The heat transfer resistance between a building canyon and the overlaying atmospheric boundary layer was evaluated in a long-term field observation. The recorded case is of a courtyard canyon of a university campus. The canyon has an aspect ratio of 0.22 and height of 13 m. The resulting resistance was observed to be clearly dependent on the wind speed and atmospheric stability in the range of 1.6 to 20 m-1s. The random error in the measurement of resistance was estimated to be 27%; however, this does not nullify the above findings. The adequate lower-level reference temperature for the transfer parameterization was found to be the average for the entire canyon volume. This is because vortex-type airflow controls the heat transfer.
Tropical cyclones (TCs) in the western North Pacific are examined to present climatology of their extratropical transition (ET) during the period of 1979–2004. Two parameters of the cyclone phase space (CPS) are calculated using the Japanese 25-year Reanalysis (JRA-25) dataset as indices of ET for the TCs in the best track data from the Japan Meteorological Agency. The onset and the completion of ET are defined as the beginning of an increase in thermal asymmetry and the evolution from the warm to the cold-core structure, respectively. Forty percent of all TCs are assumed to complete ET. The mean transition period from the ET onset until ET completion is estimated to be about 18 hrs, although 16.8% of all ET storms are transformed into the cold-core structure before the increase in thermal asymmetry. Meridional fluctuation in the location of ET completion by season attains its peak in August, while the peak of ET events occurs in September-October with regards to the ratio of ET frequency to all TCs, the transition period, and intensity at the time of ET completion. The background environment of ET events are also examined using the monthly mean of the JRA-25 and sea surface temperature of the centennial in situ observation-based estimates (COBE). Both air-sea thermal contrast and the tropospheric vertical shear are greater in autumn than in the Baiu season and summer. The results suggest that, whereas a TC can translate into higher latitudes with a warm-core structure in an environment of weak vertical shear and small air-sea contrast in summer, it tends to complete ET in lower latitudes in an environment of strong vertical shear and large air-sea contrast in autumn-winter. These characteristics of ET affect the seasonal variation in the structure of landfalling TCs on the main islands of Japan.
A climatological and numerical study of “Karakkaze,” a type of local wind in Japan, was conducted. First, winter days under a winter-type synoptic pressure pattern with daily minimum relative humidity of less than 40% were classified according to strong wind (wind speed ≥ 9 ms-1, Karakkaze day), medium wind (6 ms-1 ≤ wind speed < 9 ms-1), and weak wind (wind speed < 6 ms-1). Secondly, the spatial patterns of the surface wind in each category are confirmed by the Japan Meteorological Agency (JMA)-Automated Meteorological Data Acquisition System (AMeDAS) observation data. In addition, we compared the boundary-layer wind of the three categories using wind speed data from the observation tower of the Meteorological Research Institute (MRI) in Tsukuba and from the JMA wind profiler in Kumagaya. Finally, we performed one-dimensional numerical experiments using a column Planetary Boundary Layer (PBL) model to evaluate the impact of solar radiation and upper-level wind on the formation of the Karakkaze. The results are summarized as follows. On the strong-wind days, strong northwesterly winds appear in the area along the Arakawa River and the Tonegawa River from Maebashi. The surface wind speed has a clear diurnal variation with a peak in the early afternoon. Such a diurnal variation is observed up to a 200 m level, but this diurnal pattern nearly reverses itself between 200 m and 400 m levels. On weak-wind days, the diurnal variation pattern is similar to that in other two categories, but the reversed pattern appears at a 100 m level, not at a 200 m level. Stronger surface winds appear under a clearer winter-type pressure pattern. The correlation coefficient is 0.632 between the daily maximum surface wind speed and the daily mean wind speed at a height of 2759 m, whereas the correlation is 0.284 between the surface wind speed and daily sunshine duration, which, in a previous study, was suggested to be the most significant factor. Numerical experiments indicate that a large amount of solar radiation is a necessary condition for a strong daytime wind, but this is not a sufficient explanation for the difference in the surface wind speed between weak and strong-wind days.
This study describes the three-dimensional structure and migration process of a westward-migrating precipitation system with a diurnal cycle observed on 10 November 2006 in west Sumatera as based on dual-Doppler radar analysis, rawinsonde data, and surface data. The location of convective cell generation over land in the daytime and the timing of an evening change in the migration direction of precipitation systems from landward to seaward were influenced by local circulation: in the morning, an isolated convective cell generated near the west coast propagated inland (toward the mountains) via the successive generation of new convective cells over the sea breeze. In the afternoon, convective cells in a precipitation system were generated over the western slopes of the mountains surrounding Lake Maninjau by thermally induced local circulation and the slope effect. In the evening, precipitation systems located over the mountains started to propagate toward the sea (westward) in response to a change in local circulation from landward to seaward winds. Subsequently, precipitation systems were newly generated over the sea near the west coast of Sumatera Island, merging with the systems that originated over land. These two sets of systems formed a larger system (long axis > 100 km) than that over land (long axis of several tens of kilometers). The expanded precipitation system had a convective region with a long axis oriented parallel to the west coast of Sumatera Island and a short axis (∼7 km) oriented perpendicular to the west coast at the leading edge of the system. The echo top height of the system was located at approximately 13 km and an anvil moved faster than the convective region above 6 km in height. The convergence was formed by an easterly wind component in the system and a southerly wind component over the sea around the leading edge of the system at the lowest layer. The expanded system migrated farther offshore at a speed of about 5 m s-1 via the self-replication of convective cells over the convergence at the leading edge of the system and via advection by ambient wind in the lower troposphere. The convergence regions at the leading edge of the system were continuously strengthened by downward transportation of horizontal momentum below 4 km, meaning that the system could be maintained for a long time and migrate offshore for a long distance.
Dynamical contributions to past long-term changes in the lower stratospheric ozone over the northern mid-latitudes are evaluated using a chemical transport model (CTM) forced by the horizontal wind of the Japanese 25-year Reanalysis (JRA-25). Two simulations (i.e., one is a simulation that prescribes the time-dependent vertical profile of halogens and the other is a simulation which uses the fixed vertical profile of halogens at 1979.) were conducted to estimate chemical and dynamical contributions to the long-term changes in stratospheric ozone during the last three decades. Different from previous similar studies using meteorological data of ECMWF (European Centre for Medium Weather Forecast) 40-year re-analysis (ERA-40), our current simulation does not show a large positive anomaly of simulated total ozone over northern mid-latitudes in the late 1980s, which is consistent with the observation. Because the trend of the fixed halogen simulation amounts to about two-third of that of the time-dependent halogen simulation during 1980–1993 in the northern mid-latitudes, it is evaluated that about two-thirds of the negative trend in total ozone comes from dynamics in the northern mid-latitudes. Since the increasing ozone from 1994 to 1998 is also represented in the fixed halogen simulation, it is considered that the increase of ozone was mainly due to dynamics as pointed out in previous studies. However the dynamical contribution to the trend after 1994 could not be evaluated in our simulation because of simulated ozone gap in 1998. In the same manner, it is evaluated that about two-thirds of the negative ozone trend in the lower stratosphere comes from dynamics in the northern mid-latitudes from 1980 to the mid-1990s. The simulation results indicate that the effect of transport (dynamical influence) is predominant for the negative ozone trend in the lower stratosphere from 1980 to mid-1990s, while the upper stratospheric ozone trend is strongly influenced by long-term changes in halogens (chemical influence).
We investigated the climatological shape of the North Pacific subtropical high (NPSH) and its interannual and submonthly variability (variability with a period of less than 31 days) in August during the twentieth-century using the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset. The climatological NPSH in a CMIP3 multimodel ensemble mean (MMEM) is similar to that in the real atmosphere. However, intermodel differences of the NPSH are quite large in the southwestern North Pacific where some models simulate stronger NPSH with positive geopotential height (Z) anomalies, and other models simulate weaker NPSH with negative anomalies compared with the MMEM NPSH. An NPSH index was defined as the deviation of the climatological Z (Z) in each model from the MMEM Z, averaged over the region 123.75°–151.25°E, 16.25°–31.25°N at 850 hPa. Composite analyses using data from the four most positive (P-type) and negative (N-type) models revealed that the Indian monsoon westerly is weaker, precipitation to the east of the Philippines is smaller, and the sea surface temperature (SST) to the west of the warm pool is greater in the P-type than in the N-type models. We suggest that better simulation of the east–west gradient of the SST around the warm pool is related to improved simulation of the climatological NPSH in the southwestern North Pacific. Models with positive (negative) values of the NPSH index tend to simulate small (large) interannual and submonthly variability in the index area.
Perturbations generated by symmetric instability can be characterized, in terms of growing normal modes, by slantwise vertical motion bands similar to those observed in frontal rainbands. Nonmodal growths of symmetric perturbations, characterized also by slantwise vertical motion bands, can be produced by linear combinations of normal modes even before the basic state becomes symmetrically unstable in order to generate growing modes. In this paper, normal modes for nonhydrostatic symmetric perturbations in a vertically bounded domain are revisited and constructed by free modes obtained in unbounded domain. The constructed modes form a complete set in the full-solution space, and thus, can construct any admissible solutions to further explore nonmodal growths of symmetric perturbations in the vertically bounded domain beyond previous studies.