Impact analysis of dynamical downscaling on the treatment of convection in a regional NWP model – COSMO: a case study during the passage of a very severe cyclonic storm OCKHI

S. Roshny1, D. Bala Subrahamanyam1, T. J. Anurose2, and Radhika Ramachandran1 S. Roshny et al.
  • 1Space Physics Laboratory, Vikram Sarabhai Space Centre, Dept. of Space, Govt. of India, Indian Space Research Organization, Thiruvananthapuram – 695 022, India
  • 2Institute for Atmospheric and Environmental Sciences, Goethe University, Frankfurt 60438, Frankfurt a. Main, Germany

Abstract. A significant source of uncertainty in Numerical Weather Prediction (NWP) models arises from the parameterization of sub-grid scale convection, whose inherent nature of complexity is amplified while applied to tropical regions where weather systems are controlled by many intricate factors. However, as the model resolution becomes finer, it is possible to switch off the convection parameterization, although it is still unclear at what resolution this can be achieved. Ambiguity arises due to the inter-linking of various parameterization schemes within a model, and efficiency of one scheme depends on the output of another. In order to explore these issues, an intense convective episode with very heavy precipitation over the coastal Arabian Sea associated with the passage of OCKHI, one of the very severe cyclonic storms, is chosen as a case study. A set of distinct numerical simulations are carried out using Consortium for Small-scale Modelling (COSMO) to assess the direct and indirect impacts of dynamical downscaling on the treatment of convection. Results obtained from the present investigation indicate dynamical downscaling together with switching off the convection parameterization could simulate the magnitudes of CAPE, one of the proxies for characterizing the occurrence of tropical convection, more realistically. But the downscaling did not improve the rainfall prediction, which were seen to deteriorate in the absence of convection parameterization.

S. Roshny et al.
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Short summary
Dynamical downscaling of an NWP model is closely related to the parameterization of convection. This is an unique study on a very severe cyclonic storm OCKHI over the Arabian Sea in which we explore the impact of downscaling of grid resolution in COSMO regional NWP model and investigate the effect of explicit treatment of convection on the performance of model. Switching off the convection parameterization yield good values of CAPE but lead to poor performance in terms of rainfall simulations.
Dynamical downscaling of an NWP model is closely related to the parameterization of convection....
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