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1. Causes of Differences in Depiction of Convective Lines in 1-km and 3-km CM1 Simulations

   https://doi.org/10.1175/MWR-D-23-0240.1  

   Luthi, Samuel T; Gallus, William A (January 2025, Monthly Weather Review)

   Abstract Previous research has shown that 3-km horizontal grid spacing simulations depicting clusters of cells often change to showing linear structures when grid spacing is refined to 1 km. This increase in linear structures at finer horizontal grid spacings may be due simply to the resolving of stronger vertical motion along the leading edge of the MCS cold pool resulting in more continuous zones of convection in higher-resolution runs. However, prior work has suggested that the cold pools themselves are stronger with finer grid spacing, enhancing lift to grow linear morphologies faster. In the present study, Cloud Model 1 was used to simulate an array of MCSs with varying wind profiles and a constant thermodynamic profile (Weisman–Klemp analytic sounding) at both 1- and 3-km horizontal grid spacings and with 50 and 100 vertical levels. A line of seven randomly spaced warm bubbles was used to initiate convection. In 1-km Δxsimulations, gravity waves dominated in initiating new convection for growth into lines, and the ascent associated with them was much greater than in 3-km runs. Upscale growth into lines in 3-km Δxsimulations was driven more by ascent caused by the collision of convective cold pools. 

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2. Intensity Change of Binary Tropical Cyclones (TCs) in Idealized Numerical Simulations: Two Initially Identical Mature TCs

   https://doi.org/10.1175/JAS-D-20-0116.1  

   Liu, Hao-Yan; Wang, Yuqing; Gu, Jian-Feng (January 2021, Journal of the Atmospheric Sciences)

   null (Ed.)

   Abstract This study investigates the intensity change of binary tropical cyclones (TCs) in idealized cloud-resolving simulations. Four simulations of binary interaction between two initially identical mature TCs of about 70 ms −1 with initial separation distance varying from 480 to 840 km are conducted in a quiescent f -plane environment. Results show that two identical TCs finally merge if their initial separation distance is within 600 km. The binary TCs presents two weakening stages (stages 1 and 3) with a quasi-steady evolution (stage 2) in between. Such intensity change of one TC is correlated with the upper-layer vertical wind shear (VWS) associated with the upper-level anticyclone (ULA) of the other TC. The potential temperature budget shows that eddy radial advection of potential temperature induced by large upper-layer VWS contributes to the weakening of the upper-level warm core and thereby the weakening of binary TCs in stage 1. In stage 2, the upper-layer VWS first weakens and then re-strengthens with relatively weak magnitude, leading to a quasi-steady intensity evolution. In stage 3, due to the increasing upper-layer VWS, the non-merging binary TCs weaken again until their separation distance exceeds the local Rossby radius of deformation of the ULA (about 1600 km), which can serve as a dynamical critical distance within which direct interaction can occur between two TCs. In the merging cases, the binary TCs weaken prior to merging because highly asymmetric structure develops as a result of strong horizontal deformation of the inner core. However, the merged system intensifies shortly after merging. 

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3. An Observational Study of the Symmetric Boundary Layer Structure and Tropical Cyclone Intensity

   https://doi.org/10.3390/atmos11020158  

   Ren, Yifang; Zhang, Jun A.; Vigh, Jonathan L.; Zhu, Ping; Liu, Hailong; Wang, Xiang; Wadler, Joshua B. (February 2020, Atmosphere)

   This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity. 

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4. On the Changes in Convection-Allowing WRF Forecasts of MCS Evolution due to Decreases in Model Horizontal and Vertical Grid Spacing. Part I: Changes in Cold Pool Evolution

   https://doi.org/10.1175/WAF-D-22-0041.1  

   Squitieri, Brian J.; Gallus, William A. (October 2022, Weather and Forecasting)

   Abstract The degree of improvement in convective representation in NWP with horizontal grid spacings finer than 3 km remains debatable. While some research suggests subkilometer horizontal grid spacing is needed to resolve details of convective structures, other studies have shown that decreasing grid spacing from 3–4 to 1–2 km offers little additional value for forecasts of deep convection. In addition, few studies exist to show how changes in vertical grid spacing impact thunderstorm forecasts, especially when horizontal grid spacing is simultaneously decreased. The present research investigates how warm-season central U.S. simulated MCS cold pools for 11 observed cases are impacted by decreasing horizontal grid spacing from 3 to 1 km, while increasing the vertical levels from 50 to 100 in WRF runs. The 3-km runs with 100 levels produced the deepest and most negatively buoyant cold pools compared to all other grid spacings since updrafts were more poorly resolved, resulting in a higher flux of rearward-advected frozen hydrometeors, whose melting processes were augmented by the finer vertical grid spacing, which better resolved the melting layer. However, the more predominant signal among all 11 cases was for more expansive cold pools in 1-km runs, where the stronger and more abundant updrafts focused along the MCS leading line supported a larger volume of concentrated rearward hydrometeor advection and resultant latent cooling at lower levels. 

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5. Challenges in Numerical Weather Prediction of the 10 August 2020 Midwestern Derecho: Examples from the FV3-LAM

   https://doi.org/10.1175/WAF-D-23-0019.1  

   Gallus, William A.; Harrold, Michell A. (June 2023, Weather and Forecasting)

   Abstract A severe derecho impacted the Midwestern United States on 10 August 2020, causing over 12 billion dollars in damage, and producing peak winds estimated at 63 m s −1 , with the worst impacts in Iowa. The event was not forecast well by operational forecasters, nor even by operational and quasi-operational convection-allowing models. In the present study, nine simulations are performed using the Limited Area Model version of the Finite-Volume-Cubed-Sphere model (FV3-LAM) with three horizontal grid spacings and two physics suites. In addition, when a prototype of the Rapid Refresh Forecast System (RRFS) physics is used, sensitivity tests are performed to examine the impact of using the Grell-Freitas (GF) convective scheme. Several unusual results are obtained. With both the RRFS (not using GF) and Global Forecast System (GFS) physics suites, simulations using relatively coarse 13 and 25 km horizontal grid spacing do a much better job of showing an organized convective system in Iowa during the daylight hours of 10 August than the 3-km grid spacing runs. In addition, the RRFS run with 25-km grid spacing becomes much worse when the GF convective scheme is used. The 3-km RRFS run that does not use the GF scheme develops spurious nocturnal convection the night before the derecho, removing instability and preventing the derecho from being simulated at all. When GF is used, the spurious storms are removed and an excellent forecast is obtained with an intense bowing echo, exceptionally strong cold pool, and roughly 50 m s −1 surface wind gusts. 

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