WRF Forecasts of Great Plains Nocturnal Low-Level Jet-Driven MCSs. Part II: Differences between Strongly and Weakly Forced Low-Level Jet Environments

Abstract

The classic Great Plains southerly low-level jet (LLJ) is a primary factor in sustaining nocturnal convection. This study compares convection-allowing WRF forecasts of LLJ events associated with MCSs in strongly and weakly forced synoptic environments. The depth of the LLJs and magnitude, altitude, and times of the LLJ peak wind were evaluated in observations and WRF forecasts for 31 cases as well as for case subsets of strongly and weakly forced synoptic regimes. LLJs in strongly forced regimes were stronger, deeper, and peaked at higher altitudes and at earlier times compared to weakly forced cases. Mean error MCS-centered composites of WRF forecasts versus RUC analyses were derived at MCS initiation time for the LLJ atmospheric water vapor mixing ratio, LLJ total wind magnitude, convergence, most unstable convective available potential energy (MUCAPE), and most unstable convective inhibition (MUCIN). In most configurations, simulated MCSs in strongly and weakly forced regimes initiated to the north and east of observations, generally in a region where LLJ moisture, MUCAPE, and MUCIN fields were forecast well, with larger errors outside this region. However, WSM6 simulations for strongly forced cases showed a southward displacement in MCS initiation, where a combination of ambient environmental factors and microphysics impacts may simultaneously play a role in the location of forecast MCS initiation. Strongly forced observed and simulated MCSs initiated west of the LLJ axis and moved eastward into the LLJ, while observed and simulated MCSs in weakly forced environments traversed the termini of the LLJ. A northward bias existed for simulated MCS initiation and LLJ termini for weakly forced regimes.

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.…”

Ensemble cloud-resolving modelling of a historic back-building mesoscale convective system over Liguria: the San Fruttuoso case of 1915

“…In a much more recent set of cases, Duda and Gallus (2013) found an average displacement distance (absolute error) of 105 km for initiation of systems. Squitieri and Gallus (2016) show that centroids of forecasted MCSs in their sample of 31 relatively recent events in the US Great Plains are usually over 100 km or more removed from the centroids of the observed MCSs. Similarly good performance of the ensemble exists for areal coverage, rainfall intensity (although there is a 30-40 % underestimate) and overall characteristics of the forecasted objects as implied by the interest value.…”

WRF-Simulated Low-Level Jets over Iowa: Characterization and Sensitivity Studies

“…The specific WRF configuration (e.g. selection of the planetary boundary laver (PBL) scheme) and horizontal and vertical resolution has a clear impact on simulated flow within the atmospheric boundary layer and LLJ properties, but in general WRF has been demonstrated to exhibit skill in simulating LLJ events and the near-surface wind climate (Storm et al, 2008;Vanderwende et al, 2015;Squitieri et al, 2016;Smith et al, 2018;Schepanski et al, 2015;Pryor et al, 2020a). 75…”