A modified atmospheric non-hydrostatic model on low aspect ratio grids

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tellus A 64 (2012): 17516, doi:10.3402/tellusa.v64i0.17516.It is popular to use a horizontal explicit and a vertical implicit (HE-VI) scheme in the compressible nonhydrostatic (NH) model. However, when the aspect ratio becomes small, a small time-interval is required in HE-VI, because the Courant-Fredrich-Lewy (CFL) criterion is determined by the horizontal grid spacing. Furthermore, simulations from HE-VI can depart from the forward–backward (FB) scheme in NH even when the time interval is less than the CFL criterion allowed. Hence, a modified non-hydrostatic (MNH) model is proposed, in which the left-hand side of the continuity equation is multiplied by a parameter d (45d516, in this study). When the linearized MNH is solved by FB (can be other schemes), the eigenvalue shows that MNH can suppress the frequency of acoustic waves very effectively but does not have a significant impact on the gravity waves. Hence, MNH enables to use a longer time step than that allowed in the original NH. When the aspect ratio is small, MNH solved by FB can be more accurate and efficient than the NH solved by HE-VI. Therefore, MNH can be very useful to study cloud, Large Eddy Simulation (LES), turbulence, flow over complex terrains, etc., which require fine resolution in both horizontal and vertical directions

Time-Lagged Ensemble Quantitative Precipitation Forecasts for Three Landfalling Typhoons in the Philippines Using the CReSS Model, Part II: Verification Using Global Precipitation Measurement Retrievals

In this study, high-resolution quantitative precipitation forecasts (QPFs) in lagged runs with a cloud-resolving model are evaluated for three typhoons in the Philippines: Mangkhut (2018), Koppu (2015), and Melor (2015), hitting northern Luzon, central Luzon, and the middle section of the Philippine archipelago, respectively. In Part I of this study, the QPFs were verified using 56 gauge observations on land over the Philippines. Here, in Part II, they are verified against the Global Precipitation Measurement (GPM) satellite estimates (also covering nearby oceans), using categorical scores in the same way. For each typhoon, rainfall valid at a selected 24 h period and the whole event (48 or 72 h) is examined. For 24 h rainfall inside the short range (lead time ≤ 72 h), good QPFs (with a threat score of ≥0.2) were produced for Koppu at 200 mm by almost all runs, and at 100 mm by all runs for Mangkhut, but only 22% of the runs for Melor. At longer lead times, good QPFs at 100 mm were also produced by all runs for Koppu, half of the runs for Mangkhut, and only 1 out of 16 runs for Melor. For whole events (48 or 72 h), the QPFs were similarly the best for Koppu, followed by Mangkhut, and least ideal for Melor. The quality of the GPM data during the three typhoons was found to be generally good and suitable for QPF verification, and the results were more stable and, thus, more reliable for the assessment of bias. However, the threat scores using the GPM dropped lower at high thresholds, and the results could become different from those obtained against the gauges (Part I), suggesting a much higher skill. Thus, verification using rain gauges is still needed toward high thresholds, especially over mountain regions where satellite estimates tend to exhibit larger errors

Time-Lagged Ensemble Quantitative Precipitation Forecasts for Three Landfalling Typhoons in the Philippines Using the CReSS Model, Part II: Verification Using Global Precipitation Measurement Retrievals

In this study, high-resolution quantitative precipitation forecasts (QPFs) in lagged runs with a cloud-resolving model are evaluated for three typhoons in the Philippines: Mangkhut (2018), Koppu (2015), and Melor (2015), hitting northern Luzon, central Luzon, and the middle section of the Philippine archipelago, respectively. In Part I of this study, the QPFs were verified using 56 gauge observations on land over the Philippines. Here, in Part II, they are verified against the Global Precipitation Measurement (GPM) satellite estimates (also covering nearby oceans), using categorical scores in the same way. For each typhoon, rainfall valid at a selected 24 h period and the whole event (48 or 72 h) is examined. For 24 h rainfall inside the short range (lead time ≤ 72 h), good QPFs (with a threat score of ≥0.2) were produced for Koppu at 200 mm by almost all runs, and at 100 mm by all runs for Mangkhut, but only 22% of the runs for Melor. At longer lead times, good QPFs at 100 mm were also produced by all runs for Koppu, half of the runs for Mangkhut, and only 1 out of 16 runs for Melor. For whole events (48 or 72 h), the QPFs were similarly the best for Koppu, followed by Mangkhut, and least ideal for Melor. The quality of the GPM data during the three typhoons was found to be generally good and suitable for QPF verification, and the results were more stable and, thus, more reliable for the assessment of bias. However, the threat scores using the GPM dropped lower at high thresholds, and the results could become different from those obtained against the gauges (Part I), suggesting a much higher skill. Thus, verification using rain gauges is still needed toward high thresholds, especially over mountain regions where satellite estimates tend to exhibit larger errors

The formation mechanism of the Bonin high in August

The Bonin high is a subtropical anticyclone that is predominant near Japan in the summer. This anticyclone is associated with an equivalent-barotropic structure, often extending throughout the entire troposphere. Although the equivalent-barotropic structure of the Bonin high has been known for years among synopticians because of its importance to the summer climate in east Asia, there are few dynamical explanations for such a structure. The present paper attempts to provide a formation mechanism for the deep ridge near Japan. We propose a new hypothesis that this equivalent-barotropic ridge near Japan is formed as a result of the propagation of stationary Rossby waves along the Asian jet in the upper troposphere (‘the Silk Road pattern’). First, the monthly mean climatology is examined in order to demonstrate this hypothesis. It is shown that the enhanced Asian jet in August is favourable for the propagation of stationary Rossby waves and that the regions of descent over the eastern Mediterranean Sea and the Aral Sea act as two major wave sources. Second, a primitive-equation model is used to simulate the climatology of August. The model successfully simulates the Bonin high with an equivalent-barotropic structure. The upper-tropospheric ridge is found to be enhanced by a height anomaly of more than 80 m at 200 hPa, when a wave packet arrives. Sensitivity experiments are conducted to show that the removal of the diabatic cooling over the Asian jet suppresses the Silk Road pattern and formation of an equivalent-barotropic ridge near Japan, while the removal of the diabatic heating in the western Pacific does not. Copyright © 2003 Royal Meteorological Societ