A rain observation by an X- and Ka-band dual-wavelength radar has been performed. The objective of this experiment is to explore the possibility of multi-wavelength radar for measuring rainfall rate. Simultaneous rain observations by C- and Ku-band radars were also performed. Correlations among measured Z- factors show that rain attenuation is significant only in the Ka-band for a short range when the observing range is a few km. After a method of obtaining an optimum averaging hit number is proposed, a dual-wavelength analysis using rain attenuation is applied to Ka- and X-band radar data and the result confirms the capability of providing an estimation of rainfall rate other than that by the conventional method. The dual-wavelength analysis is compared with a single wavelength analysis using rain attenuation and an advantage of dual-wavelength analysis is verified. The capability of the dual-wavelength analysis for retrieving a vertical structure of rain is demonstrated. A radar calibration using the dual-wavelength analysis is tested and a more accurate calibration than that comparing the rain gauge data is obtained.
This paper presents a case study of westerly wind bursts in the equatorial western Pacific (EWP) during May 1982. Using 12-hourly island, atoll, and ship observations, the latitudinal extent of the 11-day long burst was determined to be within about five degrees from the equator, whereas its longitudinal domain varied considerably, raging from 1000 km when the burst was first recognized to more than 3500 km when the burst was fully developed. Consequently, Rossby numbers changed appreciably throughout the burst period, and the acceleration term, as expected, dominate the Coriolis force term at the initial stage of the burst. A pressure increase in the far western equatorial Pacific appeared to follow midlatitude forcing. The imposed zonal pressure distribution there greatly enhanced the otherwise slack background pressure gradient in the EWP and thus favored westerly wind bursts. Westerly winds averaged in the box bounded by 5°N-5°S and 135°-155°E peaked at 12-h and 2 days after the establishment of the equatorial zonal pressure gradient. In addition to the pressure rise in the extreme western Pacific, a pressure plunge in the core of the EWP (-150-160°E) resulted in a strong eastward-directed pressure gradient that acted to prolong the burst. Hovmoller diagrams of the meridionally averaged zonal wind and horizontal divergence in the equatorial waveguide indicate that the major axis of the surface divergence was located on the west of maximum westerlies throughout the burst period. The region of maximum surface convergence was on the eastern terminus of the burst, where convection and ascending motion were strong. Deep convective clouds were sometimes found east of strong westerlies. During the burst period, the zonal circulation in the EWP was marked by two cells, linked by strong convection on the eastern extreme of the burst. While the eastern cell resembled a local Walker circulation, the western cell resembled a reverse one. The mean SST in the aforementioned box fell by 1°Cduring the burst period. The equatorial atmosphere in the same area gained a small amount of heat and a relatively large amount of moisture from the warm ocean during the burst period. These conditions were favorable for the enhancement of moist convection over the equatorial ocean.
Interactions among stationary planetary waves in a low-order spherical stratospheric model are studied both analytically and numerically. A triad of interacting waves is examined using a steadystate hemispheric quasi-geostrophic model with a mean zonal wind in solid body rotation. Due to the structure of the model basic state, dissipation is required for the waves to interact. It is found that the nonlinear structure of the two gravest forced modes is independent of their relative position only if the third mode is unforced at the lower boundary. In this case the degree to which the nonlinearities act to amplify or phase shift the linear waves is shown to be dependent upon the vertical propagation characteristics of the waves. Large horizontal scale waves and weak zonal westerlies are found to be conditions which can result in significant amplitude changes due to the wave-wave interaction. As the scale of the wave is reduced and/or the speed of the mean zonal westerlies is increased, the predominant changes occur in the wave phases. Numerical solutions using realistic boundary forcing amplitudes and realistic dissipation are found to be only weakly nonlinear, despite the fact that the planetary waves distort considerably the polar vortex.
During the observation period (one month from 23 December 1987) of low-temperature types of snow crystals at Kautokeino, Norway, 68 twelve-branched snow crystals were collected and photomicrographed. Side views of twelve-branched snow crystals (from the direction perpendicular to the c-axes) were analyzed. The angles φ between branches were measured from the photomicrographs. Most of the twelve-branched snow crystals were composed of two double plates (or six-branched snow crystals). In the histogram of the angle φ, no evidence of the formation of twelve-branched snow crystals by the so-called rotation twinning mechanisms was found. The formation mechanisms of twelve-branched snow crystals are explained by snowflake formations for the most part, although a slight possibility of a formation by another mechanism exists.
In part I of this study, cloud variations in/around the Baiu front in July 1982 were studied. The purpose of Part II is to clarify inclusively the structure of the Baiu frontal zone and features of the Baiu frontal disturbances which would be related to the cloud variations studied in Part I, analyzing relative vorticity fields in July 1982. Attention is focused on the relationship and interaction between the upper westerly jet (W-jet) in 40°N-50°N and the Baiu low-level jet (B-jet) in 30°N-40°N. In monthly mean, the Baiu frontal cloud zone, which was formed in 30°N-40°N, coincided with a low-level positive vorticity zone associated with a B-jet. Over the continent, the positive vorticity zone was confined to the lower layer. The zone merged with the positive vorticity zone associated with the W-jet around the east coast (-120°E) of the continent. Over the region east of 120°E (Japan-the northwestern Pacific), the deep positive vorticity zone of a baroclinic structure was formed in the Baiu frontal zone. This will account for the difference in cloud features of the Baiu frontal mesoscale systems between the continent and the Japan-northwestern Pacific region described in Part I. With the change of large-scale circulation in East Asia, the relationship between the W-jet and the B-jet changed during the analysis period. Concurrently, the frontal structure, features of disturbances and cloud amounts in the Baiu front, also changed significantly in the 120°E-140°E region. Three situations of the Baiu front ((1) active-deep, (2) active-shallow and (3) inactive phases) are classified. The vertical structure of the Baiu front and features of the disturbances in these three phases are studied in detail. Over 120°E-140°°E, the Baiu frontal mesoscale disturbances showed a deep-baroclinic structure in the active-deep phase, while in the active-shallow phases they showed a shallow structure and located to south of the deep mesoscale disturbances in the northern frontal zone of the W-jet. In both active phases, the mesoscale disturbances in the Baiu frontal zone began to develop around the east coast (-120°E) of the continent under the influence of the upper-level disturbances in the W-jet propagated from Central Asia.
The relationship between the unusual summer weather over Japan and the tropical conditions in 1988 was investigated. It was shown that the cool and rainy summer over Japan can be directly explained by the weak convective activity in the tropical western Pacific around the Philippines. The subtropical high in the western Pacific did not fully develop due to the inactive tropical convection and the low-pressure zones prevailed around Japan for July and August. Analyses of wind and Outgoing Longwave Radiation (OLR) data show that the convection center corresponding to the rising center of the tropical east-west circulation (Walker circulation) was abnormally shifted westward from the normal position over the tropical western Pacific towards the Bay of Bengal. This westward shift of the convection center may result in the lack of convective activity in the tropical western Pacific probably due to the compensating downward motion. The tropical regions during the 1988 summer were characterized by the strongest La Nina condition since 1975 and above-normal SST over the Indian Ocean. It is speculated that the warmer SST over the Indian Ocean together with the extreme La Nina condition might force the convection center to move westward toward the Bay of Bengal.
The local associations between convective activity and sea surface temperature (SST), and also water vapor amount have been investigated over the western tropical Pacific using split window data (11μm, 12μm) from the NOAA-7 Advanced Very High Resolution Radiometer (AVHRR) for the period September 1983 to August 1984. SST and indices of convective activity and water vapor amount are all estimated from the split window data. Monthly means of each variable, determined over 2.5°×2.5°latitude-longitude areas, are used for this study. It is revealed that the mean depth of convection is mostly correlated with water vapor amount while convection becomes more active with the increase of both SST and water vapor amount. The convection is slightly dependent on SST for a fixed water vapor amount, while convective activity is highly related with the increase of water vapor amount at a specific SST. Deep convection areas coincide with larger amounts of water vapor rather than warmer SSTs, and convectively suppressed areas over warmer SST areas are found to have smaller amounts of water vapor. For the same SST and water vapor amount, the convection over region I (20°N-Equator, 130°E170°W) where the intertropical convergence zone (ITCZ) is dominant, is less deep than that over region II (Eq.-20°S, 150°E-170°W) where the south Pacific convergence zone (SPCZ) prevails. Deep convection (cloud tops higher than 10 km) occurs over region I when the brightness temperature difference of the split window data is greater than 2.1°C(2.0°C over region II) and the SST warmer than 28.2°C(27.2°C over region II).
In situ measurement of the solar radiation was made during 166 days from August 1989 to January 1990 over the sea south of Japan, by using a small-sized pyranometer which was attached to the moored surface buoy system. In this prompt report, the daily mean values during 135 days from August to December 1989 were reported and the monthly mean values were compared with the available data which were mostly estimated by other authors using the various empirical formulae for the experimental area.
The African atmospheric circulation as related to the long-term Sahelian drought from the late 1960's to the mid-1980's is investigated. Quantitative analysis of the tropical African rainbelt for August shows that the reduction in total rainfall is more responsible for the decreasing trend in the Sahelian rainfall over the period than the equatorward retraction of the rainbelt. This trend of the rainbelt is accompanied by increasing trends in both 700 mb heights and temperatures throughout the tropical zone for August. These trends are most pronounced over the Sahelian zone where the average trend values during the period 1959-1984 are 14.7 gpm/decade and 0.62°C/decade for geopotential and temperature, respectively. The concurrent increases in the 700 mb heights and temperatures are likely caused by the intensification of subsidence, primarily south of the Northern Hemisphere subtropical high, without a substantial displacement of its axis.