WRF Forecasts of Great Plains Nocturnal Low-Level Jet-Driven MCSs. Part I: Correlation between Low-Level Jet Forecast Accuracy and MCS Precipitation Forecast Skill

Abstract

The Great Plains low-level jet (LLJ) fosters an environment that supports nocturnal mesoscale convective systems (MCSs) across the central United States during the summer months. The current study examines if LLJ forecast accuracy correlates with MCS precipitation forecast skill in 4-km WRF runs. LLJs were classified based on their synoptic background as either strongly forced, cyclonic flow (type C) or weakly forced, anticyclonic flow inertial oscillation driven (type A). Large-scale variables associated with the LLJ were examined. For all LLJs inclusive and the subset of type C LLJs alone, the forecast accuracy of the LLJ total wind direction significantly correlated with MCS precipitation forecast skill. For type C LLJ cases, where predictive skill for MCSs was higher overall, the LLJ ageostrophic wind direction forecast accuracy significantly correlated with MCS precipitation forecast skill during the LLJ and MCS developmental stages, with potential temperature and moisture forecast accuracy correlating well with the forecast skill of mature MCSs. Statistically significant correlations were mainly absent between MCS precipitation forecast skill and LLJ forecast accuracy for type A cases. It is thus suggested that either non-LLJ factors like most unstable convective available potential energy (MUCAPE) or most unstable convective inhibition (MUCIN) fields within close proximity of MCSs, or factors on smaller scales than analyzed (such as gravity waves or bores), may have the greatest potential influence on MCS precipitation forecast skill in LLJ-induced MCS cases in an ambient weakly forced synoptic regime.

Impact of Synoptic‐Scale Factors on Rainfall Forecast in Different Stages of a Persistent Heavy Rainfall Event in South China

“…Low‐level jet can not only transport moisture but also determine the spatial distribution of surface rainfall. Previous studies have shown that a low‐level jet is strongly correlated with heavy rainfall and associated MCSs (e.g., Chen et al, , ; Nicolini et al, ; Squitieri & Gallus, , ). As mentioned in section 3, during this persistent heavy rainfall, there was a strong southwesterly low‐level jet along the southeast flank of the low‐level vortex.…”

A Case Study of Low‐Level Jets in Yerevan Simulated by the WRF Model

“…It is very interesting and relevant issue to examine the impact of urban effect on dynamical characteristics of LLJs over Yerevan, as well as, the opposite issue, i.e., study of LLJs impact on the nocturnal urban heat island intensity. Squitieri and Gallus () and Dezfuli et al () showed that LLJs may play significant role in initiation and sustaining of nocturnal convection over Great Plains and regional‐scale transport of moisture and formation of precipitation over the Middle East. The setup and configuration of the WRF model may be further improved in future studies to improve the simulation of LLJs over Yerevan.…”

“…It is worth noting that the WRF model is the most popular one used in more recent studies (Giannakopoulou & Toumi, ; Schepanski et al, ; Squitieri & Gallus, ). In particular, various WRF configurations have been tested in order to understand the capabilities and limitations of mesoscale numerical weather prediction models (NWPs) to simulate general characteristics of LLJs through sensitivity studies.…”

Where, When, and Why Did It Rain during PECAN?

“…Some of the complex and interacting factors affecting nocturnal CI and MCS maintenance include a surface-based stable layer (e.g., Carbone et al 2002;Billings and Parker 2012), the LLJ (e.g., Trier et al 2006;Tuttle and Davis 2006;French and Parker 2010;Shapiro et al 2016;Trier et al 2017;Gebauer et al 2018;Reif and Bluestein 2018), frontal boundaries (e.g., Maddox et al 1979;Trier and Parsons 1993;Horgan et al 2007;Reif and Bluestein 2017), gravity waves (e.g., Wilson et al 2018;Reif and Bluestein 2018), and nocturnal bores (e.g., Koch et al 2008a,b;Marsham et al 2011;Coleman and Knupp 2011;. In contrast to relatively high skill in warm season quantitative precipitation forecasts (QPFs) under strong synoptic-scale forcing (e.g., Jankov and Gallus 2004;Squitieri and Gallus 2016), the forecasting skill is relatively low for weakly synoptically forced systems in the central United States (e.g., Fritsch and Carbone 2004). This is partially due to the poor forecast skill of nocturnal CI (e.g., Davis et al 2003;Clark et al 2007;Surcel et al 2010;Pinto et al 2015;Stelten and Gallus 2017).…”