Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Kunio Yoneyama x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: All Content x
Clear All Modify Search
Kunio Yoneyama and David B. Parsons

Abstract

Recent studies using data from the Tropical Ocean and Global Atmosphere program’s Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) have shown that synoptic-scale areas of extremely dry air can occur in the troposphere over the equatorial western Pacific. These layers of extremely dry air modify convective activity and the vertical profile of radiation in clear air. At the present time there is some disagreement as to the dynamic mechanism responsible for these events and a number of their characteristics are relatively unknown. In this study, the origin and characteristics of the dry air events were investigated through analysis of TOGA COARE rawinsonde data and examination of global analyses from two different forecast centers. These drying events were found to be very common and evidence was presented that their intensity was underestimated in the global analyses. These dry events were shown to most often originate in the Northern (winter) Hemisphere as troughs associated with baroclinic waves intensified and expanded equatorward, leading to a process analogous to Rossby wave breaking. In these cases, the dry air at the edge of the westerlies at upper levels was incorporated into the equatorward extension of thin NE–SW tropospheric troughs, where it subsided and was subsequently advected equatorward. If sufficient subsidence took place, the dry air continued flowing equatorward on the eastern edge of well-defined anticyclones in the lower troposphere. The dry air in one case originated in a Southern (summer) Hemisphere trough that was associated with midlatitude baroclinic waves that propagated equatorward and developed into a series of distinct disturbances along a subtropical jet. In both the Northern and Southern Hemisphere events, the subsiding dry air in the midtroposphere was injected into the fringes of the Tropics, where it was able to reach equatorial regions if it interacted with favorable meridional flow in the Tropics. Past studies have proposed that these intrusions of dry air could induce droughts in the Tropics through decreasing deep convective activity. The implication of this study is that these droughts are actually induced by midlatitude processes.

Full access
Tsutomu Takahashi, Kunio Yoneyama, and Yukimasa Tsubota

Abstract

Data obtained from aircraft observations conducted around the island of Hawaii during the 1985 Joint Hawaii Warm Rain Project was analyzed pertaining to the microphysical, thermodynamic and dynamic characteristics of rainbands. Analysis focused upon rain duration and rain accumulation.

The most outstanding feature of long-lasting trade-wind rainbands is that unlike other storm systems, dry air does not intrude into the main cloud system at low levels.

A certain rainwater accumulation process seems to occur that produces heavy rainfall. This is accompanied by the production of a large amount of drizzle in the middle levels of the cloud. An increase in the drop growth rate within cloud cells above zones of low level convergence, as well as drop recirculation, influence efficient production of a “drizzle pool”.

Full access
Hiroyuki Yamada, Kunio Yoneyama, Masaki Katsumata, and Ryuichi Shirooka

Abstract

The multiscale structure of a super cloud cluster (SCC) over the equatorial Indian Ocean, observed in November and December 2006, was investigated using data from satellite microwave sensors and surface-based radars. The smaller-scale structure of this SCC was marked by a complicated relationship between rainfall systems and upper-tropospheric cloud shields, which moved eastward and westward, respectively, with a cycle of 2–4 days. In the analyses, attention was given to the structure of slow eastward-propagating (5–11 m s−1) precipitating systems and related synoptic-scale (∼2000 km) disturbances. A case study of one of the systems revealed that it consisted of several lines of convective cells with a depth that was usually shallower than 10 km unless the cells encountered the westward-moving cloud shields. The environment of the convective lines was characterized by persistent unstable conditions with an increase of the westerly flow in the lower troposphere, suggesting the existence of a synoptic-scale upward motion. Composite analyses revealed that each rainfall system formed in a region of zonal flow convergence near the surface and divergence near 300 hPa. The vertical temperature structure tilted westward with height below this pressure level and eastward aloft, similar to that of a convectively coupled Kelvin wave. These results suggest that a SCC involves a group of synoptic-scale shallow waves propagating eastward. An additional analysis over the western Pacific also showed the predominance of eastward propagation in a SCC, demonstrating the advantage of satellite microwave sensors over infrared ones in monitoring the multiscale structure of tropical convection.

Full access
Kazuaki Yasunaga, Kunio Yoneyama, Hisayuki Kubota, Hajime Okamoto, Atsushi Shimizu, Hiroshi Kumagai, Masaki Katsumata, Nobuo Sugimoto, and Ichiro Matsui

Abstract

In this study, cloud profiling radar and lidar were used to determine the frequency distribution of the base heights of cloudy layers with little (or no) falling condensate particles. The data were obtained from stationary observations conducted from Research Vessel Mirai over the tropical western Pacific (around 1.85°N, 138°E) from 9 November to 9 December 2001. The observed cloudy layers had base heights predominantly in the range of 4.5–6.5 km. Almost all cloudy layers with a base in the range of 4.5–6.5 km had thickness thinner than 500 m, and the frequency peak of the base heights of measured cloudy layers is considered to represent the common occurrence of midlevel thin clouds.

Midlevel thin clouds were frequently observed even during the active phase of the Madden–Julian oscillation (MJO). Composite analysis of radiosonde-derived relative humidity and temperature lapse rate indicates that the midlevel thin cloud in the MJO active period is generated via melting within the stratiform cloud, rather than by detrainment of surface-based convection.

Full access