Ensemble predictability of an isolated mountain thunderstorm in a high‐resolution model

Abstract

Convective-scale ensemble simulations with perturbed initial conditions are performed to investigate the physical mechanisms, sensitivities and predictability of convective precipitation for a well-observed event from the Convective and Orographically induced Precipitation Study (COPS). On this day an isolated thunderstorm developed over the Black Forest mountains in southwest Germany that was initiated by thermally driven low-level convergence. With the default ensemble configuration, none of the members simulated any deep convection. A soil moisture and atmospheric moisture deficiency was found to be responsible for a lack of boundary layer moisture in the analysis. Correction of this moisture bias yielded an ensemble in which some members produced a realistic convective storm. In contrast, other members only produced shallow convection. A robust feature of the members that simulated the storm was that an impinging upper-level disturbance to the west had progressed further east towards the COPS region. This led to increased westerly winds aloft, which when mixed down into the boundary layer acted to feed a deeper layer of moist valley air into the convergent mountain circulation. The simulations reveal a very strong sensitivity to uncertainties in the large-scale flow as well as the boundary layer structure and stability.

Factors Influencing the Development and Maintenance of Nocturnal Heavy-Rain-Producing Convective Systems in a Storm-Scale Ensemble

“…Martin and Xue (2006), Ancell and Hakim (2007), Hawblitzel et al (2007), Sippel andZhang (2008, 2010), and Torn (2010) all used these or similar methods to examine the dynamics and sensitivities of high-impact weather systems. Constructing subsets of members that have contrasting properties and analyzing the differences between those subsets can also reveal important information about the weather systems of interest; this method was used by Reinecke and Durran (2009), Schumacher (2011), and Hanley et al (2011. Herein, we will use a combination of manual and statistical diagnosis, along the same lines as the analysis of Clark et al (2010b), to investigate the processes that were most favorable for, or detrimental to, the development of extreme precipitation in association with an MCV on 9-11 June 2010.…”

Separation of upslope flow over a uniform slope

“…In particular, heat carried by the separated flow as well as small-scale features associated with flow separation are crucial for predicting deep convection, precipitation, moisture distribution and unsteady phenomena surrounding orographic features (Banta 1984;Hanley et al 2011). Excessive precipitation over steep and high mountains in mesoscale as well as climate models is a well-known problem, which has been attributed to the difficulties of parameterizing flow separation over sloping boundaries, especially the sub-grid heat ventilation by separated upslope flows (Chao 2012).…”

A Spatial View of Ensemble Spread in Convection Permitting Ensembles

“…Ensemble prediction systems strive to represent the meteorological uncertainty present in a particular forecast and have been widely used to assess error growth in a variety of high-resolution situations (Walser et al 2004;Walser and Schär 2004;Hohenegger and Schär 2007b;Hanley et al 2011Hanley et al , 2013. Further investigations have been conducted into different ensemble perturbation strategies for high-resolution ensembles including initial condition perturbations (Migliorini et al 2011;Caron 2013;Kühnlein et al 2014), physics perturbations (Stensrud et al 2000;Hacker et al 2011;Gebhardt et al 2011;Vié et al 2012;Baker et al 2014), perturbation of boundary layer parameters (Martin and Xue 2006;Leoncini et al 2010;Done et al 2012), and the use of different physics schemes (Berner et al 2011;Leoncini et al 2013).…”