A 20-year integration by the nonhydrostatic icosahedral atmospheric model (NICAM) with a 14 km mesh was conducted for the first time to obtain a climatological mean and diurnal to interannual variability of a simulated atmosphere. Clouds were explicitly calculated using a cloud microphysics scheme without cumulus convection scheme. The simulation was performed under the atmospheric model intercomparison project-type conditions, except that sea surface temperature was nudged toward observed historical values using the slab ocean model. The results are analyzed with a focus on tropical disturbances, including tropical cyclones (TCs) and the Madden-Julian oscillation (MJO). NICAM simulates many aspects of atmospheric climatological mean state and variability. The geographical distributions of precipitation, including interannual, seasonal, and diurnal variations, are well reproduced. Zonal mean basic states, clouds, and top-of-atmosphere radiation are qualitatively simulated, though some severe biases such as underestimated low clouds, shortwave reflection, warmer surface, and tropical upper troposphere exist. TCs and MJO are the main focus of the simulation. In the simulation, TCs are detected with the objective thresholds of maximum wind speed due to the realistic intensity of simulated TCs. The seasonal march of TC genesis in each ocean basin is well simulated. The statistical property of the MJO and tropical waves is well reproduced in the space-time power spectra, consistent with previous NICAM studies. This implies that the stratospheric variability is also reproduced, as partially revealed in this study. Asian monsoon analysis shows that climatological western North Pacific monsoon onset occurs near the observed onset, and that the Baiu front is reproduced to some extent. Some significant model biases still exist, which indicates a need for further model improvements. The results indicate that a high-resolution global nonhydrostatic model has the potential to reveal multiscale phenomena in the climate system.
The interaction between Rossby and gravity waves is examined in a vertical-zonal two-dimensional model, in which the basic state has an upward gradient of buoyancy at a lower level and a downward gradient of horizontal vorticity at a higher level. Because of the gradients, there exist westward-propagating upper Rossby and westward- and eastward-propagating lower gravity waves, where the propagation is relative to the fluid. The initial value problem for the disturbance is analytically solved. The temporal evolution of the analytical solution from an initial value shows the following characteristics. Resonant interaction between the westward-propagating upper Rossby wave and the eastward-propagating lower gravity wave is possible, in the same way as between two counter-propagating Rossby waves in barotropic and baroclinic problems. On the boundary in the parameter space between the unstable region, where resonant exponential growth occurs, and the stable region, where the solution oscillates, the marginal solution grows as a linear function of time. As in other instability problems, the marginal linear growth in the present model is not trivial. For small horizontal wave numbers, the westward-propagating gravity wave makes a non-negligible contribution to the resonant interaction between the westward-propagating Rossby wave and the eastward-propagating gravity waves.
This study investigates the asymmetry of forecast errors in the Northern winter stratosphere between vortex weakening and strengthening conditions. Previous studies suggest that the stratospheric forecast errors for medium-range time scales of about two weeks are larger for weakening conditions of the polar vortex than for strengthening conditions even though they have anomalies of similar magnitudes. We explore the asymmetry by comparing the one-month hindcast data of the Japan Meteorological Agency to the Japanese 55-year Reanalysis data. We define the vortex weakening and strengthening conditions of anomalies of similar magnitudes using an empirical orthogonal function analysis of polar stratospheric temperatures. Results indicate that the larger forecast errors in the stratosphere for the vortex weakening conditions originate from those of the planetary wave forcing in the upper troposphere. In particular, it is more difficult to forecast planetary wave amplification that leads to the vortex weakening conditions than forecasting wave attenuation that leads to the vortex strengthening conditions. Examining forecast errors between major stratospheric sudden warming events (MSSWs) and vortex intensification events (VIs) defined by the zonal mean zonal wind in the high latitude stratosphere, we also show that it is more difficult to forecast the MSSWs than the VIs and further discuss the cause for the difference.
Atmospheric motion vectors (AMVs) derived from 5-min rapid scan (RS) imagery of the Multi-functional Transport Satellite are expected to capture small-scale distributions of airflows better than typical AMVs derived from 30-min imagery because the observation interval of RS-AMV is shorter. The impact of these high-frequency data on the numerical forecasting of a heavy rainfall near a stationary front was investigated by conducting data assimilation experiments. As a part of preparation for the assimilation, RS-AMVs were compared with the first-guess field obtained from the Japan Meteorological Agency (JMA) nonhydrostatic model (NHM). The comparison result indicated that the RS-AMVs were of good quality and could be used in the JMA’s operational NHM with 4D variational data assimilation (JNoVA). Assimilation experiments investigating a heavy rainfall event were conducted using different lengths of assimilation time slot and time intervals of spatial thinning for the assimilation of the RS-AMV data. The assimilation of RS-AMVs caused the initial wind fields to enhance the upper-level divergence and low-level convergence around the front. Consequently, the forecast of the rainfall amount was increased near the front, and the verification scores were slightly improved over the control experiment in the early forecast hours.
In this study, an analytical solution of nocturnal low level jets (LLJs) is presented. The present model is an extension of Blackadar, who described the nocturnal LLJ as a result of an inertial oscillation. In the present model, the momentum equation in the daytime atmospheric boundary layer includes a term representing convective mixing in addition to mixing with a constant diffusion coefficient. With the convective mixing, the daytime equilibrium wind velocity becomes vertically more uniform than the Ekman solution. In the nighttime atmospheric boundary layer, the convective mixing is assumed to be absent and the diffusion coefficient, which is assumed to be a constant, is smaller than that in the daytime. Without the convective mixing, the nighttime equilibrium wind velocity is the same as that of the Ekman solution. The analytical solution describes the temporal evolution of nighttime wind velocity as a damped inertial oscillation around the nighttime equilibrium wind velocity, starting from daytime equilibrium wind velocity. By appropriately selecting the values of parameters in the analytical solution, some previously published results are reproduced. For example, the height of maximum wind speed decreases as time goes on. There exist backward inertial oscillations in addition to the well-known forward inertial oscillations. In the lower levels, the oscillations are rapidly damped.
Although future reduction of the global frequency of tropical cyclones is known to be a robust response of dynamical numerical projections to global warming, its mechanism is not yet fully explained. We propose a diagnostic relation based on the convective mass flux to constrain the global frequency of tropical cyclones. Simulation results using a high-resolution global non-hydrostatic model showed that the reduction in the global frequency is much larger than that in the total tropical convective mass flux. Either a future increase in the frequency of stronger tropical cyclones or an areal increase in strong updrafts explains this difference. A reduction in the contribution of convective mass flux of tropical cyclones to total tropical convective mass flux also contributes to the difference. This study suggests that future intensification of tropical cyclones leads to future reduction in their frequency under the condition that the contribution of tropical cyclone remains the same or smaller.
Chen et al. (2014, J. Meteor. Soc. Japan, 92A, 157-165) estimated the global distribution of anthropogenic heat release (AHR) using satellite observed night lights, and showed a rapid increase of AHR from 2000 to 2009 in many regions, including Europe and North America. From model simulation based on this estimation, they showed a possibility of substantial influence of AHR on the climate over some regions of the world. However, existing data indicate that neither energy consumption nor night lights changed largely from 2000 to 2009. These facts raise serious doubts about the reliability of their results.