By using hourly data at 808 automated stations for 19 years, the frequency distribution of surface air temperature was analyzed, with attention to the near-0 °C frequency maximum resulting from the melting of snow particles. Temperature in precipitation cases has a distinct frequency maximum just above 0 °C (0.3 °C on the average), whereas no peak exists in no precipitation cases. The peak height, defined by the maximum frequency deviation from neighboring temperature ranges, is 122% for the average over all the precipitation cases. The peak height is dependent on precipitation intensity and strongly on wind speed, and reaches several hundred or a thousand percent for calm cases with heavy precipitation (4-6 mmh-1 and ≥7 mmh-1). There are some regional differences in peak height, with relatively high values in plains facing the Pacific coast, especially in the inland part of the Kanto plain. Finally, the depth of the isothermal layer due to melting of snow particles was estimated by applying some simple assumptions to the present results. It was shown that the average case corresponds to an isothermal layer of a few hundred meters depth, in rough agreement with some vertical sounding data.
The East Asian countries experienced an extremely wet summer in 1998. More than 150% of normal rainfall has been observed over a large portion of East Asia extending from southern China, Korean peninsula to Japan. A record-breaking flood occurred over the Changjiang River basin of China, and lasted for almost three months. Heavy rainfalls hit northern and eastern Japan, and the Korean peninsula in July and August. Observational studies indicate that the 1998 East Asian summer monsoon was characterized by suppressed convection and persistent low-level anticyclonic circulation anomalies over the subtropical western Pacific. It is confirmed by a moisture budget analysis that the seasonal mean, rather than transient, component of the moisture transport anomaly contributed mainly to the wet summer in East Asia. An atmospheric general circulation model (AGCM) was integrated with observed sea surface temperature anomalies to study the 1998 East Asian summer monsoon. The large-scale features over the East Asian monsoon region were well reproduced by the model. Experiments indicate that SST anomalies over the two key regions, the southeastern Indian Ocean and the equatorial eastern Pacific, were most influential in forming the subsidence anomaly over the subtropical western Pacific and associated low-level anticyclonic anomaly. It is indicated that high SSTs over the southeastern Indian Ocean enhanced local convection and weakened the local Hadley circulation, and that associated subsidence contributed to strengthen the low-level anticyclonic anomaly over the subtropical western Pacific. On the other hand, SSTAs over the equatorial eastern Pacific helped enhance the local convective activity and weaken the Walker circulation. The AGCM experiments indicated that the dual effect made the persistent and strong low-level anticyclonic anomaly in the subtropical western Pacific, and thereby was responsible for the anomalous 1998 East Asian summer monsoon.
A comparison of midsummer (August 1 to 10) daily sunshine duration averaged over 10 years for the periods 1986∼1995 and 1959∼1968 indicates that it dropped by as much as 2 hr day-1 in some observational sites in central Japan. The diurnal temperature range and water vapor pressure in midsummer also decreased. A long-term examination of the Baiu frontal zone location suggests that its slower northward movement in recent years affected the midsummer weather. Furthermore, the sea level pressure increased in northern Japan, while the equivalent potential temperature in the lower atmosphere dropped in central Japan. The change in the northward movement of the Baiu frontal zone is likely to be associated with the strengthening of the polar airmass around Japan.
Using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data and satellite-observed outgoing long-wave radiation (OLR) data, we examined the westward extension and eastward contraction of the North Pacific subtropical high in summer (NPSH). It was found that the NPSH shows a great variability in its western extent, both on the seasonal and interannual time scales. In order to examine the interannual variations of NPSH, we defined a NPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850 hPa averaged over the west edge (110∼150°E, 10∼30°N) of NPSH. This index describes the year-to-year zonal displacement of NPSH. Composite analysis based on this NPSH index showed that there is a significant relation between zonal displacement of NPSH and intensity of atmospheric convection over the warm pool. A low-level cyclonic (anticyclonic) anomaly that is closely associated with the zonal shift of NPSH appears north of enhanced (weakened) atmospheric convection, i.e., the vorticity anomaly is found north of the divergence one. Climatologically, the NPSH contracts eastward swiftly after pentad 40 (July 15 to 19). Such an eastward contraction is closely associated with the poleward shift of both NPSH and atmospheric convection over the tropical western Pacific warm pool. However, such seasonal variations of both NPSH and convection show distinct features between the summers with positive and negative NPSH indexes. During summers with positive NPSH index, NPSH and convection over the warm pool do not show an appreciable seasonal evolution. During summers with negative index, by contrast, they show a swift seasonal evolution after pentad 40. Finally, we performed a vorticity analysis to explain the relation between the divergence and vorticity anomalies on the interannual time scale. The analysis shows that in the lower troposphere (925 hPa), the advection of relative vorticity is comparable to the stretching and is responsible for the northward shift of the circulation anomaly relative to anomalous atmospheric convection. The difference from the theory of Gill (1980) is discussed. In the upper troposphere (200 hPa), the advection is slightly smaller than the stretching with opposite signs in East Asia and the western North Pacific, and thus the position of the vorticity anomaly is consistent with that of the stretching anomaly.
The Mackenzie River of Canada is one of the great rivers of the world. Its basin is characterized by a highly variable topography and its climate is subject to many important cold-region phenomena. Over the last few decades, the Mackenzie Basin has also been experiencing a pronounced winter warming. In this study, a number of extreme basin warming events and related processes during the winter are investigated using surface and rawinsonde data. By documenting these events, we have found that the basin warming is mainly associated with low pressure systems in and near the basin, and with extratropical and subpolar high pressure systems in the vicinity of the basin. The dynamic and thermodynamic contributions (due to atmospheric circulations, topography, low-level temperature inversion and their interactions) to the basin warming are also examined. It is shown that basin warming events occur through the horizontal advection of warm air from west and south of the basin, and through adiabatic descent induced by both the topography and the low and/or high pressure system, particularly when the low-level temperature inversion occurs.
Many previous papers stressed that the Meiyu-Baiu rain zone exhibits the quasi-stationary nature, except the abrupt transitional period. This is true only for the 10-day averaged large-scale field. The actual Meiyu-Baiu front indicates complicated variations even in “the quasi-stationary period”. The present paper will study a large λ-shaped cloud zone seen around July 6, 1991 as one of the cases of significant variations of the Meiyu-Baiu front. This λ-shaped cloud zone is formed when a north-south oriented cloud zone elongated from the intense rainfall area of Meiyu-Baiu front with pole-ward moisture transport. This process is quite different from “the abrupt transition of the Meiyu-Baiu front”, since the Meiyu-Baiu frontal zone itself remains in almost the same latitude belt. The λ-shaped cloud zone is associated with a north-south extending trough, which resulted from coupling of the short-wave troughs in northern (40-50°N) and southern (30-40°N) latitude. When the extending trough passed over the Tibetan Plateau and approached a quasi-stationary ridge over ∼120°E, the narrow north-south elongated cloud zone, which is associated with southerly wind and ascending motion, is formed in front of the trough. Convective precipitation occurs within the north-south elongated cloud zone. However, subsidence to the west of the trough and that to the east of the ridge block the northward shift of the Meiyu-Baiu front. The north-south elongating cloud zone disappears when the eastward propagating southern short-wave trough is de-coupled from the northern short-wave trough by the blocking ridge. The λ-shaped cloud zone occurs only under the specific phase relations among the short-wave troughs in northern and southern latitudes, quasi-stationary ridge and intense rainfall area in the Meiyu-Baiu front. Typical large λ-shaped cloud zone appears only a few times in a Meiyu-Baiu season.
Diurnal variations of tropical convection in the western Pacific are examined by using the data observed during the Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) Intensive Observing Period (IOP) (November 1992 to February 1993). High resolution data of satellite infrared histograms, MIT radar from the TOGA project at NASA/GSFC, upper-air soundings, and improved meteorological surface mooring (IMET) buoy data by the WHOI are utilized for analyses. Over tropical western Pacific, maximum convective activity occurs during the evening hours to midnight on large islands. On the other hand, in the vicinity of large islands, the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), the peak activity occurs during the morning hours to noon. The local time of peak activity varies depending on the focused cloud top height. Comparing the results in different regions, local time of peak convective events of different cloud top heights change little over large islands, but it varies among the surrounding ocean, ITCZ and SPCZ. Diurnal variations of precipitation tend to be more prominent in the case of heavy rainfall with a nocturnal maximum from the TOGA-COARE special data. Therefore, we focused on the days with nocturnal precipitation maxima and examined the diurnal variations of atmospheric vertical structures over the TOGA-COARE region. At the onset of convection in the evening, water vapor is increased in lower troposphere. During this time low-level clouds appear, and upward motion is observed in the lower layers. Convective activity reaches its peak around 00-03 LT, which coincides with the maximum precipitation. Large-scale upward motion, apparent heat source, and moisture sink are also observed at the peak of convective activity. While precipitation decreases gradually in the morning to noon, the activity of high-level clouds decay. It is suggested that the water vapor increase at low levels in the evening plays a role in the development of nocturnal convection. The maximum activity of convection over the TOGA-COARE region however, is observed about 6 hours prior to the peak of other convergence regions over the tropical western Pacific.
The structure of a downdraft under a melting layer of a front was analyzed. On 11-12 May 1994, during the TAPS period, a frontal system accompanied by a weak cyclone was observed at Tsukuba as it moved along the southern coastal line of the Kanto district. The upper front was observed as a stable layer at Tsukuba. The front was observed at an altitude close to a 0 °C layer. This situation lasted for about 12 hours. It is suggesting that cooling due to melting of ice particles maintained and intensified the stable layer. This resulted to fix the front to the altitude close to the 0 °C layer. In the stratiform cloud area, a precipitation band appeared at around 20:45UTC and reflectivity of the melting layer increased to 45 dBZ. This precipitation band was narrow with a width of about 20 km in the direction perpendicular to the band. Downdraft was produced under the layer of intense reflectivity of the precipitation band. This downdraft was generated mainly as a result of cooling due to melting of falling ice particles. The maximum speed of the downdraft was estimated to be 0.2 ms-1. This was the same order of magnitude as the simulated value in the stratiform cloud band behind a squall line (Biggerstaff and Houze, Jr. 1991, 1993, Caniaux et al. 1994). The downdraft induced convergence around the height of the melting layer and divergence at lower layers. With this convergence and divergence across the precipitation band, wind fields under the melting layer were deformed. Due to this deformation, wind speed before and after passage of the precipitation band was not uniform over the Tsukuba point. At 3.5 km altitude, a SW wind intensified from 12-13 ms-1 to 19-21 ms-1 due to the convergence. At altitude 0.5 km, an ENE-NE wind intensified from 2-3 ms-1 to 7-8 ms-1, which corresponded to the divergence. Horizontal wind fields calculated from Doppler velocity also shows discontinuity of wind across the precipitation band, which extended from SSE to NNW. At 0.5 km altitude, an ENE wind of about 10 ms-1 was detected on the western side of the band and a weak easterly on the eastern side. This change corresponded to wind divergence. At 3.5 km altitude, a SW wind of about 15-30 ms-1 occurred on the western side and one of 5-10 ms-1 on the eastern side of the band. This corresponded to the convergence.
The behaviour of wave energy (projected onto a wave vector) at the lowest critical level (corresponding to the wave vector) is investigated analytically in a linear 3-dimensional framework by the ray tracing method. Both the magnitude and direction of the environmental flow are arbitrary functions of the vertical coordinate, with a slowly varying assumption. The Coriolis parameter is constant. The obtained results show the following. The wave energy infinitely increases only at such a critical level that the magnitude of environmental wind vanishes as a linear function of vertical coordinate, and that the direction of environmental wind does not become perpendicular to the wave vector, and only when the Coriolis effect is ignored. The wave energy remains finite at all other types of critical level. The well-known 2-dimensional non-rotating result that the wave energy infinitely increases at the critical level can not be realized in the 3-dimensional rotating system.
Century-long observations enable us to uncover an interesting out-of-phase variability between the first principal component of Baiu rainfall over the Japanese archipelago and the monsoon rainfall over India during the early summer season (June and July). The signatures of this contemporaneous relationship are clearly evident from analysis of long-period multi-source climate datasets. One of the findings suggest that the circulation near the subtropical region of the west Pacific Ocean tends to vary in-phase with that over the Indian subcontinent, so that an intensified (weakened) west Pacific subtropical high is accompanied by an intensified (weakened) Baiu circulation over Japan and a weakened (intensified) monsoon circulation over India. Additionally, the Baiu-Indian Monsoon relationship is well supported by middle and upper tropospheric circulation anomalies extending from the mid-latitude region of West-Central Asia up to the Far East. A pattern consisting of an anomalous low over the Caspian and Aral Sea region, a high over Mongolia and an anomalous low over Korea and Japan, tends to be associated with increased Baiu rainfall over Japan and decreased monsoon rainfall over India. An opposite configuration of the mid-latitude circulation pattern tends to accompany below normal Baiu rains over Japan and above normal monsoon rains over India. It is suggested that the two patterns oriented along the (a) west Pacific southern oceanic route and the (b) mid-latitude continental route yield a consistent dynamical basis for inferring the Baiu-Indian Monsoon rainfall teleconnections.