To understand the basic mechanism governing the size evolution of tropical cyclones (TCs), we systematically perform numerical experiments using the primitive equation system on an f-plane. A simplified, TC-like vortex is initially given and an external forcing mimicking cumulus heating is applied to an annular region at a prescribed distance from the vortex center. Moist process and surface friction are excluded for simplification. We focus on the sensitivity of size evolution to the location of the forcing. The vortex size is defined as the radius of 15 m s-1 lowest-level wind speed (R15). The evolution of R15 depends on the forcing location, and its dependence can be understood by considering radial transport of the absolute angular momentum (AAM) at R15 due to the heat-induced secondary circulation (SC), whose structure is governed by the distribution of inertial stability. When the forcing is applied to the outer part of a vortex but still inside R15, where inertial stability is weak, the SC extends to the outside of R15 and carries AAM inward. Thus, R15 increases. Conversely, when the forcing is applied near the center of the vortex, where inertial stability is strong, the SC closes inside R15 and R15 hardly increases. These results indicate that extension of the heat-induced SC to the outside of R15 is important for the evolution of the vortex size. Moreover, the further beyond R15 the SC extends, the more the vortex size increases. This relationship is consistent with the result of the parcel trajectory analysis; the larger the extent of SC, the longer distances the parcels cover, conserving larger AAM. Finally, when the forcing is applied to the outside of R15, smaller AAM is carried outward by the SC on the inward side of the heating location, resulting in the decrease of R15.
Recent studies suggest that the horizontal propagation of gravity waves (GWs) is important in the spatial distribution of gravity wave forcing (GWF), especially during winter in the high latitudes of the Southern Hemisphere (SH). However, most standard gravity wave parameterizations (GWPs) treat GW propagation simply in the vertical. In this study, a new orographic GWP that includes three-dimensional (3D) GW propagation is developed and its impact on large-scale dynamical fields is examined. Our GWP calculates the horizontal locations and changes in the 3D wavenumbers of GWs explicitly through vertical integration of the ray tracing equations. The GWF due to wave refraction, which occurs in inhomogeneous background fields even without wave dissipation, is also calculated. In addition, the computational cost of parallelization is greatly reduced by adopting a Taylor’s series approximation for the horizontal gradient of the background fields needed for the ray tracing calculation. Two numerical experiments are performed using the Model for Interdisciplinary Research of Climate (MIROC)-AGCM: one uses the new orographic GWP and the other uses a conventional GWP. In the experiment with the new GWP, the westward GWF is enhanced in the SH winter mesosphere above the core of the polar night jet. This enhancement results from the significant latitudinal propagation of the parameterized GWs toward the jet axis. The zonal wind is slightly stronger in the SH winter polar upper stratosphere, which is consistent with the differences in GWF caused mainly by refraction. However, the strength and seasonal evolution of the polar night jet is less affected by the different GWPs. This result may be because of the compensation by Eliassen-Palm flux divergence due to the resolved waves. These results suggest that 3D propagation in GWPs is potentially important for better representation of the momentum budget of the middle atmosphere in climate models.
Indices of prediction skill over the Madden-Julian oscillation (MJO) phase space are examined with reanalysis and forecast data provided by the Japan Meteorological Agency (JMA). In addition to the bivariate root-mean-square error (RMSE) and the bivariate anomaly correlation coefficient (ACC), the mean-error vector is assessed. Conventionally, the RMSE and ACC have been used, although this approach misses information on the model bias for MJO events. Moreover, the ACC is not suitable for models in which the MJO signal tends to damp in some phases, because the ACC strongly depends on the MJO amplitude. The mean-error vector compensates for this drawback by associating a model’s erroneous mean tendency with the RMSE. For example, the JMA forecast produces a leftward mean error vector field uniformly distributed over the MJO phase space with its amplitude related to the RMSE. The RMSE should be then used with the mean error vector for evaluating the MJO prediction skill.
This study investigates the quality of the Japanese 55-year Reanalysis (JRA-55), which is the second global reanalysis constructed by the Japan Meteorological Agency (JMA), by comparing it with other reanalyses and observational datasets. Improvements were found in the representation of atmospheric circulation on an isentropic surface and in the consistency of momentum budget based on the mass-weighted isentropic zonal mean method. The representation of climate variability in several regions was also examined. In the tropics, the frequencies of high spatial correlations with precipitation, which were estimated using the Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis, are clearly higher in JRA-55 than in JRA-25. The results indicate that JRA-55 generally improved the representations of phenomena on a wide range of space-time scales, such as equatorial waves, and transient eddies in the storm track regions, compared with JRA-25 during the satellite era. Moreover, JRA-55 improved the temporal consistency compared with the older reanalyses throughout the reanalysis period. In the stratosphere, we found larger discrepancies between reanalyses for the extra-tropical stratosphere during the Southern Hemisphere (SH) winter. Comparisons with radiosonde temperature revealed that JRA-55 has a smaller bias in temperature than the other reanalyses in the extra-tropical SH winter before 1979. Some issues in JRA-55 were also identified. The amplitude of equatorial waves and Madden-Julian oscillation in JRA-55 are weaker than in the other reanalyses. JRA-55 shows unrealistic strong cooling in South America and Australia, although the spatial distribution of the long-term temperature trends in JRA-55 is the closest to an observational dataset of global historical surface temperature.
Four pairs of tropical cyclones (TCs) in the vicinity of the Northwest (NW) Pacific have been restudied in this paper by decomposing a total flow into a climatic component and an anomaly as well as by a simple generalized beta-advection model (GBAM). These results contradict with previous reports that binary interactions occurred in the four pairs of TCs. The intensities of the TC pairs were not well matched to each other during their common lifetime period; thus, the result of GBAM revealed that the strong TC had a direct influence on the track of the weak TC, but the reverse was not true. All dynamical models for studying the binary interaction of two adjacent TCs depend on how to separate the TC vortex and surrounding flow from the total flow. In this paper, the anomalous component can be directly used to measure the intensities and sizes of TCs and other adjacent disturbances. The track of TC as an anomalous vortex is influenced by the climatic steering flow and interacts with other anomalous systems in the vicinity. Whether a binary interaction happens between two TCs and whether the two TCs interact with other anomalous systems can be determined by the GBAM at the optimal level near the maximum center of the vorticity anomaly.