(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 1, pp. 7-15, 2000
The temporal and spatial relationships between GPS precipitable
water vapor (GPS-PWV) and rainfall intensity (RI) on a event of
strong convective thunder storm in the Tokyo metropolitan area
were investigated. RINEX, one of the standard format of GPS raw
data and supplied from Geographical Survey Institute of Japan,
were converted to GPS-PWV by GAMIT software on 30 min basis. The
following results were obtained ; (1) A rapid increase of GPS-PWV
can be observed 1 to 2 hours before a beginning of sever rainfall,
(2) The hourly increment of GPS-PWV has a strong correlation with
the possibility and amount of increasing of one hour later RI
, and (3) GPS-PWV begins to decrease at the peak of RI. These
results suggest the promising availability of GPS-PWV as one of
the prior-signals of strong convective thunder storms.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 2, pp. 83-96, 2000
"Kanpachi Street Cloud (KSC)" is an unusual small-scale
cumulus cloud line visible during calm summer conditions over
a major street in the Tokyo metropolitan area. In order to well
understand the mechanism leading to the formation of this cloud
line, numerical simulations have been performed using RAMS ( Regional
Atmospheric Modeling System, Pielke,1992). The general characteristics
of the simulated KSC agreed well with extensive observations by
Kai et al (1995,1997). On a large scale view, KSC can be characterized
as a cumulus cloud line generated at the convergence line of two
sea breezes on the west side of Tokyo Bay, while on the microscale
view, it resembles a Benard-type thermal convections modified
longitudinally by wind shear. Additional numerical experiments
reveled that urbanization can change the intensity and position
of KSC through enhancement of local upward motions and changes
in the near-surface horizontal pressure gradient between urban
and sub-urban areas.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 3, pp. 183-188, 2000
Cloud patterns over the Japan Sea on 23-24 January 1996 were related to the evolution of cold outbreak proceeded in three stages. First, cold airmass C1 advected southeastward from Russian coastline, next, less cold airmass C2 advected eastward from Korean Peninsula, then the coldest airmass, C3, spread southeastward from Russian coastline. Mixing cloud area was formed along the front edge of each cold airmass. A band cloud, formed at the southwestern side of airmass C1, evolved into two mesoscale cloud vortices. Landfall of the front edge of each cold airmass clearly appeared as the changes of surface temperature and potential temperature aloft. The landfall was also visualized by the cloud patterns. The behavior of C2 and W suggested that the Japan Islands prevented southward advection of the airmass.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 4, pp. 245-253, 2000
One of climatological indices widely used in Japan is the number of days in some categories of daily minimum and maximum temperatures (Tmin and Tmax), such as Tmin<0ﾅ�, Tminﾅﾜ25ﾅ�, Tmax<0ﾅ�, Tmaxﾅﾜ25ﾅ� and Tmaxﾅﾜ30ﾅ�. The present study aims at evaluating their bias arising from changes in the observation time of Tmin and Tmax, which are currently defined for 0000-2400 JST (Japan Standard Time), but were observed for 2200-2200 JST at most stations until 1939 and for 0900-0900 JST for Tmin from 1953 to 1963. The analysis was based on hourly temperature data at 830 automated stations (AMeDAS) from 1979 to 1997. In comparison to the 0000-2400 observations, the 0900-0900 and 2200-2200 observations were found to result in decrease of Tmin<0ﾅ� days by 4.9 days/year (5.8%) and 2.3 days/year (2.8%), respectively, and increase of Tminﾅﾜ25ﾅ� days by 0.8 days/year (15%) and 0.3 days/year (5.1%), respectively, on the average over all the stations. For the numbers of Tmax<0ﾅ�, Tmaxﾅﾜ25ﾅ� and Tmaxﾅﾜ30ﾅ� days, the bias due to 2200-2200 observation was found to be small. Then a simple equation for the relationship between biases of the number of days and temperature was derived, and its validity was examined. Finally, a brief analysis was made about the effect of the bias on the long-term change rate of the number of days. It was shown that the number of Tmin<0ﾅ� days during the last hundred years can have a superficial trend of several days/(year100 years) unless correction is made for the bias.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 6, pp. 453-462, 2000
Field observations to investigate area-averaged turbu-lence characteristics were performed in a densely built-up residential neighborhood in Tokyo, Japan. Be-sides eddy-correlation sensors, a scintillometer was used in an urban environment. Several important results are summarized as follows : (1) Radiation balance and en-ergy balance of observation area are obtained. (2) A new technique which uses simultaneous cintillation measurements at two heights is developed to estimate directly both area-averaged zero-plain displacement height zd and heat flux QH. The estimated zd is found to be slightly dependent on the atmospheric stability parameter (lower zd under more unstable conditions). (3) Source areas for the scintillometer are 2?3 times larger than those for the eddy correlation sensors under unstable conditions. 30-min averaged, normalized standard deviations of QH measured by the two methods are compared. Whilst QH estimates agree well, their short time variability was generally larger for the eddy-correlation as compared with the scintillometer data under wide range of stability conditions. This result can be attributed to the superior spatial integration due to the longer averaging path (250 m) of the scintillometer. It is concluded that time-averaging requirements to obtain a representative flux measurement for scintillometer applications can be relaxed.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
This study uses observational data from urban sites in Setagaya, Vancouver and Ginza, to investigate the applicability of conventional Monin-Obukhov similarity functions. The following results are obtained ; (1) The ration of linear correlation coefficients of heat and momentum transfer are generally smaller than conventional values in rural sites due to bluff body effect. (2) Non-dimensional form of (a) standard deviations of velocity, (b) standard deviation of temperature, (c) dissipation ration of temperature variances, and (d) dissipation ration of kinetic energy, as a function of stability parameter, are proposed. All of these also show more or less different curves from conventional relationships in rural sites.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
This study reveals the statistical relation between precipitation
and precipitable water vapor (PWV) obtained by the Global Positioning
System (GPS) in Tokyo metropolitan area during the period from
April 1996 to December 1997. The analysis is based on three-hourly
data of PWV at 12 GPS stations and hourly precipitation and surface
temperature data at 12 meteorological stations in the area.
Probability of precipitation occurrence is significantly high
when the areal average of PWV exceeds a certain critical value
which depends on the surface temperature. On the other hand, precipitation
rarely occur when PWV is less than that value. This relation is
clear when precipitation occur continuously for whole area, while
it is less clear for scattered precipitation. For a given surface
temperature ,the critical value nearly coincides with the minimum
of saturated PWV estimated empirically from radio sonde data.
TENKI, Vol. 47, No. 9, pp. 643-648, 2000
Local heavy rainfall events often occur in the Baiu and midsummer
seasons in Japan. These rainfall events sometimes cause severe
disasters,not only in the rural areas, but also in the urban areas.
Some authors mention that global and local climate changes may
cause long-term changes in the frequency of heavy rainfall events.
In the present study, changes in the properties of summer precipitation
over 23 years (1976 - 1998) in the Tokyo area were examined, by
using AMeDAS (Automated Meteorological Data Acquisition System)
data. Our results show that the ratio of heavy precipitation to
the total amount has increased in the Tokyo area in August, although
the total amount itself has not changed significantly. The changes
in the ratio are highly localized, and cannot be seen during the
other periods of the year. The mean temperature has increased
in the center of the Tokyo area, corresponding to the changes
in the precipitation properties. Further analyses of the AMeDAS
wind data show stronger convergence of the horizontal wind in
recent years. From these results, it can be reduced that the changes
in the precipitation properties may have been caused by stronger
cumulus convection over the urban area in 1990s resulted from
the strengthening of the heat island.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 11, pp. 777-783, 2000
Three severe tornadoes occurred in Tokai District, the central
part of Japan, on 24 September 1999 when Typhoon 9918 moved northeastward
over the westernmost part of Japan. Photograph and video images
showed that their width was several hundred meters and their rotation
was cyclonic. The Doppler radar of Nagoya University observed
the parent mesoscale convective systems of the tornadoes. PPI
display of Doppler velocity showed five meso-cyclones passed over
the district during the period from 11 to 1230 JST, 24 September
1999. Three meso-cyclones of the five were accompanied by the
tornadoes. The Doppler radar observation found characteristics
of super cell in the convective systems: a hook-shaped echo and
a bounded weak-echo region. Vorticity of the meso-cyclones estimated
from the Doppler velocity was an order of 10-2s-1. The sounding
at 09 JST, 24 September 1999 at Shionomisaki showed that the lower
atmosphere was significantly unstable and the vertical shear was
strong. CAPE of the profile was 2140 J kg-1. This condition was
favorable for formation of a super cell. In order to examine whether
the profile had a potential to produce a super cell, we performed
a numerical simulation experiment using a cloud-resolving model
(ARPS). The result showed that a quasi-steady super cell was formed
with a significant vorticity at the central part of the intense
upward motion. The result suggests that the tornadoes were produced
by intense stretching of the vorticity by the intense upward motion.
(Tenki is the bulletin journal of the Meteorological Society of Japan in Japanese.)
TENKI, Vol. 47, No. 12, pp. 821-833, 2000
Under typical cold season pressure distribution conditions,
when the Japan Islands lie between the Siberian high and the Aleutian
low, the weather distribution is characterized by a contrast between
precipitation over the Japan Sea coast region and clear skies
over the Pacific Ocean coast region. However, even under such
pressure distribution, the weather in the Omachi area, which is
located inland close to the Japan Sea side, is either cloudy with
precipitation (type A) or clear and fine (type B).
A statistical analysis of cloud amount distribution revealed
that the weather conditions at Omachi were common to a broader
area of the inland Chubu district. As for synoptic conditions,
the surface pressure isobars in the type A were parallel and ran
in J- or I-shape from north to south, while the isobars in the
type B also ran parallel to each other, but were curved and formed
a S-shape. The pressure gradient was significantly stronger within
the area of 30ﾅ絲 to 40ﾅ絲 and 130ﾅ絋 to 140ﾅ絋 in the type A than
in the type B. The trough and center of the cold air mass on the
500hPa isobaric surface of the type A were located along the same
longitude as the Japan Islands whereas those of the type B were
located farther east over the Pacific Ocean. The trough was deeper
and the temperature lower in the type A.
Transition between the two types was infrequent, although the
type A sometimes changed into the type B. They did not correspond
to the conventional "mountain-snow" and "plain-snow"
types, although the majority of mountain-snow cases belonged to
the type A.