Out-of-Cloud Convective Turbulence: Estimation Method and Impacts of Model Resolution

Abstract

Convectively induced turbulence (CIT) poses both a serious threat to aviation operations and a challenge to forecasting applications. CIT generation and propagation processes occur on scales between 10 and 1000 m and therefore are best treated with high-resolution cloud-resolving models. However, high-resolution model simulations are computationally expensive, limiting their operational use. In this study, summertime convection in the North Dakota region is simulated over a 1-week period using a variety of model setups that are similar to those utilized in operational and research applications. Eddy dissipation rate and Ellrod index, both popular turbulence metrics, are evaluated across various model resolutions and compared with pilot reports from aircraft. The Ellrod index was found to be extremely sensitive to model resolution and overestimated turbulence intensity. The variability of turbulence values with respect to model resolution and distance away from convection is also examined. Turbulence probability was found to be the greatest when farther than 20 mi (32.2 km) away from convective cores. Model resolution was found to influence the intensity of predicted turbulence, and the model setup with the highest horizontal and vertical resolution predicted the highest turbulence values. However, the influence on turbulence intensity of vertical resolution and convective properties, such as storm depth, was found to be minimal for 3-km horizontal grid spacing.

Properties of Convectively Induced Turbulence over Developing Oceanic Convection

“…Additional indices that are commonly used for diagnosing turbulence including the Brown index (Brown 1973), Colson-Panofsky turbulent kinetic energy metric (Colson and Panofsky 1965), the Dutton index (Dutton 1980), the Ellrod index (T2; Ellrod and Knapp 1992), and the DTF3 index (Marroquin 1998) were also computed for this case day, but will not be discussed due to poor performance. More specifically, the Brown and Ellrod Index suffered resolution sensitivity as was found in Barber et al (2018).…”

“…has been shown to be adequate for turbulence prediction (Barber et al 2018). A 10-km damping layer is used at the model top, which is set to approximately 30 km (10 hPa).…”

“…In this study, turbulence intensity thresholds will be defined by Sharman and Pearson (2017) and Pearson and Sharman (2017), which are provided in Table 3. Barber et al (2018) found EDR to be an accurate indicator of turbulence in northern Great Plains convective simulations, but few tropical studies have included EDR as a turbulence metric. EDR is used as a turbulence diagnostic for this case day because it is a common metric used for aviation turbulence prediction (in situ and model derived), is included in GTG, and has been shown to perform well in the midlatitudes.…”

“…These studies have led to some advancement in our understanding of CIT but more research needs to be done to fully understand all physical processes that lead to convection related turbulence. Furthermore, the majority of these studies have been limited to mature convection in the midlatitudes (Lane et al 2003;Koch et al 2005;Lane and Sharman 2008;Trier and Sharman 2009;Lane et al 2009;Lane et al 2012;Trier et al 2012;Lane and Sharman 2014;Zovko-Rajak and Lane 2014;Trier and Sharman 2016;Barber et al 2018). Because the scientific research on turbulence associated with tropical convection (developing and mature) is sparse, aviation operations in tropical regions abide by thunderstorm avoidance policies designed from research of mature midlatitude convection.…”

Latitudinal and Topographical Variabilities of Free Atmospheric Turbulence From High‐Resolution Radiosonde Data Sets

“…In addition, specially equipped aircrafts can provide important observations (Cho et al, 2003;Cho & Lindborg, 2001;Frehlich & Sharman, 2010;Lindborg, 1999;Wroblewski et al, 2010) but are either confined to preplanned trajectories that generally do not provide good global coverage and altitude sampling. Pilot reports (PIREPs) provide global observations of turbulence but lack quantitative turbulence estimates (Barber et al, 2018;Wolff & Sharman, 2008).…”