A new method for the estimation of tropical cyclone (TC) intensity utilizing 10, 19, 21, 37 and 85 GHz channel TRMM Microwave Imager (TMI) data from 1999 to 2003 is developed. As a first step, we investigated the relationship between the TRMM/TMI brightness temperature (TB) parameters, which are computed in concentric circles, or annuli of different radius in different TMI frequencies, and the TC maximum wind speed from the TC best track data, and/or observed by microwave scatterometers (QuikSCAT and SeaWinds). In contrast to the previous studies, we found that the parameters with lower frequency channels of 10 or 19 GHz give higher correlation. This would be because that TBs of lower frequencies, that have less sensitivity to rain than those of higher frequencies, reflect the speed ofsea surface wind more directly in the TC case. The highest correlation coefficient obtained is 0.7, and the root mean square error (RMSE) of the regression between a parameter of highest correlation case is found to be 6 ms−1. We developed a TC intensity estimation method, based on the multiple regression equations using a few parameters. After choosing 3 parameters out of all possible combinations, we computed the regression coefficients and chose 10 regression equations, sorted by the lower RMSEs. Finally, we evaluated our estimation method using independent verification data during 2004. The RMSEs are found to be about 8 ms−1 in the entire basin for the best track data, and about 6 ms−1 for the best track data in the northwestern Pacific. Whereas, for the microwave scatterometer data in all basins RMSE is found to be about 7 ms−1. We also found that the temporal TC intensity change in our method shows good agreement with the TC best track data.
Data from the Automated Meteorological Data Acquisition System (AMeDAS), global positioning system-derived precipitable water vapor (GPS-PWV), conventional and Doppler radar observations, and results from a numerical simulation by the Japan Meteorological Agency Non-Hydrostatic Model (JMANHM) were used to investigate the evolution and structure of the convective systems that caused the Nerima heavy rainfall, which was a disastrous rainfall event in the Tokyo metropolitan area, with hourly precipitation amount reaching 110 mm. Two types of convective systems comprised the thunderstorms that caused the rainfall event: a thunderstorm developed in a warm and moist region where southerly inflow from Tokyo and Sagami bays and northeasterly flow over the northern Kanto Plain converged, and a convective band that organized to the west of the first system. The first convective system quickly decayed after the mature stage, because divergent flow from intense rainfall prevented the southerly inflow from reaching the updraft region. The middle-level airflow characterized by cold temperature invaded this system from the north after its mature stage. Because it entered the updraft region of the system, it did not enhance the convection through the intensification of the cold outflow that produces the convergence with the low-level inflow from the south. However, abundant water vapor in the region of convergence, resulted in heavy rainfall in spite of the short duration of the system. For the second convective system, water vapor of low-level southerly inflow directly fed into the band, and thus the band maintained its intensity after the first system decayed. Low-level northerly airflows that lifted up the southerly inflow were far more intense than that of the first system. These northerly airflows acted to organize the convective band and forced it southeastward. The middle-level cold airflow that also invaded the system after the mature stage entered the downdraft region, resulting in an enhancement of the convection. However, due to rapid propagation speed of the second band, the rainfall duration at a fixed point was relatively short, so that the band did not produce floods. For prediction of thunderstorms, monitoring of low-level convergence zones of moist air was found to be possible, using indexes of accumulation and convergence of water vapor, as well as the Doppler radar radial wind in the non-precipitation weak echoes.
Idealized cloud-resolving simulations with the horizontal resolution of 2 km are carried out to investigate effects of ice-phase processes on the development and structures of tropical cyclones (TCs). A comparison between cold-rain and warm-rain simulations shows that ice-phase processes delay the TC organization, and decrease the area-averaged kinetic energy. The ice-phase processes also shrink the TC size; for example, the radius of the storm-force wind area (over 25 m s−1) in the cold-rain simulation is two-thirds of that in the warm-rain one. The TC evolution depends greatly on strong cooling due to melting and sublimation of snow and graupel around the melting layer outside of the TC eyewall. The cooling reduces the pressure gradient below the melting level, and weakens the inflow toward the TC center. It suppresses the inward transport of high absolute angular momentum (AAM), and decreases the energy conversion rate from the available potential energy to kinetic energy of axisymmetric flows. As a result, the reduction of AAM around the eyewall shrinks the TC eyewall size, and the reduction of energy conversion rate delays the TC organization. The influence of the terminal fall velocities ofsnow and graupel is also examined by performing sensitivity experiments, with the horizontal resolution of 5 km. The results show that the increased terminal fall velocity lowers the actual melting altitude, and enhances the TC development through the change in the vertical profile of diabatic heating.
A millimeter-wave radiometer was installed at Rikubetsu, Japan (43.5°N, 143.8°E) in March 1999, to monitor the vertical distribution of ozone and temporal ozone variations in the stratosphere. Since November 1999, we have been monitoring venical profiles of the ozone mixing ratio in the altitude range from 22 to 60 km, with measurements at 2-km altitude intervals. The systematic error was estimated to be 10%-15% positive above 28 km, and the total random error to be 5%-21%. Comparisons of the Rikubetsu radiometer data with ozone sondes at Sapporo, and HALOE show that the ozone-mixing-ratio data, between 22 and 32 km, agree within the calculated 10% systematic error. On February 17, 2001, temporary ozone decreases of 37% and 15% at 22 and 30 km were found, respectively, when the potential vorticity at 550 K and 800 K increased. Suggesting that the ozone decrease may have been due to the arrival of a different airmass. During this event, the temporal variation of the ozone mixing ratio was clearly detected with small scatter, indicating that the Rikubetsu radiometer measurements have suflicient precision and time resolution to detect such short-term variations of stratospheric ozone.
We statistically analyzed both the reproducibility of the present climate, and future climate projections in the Asian monsoon region, using two Regional Climate Models (RCMs), nested into the MRI-CGCM2.2 to assess regional climate projections associated with global warming. Both GCM-RCM systems reproduced the present regional surface air temperature well. Also, they indicated about the same temperature increases as that of GCM for all regions over the Asian continent. The reproducibility of the present-climate precipitation amounts, in the lower-latitude regions was not as good as that of the surface air temperature, although it was better simulated in the higher-latitude regions. The future precipitation increase was not statistically significant. It was also statistically revealed that precipitation in future projections, with GCM-RCM systems, tended to converge in regions where the model biases were small. This result suggests the importance of an accurate reproduction of the present regional climate using physically based dynamical models, in order to analyze regional climate changes.
The term “convectively-coupled Kelvin waves” is misleading. That is because these waves have Rossby-wave components, comparable to their Kelvin-wave components and, in fact, bear a close resemblance to the Gill solution with a moving heating source. A better alternative would be to call these waves chimeric Kelvin waves, to signify their combined nature and the fact that they are implicitly convectively-coupled. By extension, chimeric Rossby waves and chimeric mixed Rossby-gravity waves would be better alternatives to “convectively-coupled Rossby waves” and “convectively-coupled mixed Rossby-gravity waves,” respectively. Collectively, these waves can be called chimeric equatorial waves. Recognizing the above misleading terms can help avoid confusion.