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

Abstract

The overarching goal of the Plains Elevated Convection At Night (PECAN) field campaign was to improve understanding of the processes contributing to the nocturnal precipitation maximum in the U.S. Great Plains. This study presents the precipitation pattern surrounding PECAN and addresses the origin, timing, duration, and potential causes contributing to that pattern. It is shown that the precipitation occurs most frequently at night, as expected. The maximum in the precipitation pattern occurred in the northeastern portion of the PECAN radar domain. The source of the rainfall was attributed to mountain-initiated precipitation, plains-initiated precipitation, precipitation advecting over the border of the radar domain, and episodes in which different initiation categories merged together. Through the combination of mountain-initiated, border, and merged episodes, 70% of the Great Plains precipitation was caused by episodes that formed outside of the PECAN domain and propagated into the region. The remaining 30% of the precipitation was attributed to plains-initiated storms. The mountain-initiated storms formed primarily in the afternoon and typically dissipated near the mountains. For those that survived, they propagated eastward, grew upscale, and contributed 27% of the precipitation in the plains. The plains-initiated precipitation fell mostly during the afternoon but also contributed to overnight rainfall and those locally triggered systems tended to be relatively smaller and shorter lived. For the top 10% rain-producing events, composite reanalysis fields showed that synoptic-scale features influenced the precipitation pattern and timing: an approaching trough established southwesterly moist flow throughout the region and a nocturnal low-level jet transported moisture to its terminus in the northeast corner of the PECAN domain.

The Role of Continental Mesoscale Convective Systems in Forecast Busts within Global Weather Prediction Systems

“…The long lasting MCSs in our study with enhanced southerly flow associated with the trough imply that if the evolution and longevity of the MCS in the model are not well represented, errors may arise in the model treatment of Rossby wave dynamics. The importance of a linkage between MCS and upper-level Rossby wave dynamics is consistent with the recent finding of Weckwerth and Romatchke (2019) [67] that found that the top 10% of rain-producing events during the summer 2015 period that included PECAN were linked to an approaching trough. While our study suggests a coupling of the outflow and the Rossby wave, further investigations are underway to determine the nature of the error in the model (e.g., errors in the location and timing of convective initiation, gravity waves generated by MCSs, inability of the model to maintain nocturnal convection, or inaccuracies in the vertical profile of heating and divergence).…”

Systematic Evaluation of the Impact of Assimilating a Network of Ground-Based Remote Sensing Profilers for Forecasts of Nocturnal Convection Initiation during PECAN

“…b. Case selection Weckwerth et al (2019) describe the variety of physical processes associated with nocturnal CI events in the Great Plains of the United States including frontal overrunning, events forming near a parent MCS, bores or density currents, or pristine CI events. An overview of each category is described here, and example cases simulated here are shown in Fig.…”

“…In the Great Plains, nocturnal convection initiation (CI) most commonly occurs between 0400 and 0900 UTC (Reif and Bluestein 2017), while conventional rawinsondes are typically only launched at 0000 and 1200 UTC. Moreover, most nocturnal CI events are driven by elevated ascent mechanisms such as the LLJ or undular bores (Wilson and Roberts 2006;Weckwerth et al 2019). Such features cannot be easily observed by conventional surface data, and various studies note the necessity of lowertroposphere profiling observations to improve this data gap (e.g., Wilson and Roberts 2006;Weckwerth and Parsons 2006;Keclik et al 2017).…”

“…Moreover, most nocturnal CI events are driven by elevated ascent mechanisms such as the LLJ or undular bores (Wilson and Roberts 2006;Weckwerth et al 2019). Such features cannot be easily observed by conventional surface data, and various studies note the necessity of lowertroposphere profiling observations to improve this data gap (e.g., Wilson and Roberts 2006;Weckwerth and Parsons 2006;Keclik et al 2017). Given these deficiencies, recent works show that assimilating high-frequency, remote sensing datasets can be particularly useful for improving forecasts of nocturnal convection.…”

“…While previous works show the benefits of assimilating ground-based remote sensing datasets for single cases (e.g., Chipilski et al 2020;D19;Hu et al 2019) or for single platforms (e.g., Coniglio et al 2019), many research areas remain unexplored. For example, nocturnal convection can be generated by a variety of ascent mechanisms at different scales (Reif and Bluestein 2017;Stelten and Gallus 2017;Weckwerth et al 2019) such that the impact of assimilating these instruments is likely case-and environment-dependent. While D19 show the benefits of assimilating the data for a large-scale frontal overrunning event, it remains unclear whether the impacts will be as large for smallerscale events such as those initiated by gravity waves or outflow boundaries.…”