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

Numerical Experiments of Mesoscale Cyclone Formed off the West Coast of Hokkaido

An intensive observation of snow storms was performed in January and February 1992 (Kikuchi, 1993) in the Ishikari District of Hokkaido, Japan. During the intensive observing period, a mesoscale cyclone was observed off the west coast of Hokkaido during the period from 30 January to 1 February 1992. We made numerical experi-ments of the mesoscale cyclone using JSM. The prediction experiment simulated development of the mesoscale cyclone. When the mesoscale cyclone generated, the lower troposphere was cold over Hokkaido and warm over the Sea of Japan. The cold air flowed southeastward and made a convergence with the northwesterly monsoon wind. The mesoscale cyclone was formed along the convergence zone. The me~u~cale cyclone was a shallow disturbance which was confined to below 850 hPa. When the sensible heat flux from the surface is switched off, the mesoscale cyclone was not simulated. The experiment in which both latent and sensible heat fluxes were switched off gave the similar result. In these experiments, the horizontal temperature gradient between the land and sea was significantly weakened and the convergence zone was not formed. The static stability in the lower troposphere became more stable. As a result, the mesoscale cyclone was not formed. Even though the moist processes were switched off, the mesoscale cyclone was simulated. This indicates that the moist processes are not essential for the develop-ment of the mesoscale cyclone. We could infer that a hydrodynamic instability of the dry air is essential for the mesoscale cyclogenesis. 1

Intensity Change of Typhoon Nancy (1961) during Landfall in a Moist Environment over Japan: A Numerical Simulation with Spectral Nudging

Intensity change of tropical cyclones (TCs) as they make landfall is closely linked to sustained periods of high surface winds and heavy precipitation. Few studies have investigated the intensity change of intense TCs that make landfall in middle latitudes such as Japan because few intense typhoons make landfall in middle latitudes. In this study, a numerical simulation of intense Typhoon Nancy (1961) was used to understand the intensity change that occurred when Nancy made landfall in Japan. A spectral nudging technique was introduced to reduce track errors in the simulation. During landfall, the simulated storm exhibited the salient asymmetric structure and rapid eyewall contraction. A tangential wind budget indicated that the maximum wind speed decreased concurrent with an increase in surface friction and advection associated with low-level asymmetric flows. Detailed evolution of the storm's warm core was analyzed with a potential temperature budget. In the upper part of the warm core centered at a 12-km height, cooling due to ventilation by asymmetric flows and longwave radiation overcame heating due to condensation and shortwave radiation during the contraction of eyewall clouds. In the lower part of the warm core, adiabatic cooling more than offset warm-air intrusions associated with asymmetric flows and condensational heating. The condensation was supplied by an abundance of moisture due to evaporation from the ocean in the well-developed typhoon based on a moisture budget. Sensitivity experiments revealed that environmental baroclinicity in the midlatitudes, orography, and radiative processes made minor contributions to the weakening. The weakening was instead controlled by inner-core dynamics and interactions with land surfaces