Environmental situations associated with temporary improvement or rapid changes in visibility in the presence of fog were analyzed to understand the mechanisms of these changes in visibility at Rokkasho Village, Aomori Prefecture, northeast Japan. This region is frequently affected by sea fogs. The profiles of liquid water content and drop size distribution in fogs were obtained by a direct photographic method using a tethered hot-air balloon. On the basis of the results, the temporary improvement in visibility on July 10, 2006 was attributed to convection-driven downward transport of air parcels with lower total water content than air parcels near the ground surface. The rapid changes in visibility at the same location on July 24 seemed to be related to mixing by Kelvin-Helmholtz waves. Mixing with an upper layer containing less water initially caused an improvement in visibility but subsequently, it led to the formation of a dense fog just above the ground surface in an area of upward flow. Settlement of fog drops from the dense fog above degraded the visibility in this area.
The intensities of the tropical cyclones (TCs) that approach or make landfall in Japan and the possible changes in these intensities due to global warming have been investigated on the basis of the theoretical model of Holland (1997). This theoretical model calculates the maximum potential intensity (MPI) of a TC as a lower limit of central sea level pressure for a given upper-air temperature profile and underlying sea surface temperature (SST). This study uses the climatology of the JRA-25 long-term reanalysis for the present environment, along with global warming anomalies derived from CMIP3 multi model climate experiments, which are added to the reanalysis climatology to form a warmed environment. A characteristic of the tropical atmosphere is that warming anomalies are amplified toward the upper troposphere, so that TC intensification due to a rise in SST is suppressed to some extent by greater warming aloft. This study evaluates the uncertainty ranges for the MPI changes by considering inter model differences in the warming structure. The MPI-SST relationship in the present climate is consistent with the historical minimum surface pressure in the western North Pacific when the eyewall relative humidity (RH), one of the model’s key parameters, is set at 88%. The average change in the MPI due to a 1°C rise in the SST is -6.7 hPa [-0.6 to -12.0 hPa], where a negative change indicates intensification and the estimated range in brackets reflects the uncertainty in the warming structure. The error of these MPI changes is estimated to be about 15%, assuming that the likely range of the RH parameter is 88 ± 2%. The fractional change in the surface pressure drop is 3.6%, 8.4%, and 19%, in response to a 0.5, 1.0, and 2.0C rise, respectively, in SST, which is basically comparable to existing studies based on observations and numerical simulations. While TC intensification due to global warming tends to be greater for intense TCs at low latitudes, the tendencies at relatively high latitudes near the main islands of Japan suggest that TC development occurs in a broader region over a more prolonged season in a warmed environment.
This paper describes the evolution of the convective boundary layer (CBL) and the initiation of clouds above Okinawa Island, Japan, during summer conditions with weak synoptic forcing and clear skies. An intensive observational campaign using a C-band precipitation radar (COBRA) and radio sonde was conducted on the island in early July 2005. These soundings exhibited diurnal variations in the potential temperature and water vapor in the atmospheric boundary layer. COBRA observed clear-air radar echoes (CAEs) within a 10-km radar range, and the CAE height showed a diurnal variation as well. To investigate the nature of the CAEs and to describe the dynamics of the CBL structure, a high-resolution cloud resolving model simulation was performed for two cases; a cumulus street and a distinct roll cloud. Flat and undulated surface models were used to understand the orographic effects on cloud initiation. On the basis of these investigations, we found strong consistency within the simulated radar reflectivity and observed CAE distributions. COBRA regularly observes turbulent moisture motions at cloud edges and occasionally detects moisture convergence in the CBL. The model CBL evolutions coincide well with the diurnal variations of CAE height, thereby showing that the island presents a good case for studying CBL evolution and that COBRA is useful to find the CBL depth here. The model results show that the warmed island induces horizontal convective rolls and a convergent zone above the island. A merging of the thermals, the rolls, and the sea-breeze reinforces the upward energy transfer, which generates a cumulus street and a distinct roll cloud being supported by orographic uplift. Wind direction characterizes a moist CBL formation and the cloud initiations above Okinawa Island.
The relationships between large-scale circulation and foehns observed during midsummer in Hokuriku district, located on the Japan Sea side of central Japan, are examined using Japanese long-term reanalysis project data, with additional data from the Japan Meteorological Agency climate data assimilation system. All foehn events are classified into two types: a tropical cyclone (TC)-induced foehn and an extratropical cyclone (EC)induced foehn. The occurrence of the TC type is attributed to a combination of a typhoon and its induced teleconnection pattern, the Pacific-Japan (PJ) pattern, in the lower troposphere. Local intensification of the North Pacific high just east of Japan, accompanied by the dominance of the PJ pattern, can in turn force a typhoon track to shift westward. The northward migration of the typhoon along the western periphery of the locally enhanced high strengthens a zonal pressure gradient across central Japan, thus producing a foehn. In contrast, an upper-level teleconnection along the Asian jet serves as a prominent trigger of the occurrence of an EC-type foehn. Stationary Rossby wave packets propagating eastward along the upper-level Asian waveguide facilitate not only the westward development of the North Pacific high but also the development of an extratropical cyclone in the vicinity of the Japan Sea by leading to the equatorward advection of higher potential vorticity from high latitudes. Both developments are crucial for the reinforcement of a northwest-southeast pressure gradient in the lower troposphere around Japan, thus providing a favorable condition for a foehn event.
This study investigates diurnal variations in lower-tropospheric wind over Japan during 2002-2008 using data from 31 stations of the Wind profiler Network and Data Acquisition System (WINDAS) and the Automated Meteorological Data Acquisition System (AMeDAS). The diurnal and semidiurnal components are extracted and analyzed to identify the dominant processes for each height range and for each season. Near the surface, the diurnal component is controlled by local wind systems (e.g., land-sea breezes) throughout the year. At 1-3 km, the diurnal component is primarily controlled by the return currents of local wind systems, with additional influence by other disturbances; the superposition of these two wind systems generates amplitude maxima in spring (∼0.5 m s-1) and autumn (∼0.6 m s-1). At 3-5 km, the diurnal wind in DJF-MAM is controlled by medium-scale eastward traveling waves, which generate the amplitude maximum (∼0.8 ms-1) in winter-spring. In JJA-SON, the diurnal component is controlled by a large-scale wind system with an amplitude of ∼0.3 m s-1, probably related to the diurnal tide. At stations located on small islands located south of the Japanese mainland, the diurnal wind within the lower troposphere has different characteristics from those described above throughout the year. Throughout Japan, the semidiurnal wind component is controlled by the semidiurnal migrating tide above ∼1 km, and is influenced by local wind systems below ∼1 km. The amplitude of the semidiurnal tide below 5 km is largest in DJF (∼0.4 ms-1) and smallest in JJA (∼0.2 ms-1).
Following on from the observation results obtained from Wind profiler Network and Data Acquisition System (WINDAS) data, as reported by Part I of this study, the dynamical processes responsible for the diurnal component in lower-tropospheric winds are examined using Japan Meteorological Agency (JMA) mesoscale analysis data (MANAL) and four global reanalysis data sets (JRA25/JCDAS, ERA-Interim, NCEP1, and NCEP2). Of these data sets, MANAL and JRA25 perform best in reproducing the WINDAS horizontal wind observations, including their diurnal and semidiurnal components. At 1-3 km height, Diurnal Eastward-moving Eddies (DEEs) with a phase speed of 10-15 m s-1 and diameter of ∼700 km move eastward over the Sea of Japan and over the Pacific throughout the year. The superposition of winds associated with DEEs over return currents controls the diurnal wind component over the main Japanese islands, generating diurnal amplitude maxima in spring and autumn at this height range. Analysis of global reanalysis data confirmed that the diurnal wind at 3-5 km in winter-spring is controlled mainly by medium-scale eastward-traveling waves with amplitude maxima around the tropopause. The diurnal wind at 3-5 km in summer-autumn is caused primarily by the diurnal tide with zonal wavenumbers of ≤10. For stations located on small islands south of the Japanese mainland, the diurnal wind is controlled mainly by the diurnal tide for the entire lower troposphere throughout the year.
The transformed Eulerian-mean (TEM) equations are useful in examining how the generation and/or dissipation of atmospheric waves drives the mean meridional circulation. However, the TEM equations do not provide a three-dimensional view of the transport. Several previous studies extended the TEM equation system to three dimensions but usually under the quasi-geostrophic assumption, which excludes small-scale phenomena such as gravity waves. Miyahara recently derived three-dimensional wave activity flux and the corresponding residual circulation applicable to gravity waves. However, his formulation has two flaws. First, the three-dimensional residual mean circulation does not satisfy the continuity equation. Second, the Eulerian-mean flow appears in the advection terms and the residual circulation appears in the Coriolis force term of the zonal momentum equation, unlike in the TEM one. The present study developed theoretical formulae of a three-dimensional residual mean circulation and wave activity flux on the basis of primitive equations that overcome these flaws. It is confirmed that the three-dimensional residual mean circulation accords with the sum of the Eulerian time-mean flow and the Stokes drift and that the three-dimensional wave activity flux accords with the mean tangential forces across material surfaces corrugated by the waves under an assumption similar to the TEM equations. A simple physical meaning is given for the terms including the shear of time-mean flow in the three-dimensional wave activity flux. Moreover, the time mean tracer transport equation is derived using the three-dimensional residual mean circulation. A simple case study using the new formulae was made on the three-dimensional transport of stratospheric ozone in the Southern Hemisphere. It is shown that the product of the Coriolis parameter and the strong poleward/equatorward Stokes drifts also balances the divergence/convergence of the three-dimensional wave-activity flux.
The effect of the Maritime Continent (MC) on the propagation characteristics of the boreal summer intraseasonal oscillation (ISO) over the Indo-western Pacific region were investigated by performing high-resolution (T159) atmospheric general circulation model (AGCM) simulations that remove and retain the MC. The most significant difference, as revealed by a finite domain wavenumber-frequency spectral analysis is the weakening of the northward propagation of ISO over the Asian monsoon region (65° -160°E) when the MC is removed; a less significant difference is the enhancement of the eastward propagation along the equator. The diagnosis of the vertical structure of the simulated ISO and the model mean flow indicates that the weakening of the northward propagation is primarily attributed to the reduction of the background easterly shear, low-level southerly and meridional humidity gradient, all of which contribute to the weakening of meridional asymmetries of vorticity and humidity fields with respect to the ISO convection center. The enhanced eastward propagation is possibly attributed to the strengthening of the mean convection over the MC in association with the increase of the local surface moisture and moist static energy.
As part of the Coupling Processes in the Equatorial Atmosphere (CPEA-II) Campaign, multistatic radar observations of local wind field were conducted using the Equatorial Atmosphere Radar and two auxiliary receiver arrays in West Sumatra, Indonesia, in December 2005. For obtaining velocity estimates from atmospheric echoes received by the two small arrays that have a high sidelobe level due to their small effective aperture, an adaptive clutter rejection process is required. Because of the existence of electromagnetic coupling between antennas and the ground, the relative phase, in terms of an echo received by separate antennas, cannot be accurately predicted only with given positions of antennas and the target volume. The phase error leads to degradation of desired atmospheric echoes in the output of the adaptive clutter rejection process. In order to compensate for the mutual coupling effect, we proposed a method to estimate the phase error corresponding to several directions in which atmospheric echoes were observed with particularly high signal-to-noise ratio (SNR). The estimates were interpolated at every necessary directional point using linear fit, thereby resulting in a successful removal of ground clutters without a serious loss of SNR. With this method, we have obtained a partial data set of the wind field. The wind field in the lower troposphere shows small-scale fluctuations on a horizontal scale of ∼500 m. We also present an attempt to interpret the fluctuation in a composite of multiple plane waves, employing a non-orthogonal decomposition method using a generalized inverse.
Tropical cyclone Nargis was generated over the Bay of Bengal (BoB) in late April 2008, and it devastated the coastal area of Myanmar. This study reveals the environmental and external factors in its genesis. It is found that large-scale environmental conditions around the genesis point were generally favorable for cyclogenesis. Positive lower-tropospheric relative vorticity and the weak vertical shear of horizontal wind were observed, which were distinctive features of April 2008 that are not seen in April climatology. Sea surface temperature and atmospheric thermodynamic profile were also favorable for cyclogenesis, as in climatology. Mid-tropospheric humidity was significantly low and unfavorable until six days before the genesis; however, it increased abruptly to become favorable just before the genesis. The atmospheric disturbance that seemed to trigger the genesis was lower-tropospheric easterly surge blowing into the genesis point. Horizontal convergence at the head of the surge associated with active convection was considered as an External Forced Convergence that triggered cyclogenesis. In addition, the active convection seemed to contribute to the preconditioning of mid-tropospheric humidity. The easterly surge was associated with a cold surge in the eastern coastal area of the Eurasian continent, which is a typical feature of cold surges in April.
Cyclone Nargis in 2008 was the worst natural disaster in the recorded history of Myanmar. Using observational and reanalysis datasets, this paper describes the features of Nargis’s track, intensity change, and environmental conditions. The cyclone track, recurving eastward over the Bay of Bengal and reaching Myanmar, is quite rare in the track record of recent 30 years. On the other hand, this record also shows that a limited number of cyclones with similar tracks formed in a concentrated period (i.e., April). Moreover, they are similar to Nargis in terms of undergoing rapid intensification before landfall. An analysis of microwave satellite data revealed a significant change in the structure of Nargis before and after the recurvature. In the analysis of environment, the relevance of a midtropospheric flow to the cyclone’s recurvature and structural change is focused. The flow in the north of the bay was characterized by a subtropical jet with low relative humidity in the middle troposphere due to subsidence along the southern slope of the Tibetan Plateau. A southward branch of the dry flow forced the cyclone’s movement to change from northward to eastward, while the intrusion of dry air into the cyclone center interrupted the convective development. After the recurvature, this dry flow meandered to the south of the cyclone, and this caused a decrease in vertical shear near the cyclone center, leading to the redevelopment of convection and intensification of the cyclone. The characteristic meander of a midtropospheric dry flow was also analyzed in the environment of the former cyclones that reached Myanmar, suggesting the applicability of a similar scenario to their intensification. These results suggest the importance of a midtropospheric flow as a determinant of cyclone track and intensification over the Bay of Bengal in spring.
A mesoscale data assimilation (DA) system was developed for low latitudes, and DA experiments for the tropical cyclone Nargis were conducted. A tropical cyclone bogus (TCB) procedure was developed for the Bay of Bengal, and the impact was investigated. The Meso 4D-Var system of the Japan Meteorological Agency (JMA), which was designed for operational mesoscale DA in the mid-latitudes, was modified for DA application in the tropics. Since the relationship of the geostrophic wind was not available near the equator, a weighting function for the regression coefficient matrix for the unbalanced wind was determined based on statistics. Six DA experiments were performed to produce the initial field at 1200 UTC on April 30, 2008, in order to assess the impact of the DA periods (12 h and 24 h) and two different TCBs. Analyzed initial fields were compared with the JMA global analysis (GANAL), the best track data of RSMC, New Delhi, and the cyclone track estimation of the Indian National Centre for Ocean Information Services (INCOIS). Precipitable water vapor (PWV) around the cyclone was less than 65 mm in GANAL; in DA analyses, areas with more than 60-mm PWV were more widespread, especially to the east of the cyclone, and the maximum values exceeded 70 mm near the cyclone center. The cyclone center position was also improved by DA and the implementation of TCB; however, the cyclonic circulation was underestimated in the 24-h DA without TCB (MA24). Lack of satellite observations of the Bay of Bengal in the first 12-h assimilation windows was determined to be the reason for this underestimation. Mesoscale numerical predictions using initial fields produced by Meso 4D-Var were conducted, and the results were compared with the prediction using GANAL (control run; GA). Overestimation of the cyclone speed and the resultant positional lag in the control run were ameliorated by modifying the initial field by DA. Differences between the upper flow in the GA analysis and that in DA analyses might account for the difference in cyclone speed. Cyclone development was well predicted except in the MA24 experiment, in which underestimation of cyclonic circulation at the initial time caused subsequent insufficient development. Northward biases were observed in several predicted cyclone tracks. Differences between precipitation in the MA24 and that in other experiments might be one of the reasons for the differences in cyclone track.
Over the northern Indian Ocean (NIO), a substantial number (∼60%) of tropical cyclones (TCs) form in association with significant intraseasonal oscillation (ISO) events (i.e., Nargis ). In this paper the relationship between TC genesis and ISO in the NIO was studied using 30-year (1997-2008) observations. Because NIO TCs mainly occur in transitional seasons when climatological environmental forcing favors TC genesis, two types of ISO modes, boreal summer intraseasonal oscillation (BSISO) and Madden-Julian oscillation (MJO), which represents boreal winter ISO, were objectively and quantitatively defined, and their connection with TC genesis was examined. It was found that over 70% of ISO-related genesis is associated with the northward propagating BSISO mode and up to 30% with the eastward propagating MJO mode. The BSISO mode primarily affects TC formation in May-June and September-November, while the MJO mode affects TC formation primarily from November-December. Because of their distinct structures and lifecycles, the BSISO and MJO modes affect TC formation differently. For the BSISO mode, TC formation is enhanced during its wet phases overlaying the NIO. For the MJO mode, TC formation is enhanced after the convection passes over the Malay Peninsula and when the Indian Ocean is in a dry phase. The BSISO mode enhances TC genesis by creating favorable environmental forcing for TC genesis, while the MJO mode does not. The most salient feature is that both the ISO modes favor TC genesis by providing a synoptic-scale seeding disturbance at least six days prior to TC formation. The seeding disturbance provided by the BSISO is a cyclonic vorticity anomaly to the north of the equatorial convection/westerly wind burst, whereas the seeding provided by the MJO is a convectively coupled Rossby wave that breaks away from the major body of the MJO convection. The seasonality of the NIO TC genesis, intensity, and prevailing tracks are also explained in terms of the effect of environmental forcing on TC genesis potential, steering flow, and maximum potential intensity. The results imply that monitoring the evolution of the two types of ISO modes, especially the BSISO, may provide a useful medium-range forecast for NIO cyclogenesis.
Tropical cyclone Nargis, generated over the Bay of Bengal in late April 2008, caused catastrophic destruction after making landfall in Myanmar. Here, the large-scale environment of cyclogenesis was investigated using reanalysis datasets and a cloud-system resolving model. The reanalysis datasets showed that a westerly wind axis over the Bay of Bengal shifted northward from mid-April to early May. This shift is attributed to a seasonal transition of the Asian summer monsoon and a boreal summer intraseasonal oscillation. The timing of this environmental modulation was consistent with the climatologically early tropical cyclogenesis over the Bay of Bengal. This period was also characterized by high genesis potential, which is an empirical index of the environmental field favorable for tropical cyclogenesis. An analysis of genesis potential showed that the genesis of Nargis was associated with reduced vertical shear and increased lower-tropospheric vorticity. A cloud-system-resolving model successfully reproduced the high probability of tropical cyclogenesis during the observed period of cyclogenesis in late April. The model also simulated the large-scale environment including the northward shift of the westerly wind axis, although the precise location of tropical cyclogenesis was sensitive to initial conditions in the model. The anomaly of sea surface temperature in 2008 had little influence on the simulated probability of tropical cyclogenesis. Therefore, a cloud-system-resolving atmospheric model even without ocean feedback is a promising tool for predicting the probability of tropical cyclogenesis over the Bay of Bengal around the onset of the Asian summer monsoon, which is a favorable environment for tropical cyclogenesis.
Numerical simulations of the 2008 Myanmar cyclone Nargis and the associated storm surge were conducted using the Japan Meteorological Agency (JMA) Nonhydrostatic Model (NHM) and the Princeton Ocean Model (POM). Although the JMA operational global analysis (GA) and the global spectral model (GSM) forecast underestimated Nargis’ intensity, downscale experiments by NHM with a horizontal resolution of 10 km using GA and GSM forecast data reproduced the development of Nargis more properly. Sensitivity experiments to study the effects of ice phase, sea surface temperature (SST), and horizontal resolutions to Nargis’ rapid development were conducted. In a warm rain experiment, Nargis developed earlier and the eye radius became larger. It was shown that a high SST anomaly preexistent in the Bay of Bengal led to the rapid intensification of the cyclone, and that SST at least warmer than 29°C was necessary for the development seen in the experiment. In a simulation with a horizontal resolution of 5 km, the cyclone exhibited more distinct development and attained a center pressure of 968 hPa. Numerical experiments on the storm surge were performed with POM whose horizontal resolution is 3.5 km. An experiment with POM using GSM forecast data could not reproduce the storm surge, while a simulation using NHM forecast data predicted a rise in the sea surface level by over 3 m. A southerly sub-surface current driven by strong surface winds of the cyclone caused a storm surge in the river mouths in southern Myanmar facing the Andaman Sea. Our results demonstrate that the storm surge produced by Nargis was predictable two days before landfall by a downscale forecast with a mesoscale model using accessible operational numerical weather prediction (NWP) data and application of an ocean model.
A mesoscale ensemble prediction system (EPS) employing the Japan Meteorological Agency’s (JMA’s) high-resolution global analysis and forecast for initial and boundary conditions of the control run and perturbations from JMA’s one-week global EPS for initial and boundary perturbations is developed and applied to numerical simulations of cyclone Nargis. Using the JMA nonhydrostatic model (NHM) with a horizontal resolution of 10 km, the system reproduces Nargis’ development and the associated storm surge in southwestern Myanmar with plausible ensemble spreads. In the ensemble prediction with initial boundary perturbations, predicted positions of cyclone centers are distributed in an elliptic area whose major axis is oriented east-northeast, suggesting that track forecast errors tend to increase in the moving direction of Nargis. The location of the minimum surface pressure of the ensemble mean is closer to the best track than the control run, and root mean square errors (RMSEs) of the ensemble mean against analyses are smaller than those of the control run in all forecast variables. However, ensemble spreads tend to decrease in the latter half of the forecast period, and the cyclone center does not disperse enough compared with the track forecast error without the lateral boundary perturbation. When lateral boundary perturbations are implemented in addition to the initial perturbations, dispersion of the cyclone center and spread of the center pressure increase by about 50% at forecast time (FT) ¼ 42. The location of the minimum surface pressure in the ensemble mean shifts westward, reducing the track error. RMSEs of ensemble means become smaller than the ensemble prediction without lateral boundary perturbations. Ensemble forecasts of storm surge were conducted using the Princeton Ocean Model (POM). When surface wind and sea level pressure from JMA’s global EPS were input, the maximum surge was no more than 0.6 m even in the highest ensemble member. The POM simulation driven by the mesoscale ensemble prediction with NHM predicted a storm surge near 4 m in southwestern Myanmar, where the timings of the peak surge were dispersed widely from FT ¼ 33 to FT ¼ 56. When the ensemble mean was input to POM, the maximum surge was 1.5 m, despite the better accuracy of the ensemble mean in terms of RMSE. This result shows that the scenario is more important than the ensemble mean when applying the mesoscale ensemble prediction to disaster prevention.
An ensemble simulation of cyclone Nargis was performed using the Non-hydrostatic ICosahedral Atmospheric Model (NICAM) at 14-km mesh size in order to examine the effect on cyclogenesis of disturbances associated with intraseasonal oscillations. An analysis of observational data reveals that cyclone Nargis formed during the northward propagation of low-level zonal wind, associated with active cloud areas and precipitation from the equator to 20°N in the Bay of Bengal, when the active convective region associated with the Madden-Julian Oscillation (MJO) passed through the bay and then resided over the Maritime continent. The northward migration of low-level zonal wind, outgoing longwave radiation (OLR), and precipitation are successfully simulated in the ensemble results. Each simulated tropical cyclone (TC) genesis also occurs with northward migration and with a timing such that the active convective region associated with the MJO resides over the east side of the Maritime continent. The incipient disturbances that contributed to the initiation of cyclone Nargis formed during the period when the westerly wind burst passed through the Bay of Bengal after the monsoon onset and developed to TCs in the ensemble simulation. However, for an ensemble member for which northward migration as a monsoon onset is not simulated, no TC is formed in the Bay of Bengal. It is also found that the effect of the easterlies across the northern part of the Malay Peninsula is important for TC formation in our simulation.