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1. Comparison of Biases in Warm-Season WRF Forecasts in North and South America

   https://doi.org/10.1175/WAF-D-20-0062.1  

   Piersante, Jeremiah O.; Schumacher, Russ. S.; Rasmussen, Kristen L. (May 2021, Weather and Forecasting)

   null (Ed.)

   Abstract Ensemble forecasts using the WRF Model at 20-km grid spacing with varying parameterizations are used to investigate and compare precipitation and atmospheric profile forecast biases in North and South America. By verifying a 19-member ensemble against NCEP Stage IV precipitation analyses, it is shown that the cumulus parameterization (CP), in addition to precipitation amount and season, had the largest influence on precipitation forecast skill in North America during 2016-2017. Verification of an ensemble subset against operational radiosondes in North and South America finds that forecasts in both continents feature a substantial mid-level dry bias, particularly at 700 hPa, during the warm season. Case-by-case analysis suggests that large mid-level error is associated with mesoscale convective systems (MCSs) east of the high terrain and westerly subsident flow from the Rocky and Andes Mountains in North and South America. However, error in South America is consistently greater than North America. This is likely attributed to the complex terrain and higher average altitude of the Andes relative to the Rockies, which allow for a deeper low-level jet and long-lasting MCSs, both of which 20-km simulations struggle to resolve. In the wake of data availability from the RELAMPAGO field campaign, the authors hope that this work motivates further comparison of large precipitating systems in North and South America, given their high impact in both continents. 

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2. A Synoptic Evolution Comparison of the Smallest and Largest MCSs in Subtropical South America between Spring and Summer

   Piersante, Jeremiah O.; Rasmussen, Kristen L.; Schumacher, Russ S.; Rowe, Angela K.; McMurdie, Lynn A. (June 2021, Monthly weather review)

   null (Ed.)

   Subtropical South America (SSA) east of the Andes Mountains is a global hotspot for mesoscale convective systems (MCSs). Wide convective cores (WCCs) are typically embedded within mature MCSs, contribute over 40% of SSA’s warm-season rainfall, and are often associated with severe weather. Prior analysis of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data identified WCCs in SSA and associated synoptic conditions during austral summer. As WCCs also occur during the austral spring, this study uses the 16-yr TRMM PR and ERA5 datasets to compare anomalies in environmental conditions between austral spring (SON) and summer (DJF) for the largest and smallest WCCs in SSA. During both seasons, large WCCs are associated with an anomalous midlevel trough that slowly crosses the Andes Mountains and a northerly South American low-level jet (SALLJ) over SSA, though the SON trough and SALLJ anomalies are stronger and located farther northeastward than in DJF. A synoptic pattern evolution resembling large WCC environments is illustrated through a multiday case during the RELAMPAGO field campaign (10–13 November 2018). Unique high-temporal-resolution soundings showed strong midlevel vertical wind shear associated with this event, induced by the juxtaposition of the northerly SALLJ and southerly near-surface flow. It is hypothesized that the Andes help create a quasi-stationary trough–ridge pattern such that favorable synoptic conditions for deep convection persist for multiple days. For the smallest WCCs, anomalously weaker synoptic-scale forcing was present compared to the largest events, especially for DJF, pointing to future work exploring MCS formation under weaker synoptic conditions. 

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3. A Synoptic Evolution Comparison of the Smallest and Largest MCSs in Subtropical South America between Spring and Summer

   https://doi.org/10.1175/MWR-D-20-0208.1  

   Piersante, Jeremiah O.; Rasmussen, Kristen L.; Schumacher, Russ S.; Rowe, Angela K.; McMurdie, Lynn A. (May 2021, Monthly Weather Review)

   null (Ed.)

   Abstract Subtropical South America (SSA) east of the Andes Mountains is a global hotspot for mesoscale convective systems (MCSs). Wide convective cores (WCCs) are typically embedded within mature MCSs, contribute over 40% of SSA’s warm-season rainfall, and are often associated with severe weather. Prior analysis of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data identified WCCs in SSA and associated synoptic conditions during austral summer. As WCCs also occur during the austral spring, this study uses the 16-year TRMM PR dataset and ERA5 reanalysis to compare anomalies in environmental conditions between austral spring (SON) and summer (DJF) for the largest and smallest WCCs in SSA. During both seasons, large WCCs are associated with an anomalous mid-level trough that slowly crosses the Andes Mountains and a northerly South American low-level jet (SALLJ) over SSA, though the SON trough and SALLJ anomalies are stronger and located farther northeastward than in DJF. A synoptic pattern evolution resembling large WCC environments is illustrated through a multi-day case during the RELAMPAGO field campaign (10-13 November 2018). Unique high-temporal resolution soundings showed strong mid-level vertical wind shear associated with this event, induced by the juxtaposition of the northerly SALLJ and southerly near-surface flow. It is hypothesized that the Andes help create a quasi-stationary trough/ridge pattern such that favorable synoptic conditions for deep convection persist for multiple days. For the smallest WCCs, anomalously weaker synoptic-scale forcing was present compared to the largest events, especially for DJF, pointing to future work exploring MCS formation under weaker synoptic conditions. 

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4. Impact of Assimilating GPS Precipitable Water Vapor on Simulations of Two North American Monsoon Convective Events Using Observing System Simulation Experiments

   https://doi.org/10.1029/2025JD044491  

   Shohan, Samkeyat; Koch, Steven E; Castro, Christopher L; Arellano, Avelino F; Kay, Junkyung; Risanto, Christoforus Bayu; Weckwerth, Tammy M; Pinto, James O; Adams, David K (August 2025, Journal of Geophysical Research: Atmospheres)

   Abstract This study evaluates the impact of assimilating precipitable water vapor (PWV) within an observing system simulation experiment (OSSE) framework to improve forecasts of monsoonal mesoscale convective systems (MCSs) in Arizona. Two contrasting case studies differing in convective forcing, longevity, intensity, and coverage are analyzed using a 40‐member ensemble of 1.8‐km resolution Weather Research and Forecasting (WRF) convective‐permitting model (CPM) simulations including the Data Assimilation Research Testbed (DART) system. Synthetic PWV data are derived from a nature run (NR) and bias corrected using real GPS‐derived PWV observations from a campaign during the North American monsoon (NAM) season 2021. These synthetic PWV are assimilated in an inferior model simulation called the control run (CR) to avoid the identical twin problem. Horizontal GPS station spacing experiments (e.g., superobbed, 50 km, 100 km, and 200 km) are conducted to identify configurations that maximize forecast skills. Assimilating the synthetic PWV reduces mean errors (∼2 mm) and dry bias during the first 4–6 hr of the predictions using analyses improved with PWV data assimilation. The 100‐km GPS network optimally captures convective precipitation patterns, outperforming coarser (200‐km) and finer (50‐km) grids due to an improved representation of moisture and winds afforded by PWV data assimilation at the appropriate scales. Topography strongly influences moisture distribution, with elevation‐dependent biases, overestimation in low elevations (0–500 m), underestimation in midelevations (500–2,000 m), and systematic high‐elevation (>2,000 m) biases due to vertically integrated PWV constraints. This study provides actionable insights for optimizing GPS network design and improving convective‐scale modeling in arid/semiarid regions. 

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5. Spatial bias in medium-range forecasts of heavy precipitation in the Sacramento River basin: Implications for water management

   https://doi.org/https://doi.org/10.1175/JHM-D-19-0226.1  

   Brodeur, Zachary; Steinschneider, Scott (June 2020, Journal of hydrometeorology)

   Forecasts of heavy precipitation delivered by atmospheric rivers (ARs) are becoming increasingly important for both flood control and water supply management in reservoirs across California. This study examines the hypothesis that medium-range forecasts of heavy precipitation at the basin scale exhibit recurrent spatial biases that are driven by mesoscale and synoptic-scale features of associated AR events. This hypothesis is tested for heavy precipitation events in the Sacramento River basin using 36 years of NCEP medium-range reforecasts from 1984 to 2019. For each event we cluster precipitation forecast error across western North America for lead times ranging from 1 to 15 days. Integrated vapor transport (IVT), 500-hPa geopotential heights, and landfall characteristics of ARs are composited across clusters and lead times to diagnose the causes of precipitation forecast biases. We investigate the temporal evolution of forecast error to characterize its persistence across lead times, and explore the accuracy of forecasted IVT anomalies across different domains of the North American west coast during heavy precipitation events in the Sacramento basin. Our results identify recurrent spatial patterns of precipitation forecast error consistent with errors of forecasted synoptic-scale features, especially at long (5–15 days) leads. Moreover, we find evidence that forecasts of AR landfalls well outside of the latitudinal bounds of the Sacramento basin precede heavy precipitation events within the basin. These results suggest the potential for using medium-range forecasts of large-scale climate features across the Pacific–North American sector, rather than just local forecasts of basin-scale precipitation, when designing forecast-informed reservoir operations. 

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