Preprints
https://doi.org/10.5194/wcd-2022-17
https://doi.org/10.5194/wcd-2022-17
 
28 Mar 2022
28 Mar 2022
Status: this preprint is currently under review for the journal WCD.

Impact of combined microphysical uncertainties on convective clouds and precipitation in ICON-D2-EPS forecasts during different synoptic control

Takumi Matsunobu1, Amirmahdi Zarboo2, Christian Barthlott2, and Christian Keil1 Takumi Matsunobu et al.
  • 1Meteorologisches Institut, Ludwig-Maximilians-Universität, Munich, Germany
  • 2Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

Abstract. The relative impact of individual and combined uncertainties of cloud condensation nuclei (CCN) concentration and the shape parameter of the cloud drop size distribution (CDSD) in the presence of initial and boundary condition uncertainty (IBC) on convection forecasts is quantified using the operational convection-permitting model ICON-D2. We performed 180-member ensemble simulations for five real case studies representing different synoptic forcing situations over Germany and inspect the precipitation variability on different spatial and temporal scales. During weak synoptic control, the relative impact of combined microphysical perturbations on area-averaged daily precipitation comprises about ±12 % which is around one-third the variability caused by operational IBC perturbations. The combined microphysical perturbations exceed the impact of individual CCN or CDSD perturbations. High CCN concentrations combined with a narrow CDSD show the largest decrease in precipitation. The combination of IBC and microphysical perturbations affect the extremes of daily spatially averaged rainfall of individual members by extending the tails of the forecast distribution by 5 % in weakly forced conditions. The responses are relatively insensitive in strong forcing situations. Visual inspection and objective analysis of the spatial variability of hourly rainfall rates reveal that IBC and microphysical perturbations alter the spatial variability of precipitation forecasts differently. Microphysical perturbations slightly shift convective cells but affect precipitation intensities while IBC perturbations scramble the location of convection during weak control. Cloud and rain water content is more sensitive to microphysical perturbations than precipitation but slightly less dependent on the synoptic control. In contrast to the impact on precipitation, an increase in CCN concentration and shape parameter of CDSD has a significant positive impact on the formation of cloud water. Combined microphysical perturbations play a dominant role in cloud forecasts with a relative impact ranging between +79 % and -62 % on daily averaged vertically integrated cloud water, and between +57 % and -35 % on rain water content in weakly forced conditions. Thus microphysical uncertainty exhibits a relevant impact on cloud and rain water content and precipitation and its impact largely depends on the prevailing synoptic control in mid-latitude warm-season weather forecasts.

Takumi Matsunobu et al.

Status: open (extended)

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  • RC1: 'Comment on wcd-2022-17', Anonymous Referee #1, 29 Apr 2022 reply

Takumi Matsunobu et al.

Takumi Matsunobu et al.

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Short summary
This study quantifies the impact of poorly constrained parameters used to represent aerosol-cloud-precipitation interactions in an operational weather forecasting model on precipitation and cloud forecasts. Uncertainties in these parameters have a non-negligible impact on daily precipitation amount, and largely change the amount of cloud. The comparison between different weather situations reveals that the impact becomes more important when convection is triggered by local effects.