How well are hazards associated with derechos reproduced in regional climate simulations?
Abstract. An 11-member ensemble of convection-permitting regional simulations of the fast-moving and destructive derecho of June 29 – 30, 2012 that impacted the northeastern urban corridor of the US is presented. This event generated 1100 reports of damaging winds, significant wind gusts over an extensive area of up to 500,000 km2, caused several fatalities and resulted in widespread loss of electrical power. Extreme events such as this are increasingly being used within pseudo-global warming experiments that seek to examine the sensitivity of historical, societally-important events to global climate non-stationarity and how they may evolve as a result of changing thermodynamic and dynamic context. As such it is important to examine the fidelity with which such events are described in hindcast experiments. The regional simulations presented herein are performed using the Weather Research and Forecasting (WRF) model. The resulting ensemble is used to explore simulation fidelity relative to observations for wind gust magnitudes, spatial scales of convection (as manifest in high composite reflectivity), and both rainfall and hail production as a function of model configuration (microphysics parameterization, lateral boundary conditions (LBC), start date, and use of nudging). We also examine the degree to which each ensemble member differs with respect to key mesoscale drivers of convective systems (e.g. convective available potential energy and vertical wind shear) and critical manifestations of deep convection; e.g. vertical velocities, cold pool generation, and how those properties relate to correct characterization of the associated atmospheric hazards (wind gusts and hail). Here, we show that the use of a double-moment, 7-class scheme with number concentrations for all species (including hail and graupel) results in the greatest fidelity of model simulated wind gusts and convective structure against the observations of this event. We further show very high sensitivity to the LBC employed and specifically that simulation fidelity is higher for simulations nested within ERA-Interim than ERA5.
Tristan J. Shepherd et al.
Tristan J. Shepherd et al.
Tristan J. Shepherd et al.
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Review of “How well are hazards associated with derechos reproduced in regional climate simulations?” by Shepherd et al.
This is a model evaluation study to evaluate WRF simulations downscaling to 1.3 km grid spacing with changes of cloud microphysics schemes, lateral boundary conditions (LBC), start date, and nudging. The focus is derecho induced from a mesoscale convective system (MCS). Since derechos cause significant infrastructure damages and economic loss, it is interesting to see if models can capture such extreme events and how the simulations are sensitive to different model setups and physical parameterizations. So, I advocate such studies. However, after looking at the results, I had to doubt whether the simulations were carried out correctly or the simulations were produced from a stable supercomputer/cluster. The model results swift from a convective system simulated with one microphysics scheme to the disappearance of the system with another microphysics scheme is something I never experienced as a senior cloud modeler. Particularly, switching graupel to hail in Morrison scheme also caused the disappearance of the MCS, which is not likely to occur since the change from graupel to hail renders minor changes relative to the entire scheme (mainly in fall speed and density). The hail option is recommended to use for continental deep convective cloud cases by the model developer but it does not simulate the MCS at all. I tested both options in several studies before and this never happened (the simulated convective cloud systems were generally very similar in morphology). In addition, there are so many literature studies with different microphysics schemes for a variety of cases that do not show such a result. As the authors stated this is expected result.
Also, none of the simulations can simulate both derecho and front stages of the observed system, then if the study have focused on why the simulations fail like this, it would still be useful. Furthermore, the sensitivity to two lateral boundary data (ERA5 and ERA-Interim) is also opposite compared with the previous studies (many literature studies showed ERA5 data is improved on ERA-Interim). With all these considered, it is very difficult for me to trust these model simulations thus I would recommend the rejection of the manuscript at this time.
Below I have some specific comments including the appropriate way to calculate the maximum hail size to compare with observations. Hope these comments will be useful for authors to improve the study.
“We also examine the degree to which each ensemble member differs with respect to key mesoscale drivers of convective systems (e.g. convective available potential energy and vertical wind shear) and critical manifestations of deep convection; e.g. vertical velocities, cold pool generation, and how those properties relate to correct characterization of the associated atmospheric hazards (wind gusts and hail).”
-The sentence is near the end of the abstract about it is still about the scientific approach. Suggest changing to phrasing it from the angle of describing your key findings, which is more appropriate for a scientific paper.
Furthermore, based on Figures 4-7, none of the simulations can simulate both derecho and front stages of the observed system, then why the simulations fail like this should be the first priority of the study.