Three-Dimensional Features of Thermal Convection in a Plane Couette Flow

In the spring of 2004, we implemented intensive field observations in the eastern part of the Tibetan Plateau. In this study, we use the field observation data, as well as satellite observation data and reanalysis data, to examine increases in diurnal potential temperature and variations in atmospheric conditions over the eastern part of the Tibetan Plateau. The results of intensive radiosonde observations reveal a significant temperature increase attaining to the tropopause in April. The depth of the mixing layer, estimated from the vertical profile of potential temperatures, was much smaller than the height of the temperature increase. This result indicates that the temperature rise was not solely caused by thermally induced dry convection. During the observation period, strong cloud activity was observed. Cloud-top heights derived from GOES-9 and radiosonde observations indicate that active cumulus convection coincides with the temperature rise in the upper troposphere. For small increases in temperature, lower cloud-top heights are observed.Numerical simulations were also conducted using a regional atmospheric circulation model and radiosonde observation data as initial conditions. The results of the numerical simulations with wet and dry conditions and ideal wind direction (no meridional wind component) reveal that in the wet case air is more efficiently warmed in the upper troposphere with cloud activity. By introducing the observed wind direction as an initial condition, the simulation predicts strong cloud convection over a wide region of the model domain and a greater increase in potential temperature; this result is comparable to observed data.In previous studies, sensitive heat transfer by thermally induced dry convection was thought to be the main cause of pre-monsoon atmospheric heating over the Tibetan Plateau; however, the results of the observation analysis and numerical simulations described in the present study suggest that strong cloud convection plays an important role, even during the period prior to the rainy season.

Section: Differences In Cloud Activitymentioning

confidence: 99%

“…Asai (1970) used a constant lapse rate of temperature and constant vertical shear; however, in the present study we use values averaged over 7000 to 11000 m ASL, where wind shear is large. Ri values at 0800, 1000, and 1200 UTC on 17 April are 0.022, 0.021, and 0.023, respectively.…”

Section: Differences In Cloud Activitymentioning

confidence: 99%

Increasing Atmospheric Temperature in the Upper Troposphere and Cumulus Convection over the Eastern Part of the Tibetan Plateau in the Pre-Monsoon Season of 2004

Taniguchi

Koike

2007

“…Yagi (1985) and Yagi et al (1986) analyzed GMS images and upper-air sounding data and indicated that the axes of the transversal cloud bands were parallel to the vertical wind shear vector of horizontal winds. In addition, by applying the linear theory of the thermal instability in the vertical shear flow (Asai 1970a(Asai , 1970b(Asai and 1972, Yagi (1985) proposed that the transversal cloud bands were produced from longitudinal-mode (shear-parallel mode) roll convections. Recently, Shimizu and Tsuboki (2005) analyzed the transversal cloud band near the coast of the Hokuriku district on the basis of dual-Doppler radar observations.…”

Section: Introductionmentioning

confidence: 99%

The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

Eito

Murakami

Muroi

et al. 2010

During a cold-air outbreak, a broad cloud band is occasionally observed over the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) that forms over the Sea of Japan from the base of the Korean Peninsula to the Japanese Islands. On 14 January 2001, a broad cloud band associated with the JPCZ (JPCZ cloud band) extended in a southeastward direction from the base of the Korean Peninsula to Wakasa Bay, and it stagnated for half a day. The JPCZ cloud band consisted of two cloud regions: one was a long cloud band extending along its southwestern edge (a developed convective cloud band), and the other was the region consisting of cloud bands normal to a wind direction of winter monsoon (transversal cloud bands). The structure and formation mechanism of the transversal cloud bands were examined on the basis of observations (e.g., satellite images, in situ measurement and cloud-profiling radar data from an instrumented aircraft and upper-air soundings from observation vessels) and simulation results of a cloud-resolving model with a horizontal resolution of 1 km.The transversal cloud bands had the following characteristic structures; they extended along a northeastsouthwest direction, which was parallel to the direction pointed by the vertical shear vector of horizontal wind Corresponding author and present a‰liation: Hisaki Eito, Numerical Prediction Division, Forecast Department, Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan. E-mail: heito@naps.kishou.go.jp 1 Present a‰liation: Numerical Prediction Division, Forecast Department, Japan Meteorological Agency. 6 2010, Meteorological Society of Japan in the mixed layer, they mainly consisted of convective clouds, which slanted with height toward the downshear side, and they widened and deepened toward southwest, as the depth of the mixed layer increased. An examination of simulation results presented that the transversal cloud bands were accompanied by roll circulations. The axes of rolls were oriented nearly parallel to the direction of the vertical shear vector in the mixed layer. An analysis of the eddy kinetic energy budget indicated that the roll circulations derived most of its energy from the mean vertical shear and the buoyancy.

“…This result suggests that the formation process of ice/snow particles plays an important role in the formation of the three-dimensional kinematic structures of longitudinal-mode snowbands. Although linear theories succeeded in explaining the formation of cloud streets by horizontal roll vortices (e.g., Asai 1970;Brown 1970;Kuettner 1971), they cannot explain the three-dimensional kinematic structures of longitudinal-mode snowbands. Kobayashi et al (1992) observed the merging phenomena of longitudinal-mode snowbands.…”

Section: Introductionmentioning

confidence: 99%

A Dual-Doppler Radar Study of Longitudinal-Mode Snowbands

Yoshimoto

Fujiyoshi

Takeda

2000

Two longitudinal-mode snowbands (bands I and II) were observed over the Ishikari Bay, Hokkaido, Japan during a wintertime cold-air outbreak. The three-dimensional kinematic structure of a snowband (band II) was examined in detail using dual-Doppler radar data. Band II noticeably developed over the Ishikari Bay. A high-reflectivity (approximately 35 dBZ at the maximum) zone was formed along the band axis and characterized the radar-echo structure of band II. The high-reflectivity zone of band II had the airflow structure dominated by circulations in vertical cross sections perpendicular to the band axis.The interactions between the two snowbands were discussed. Interestingly, it was found that radarecho bridges existed at the low levels between the two snowbands. The radar-echo bridges were formed in association with low-level outflows from the meso-y-scale convective cloud systems composing band I. The low-level outflows moved toward band II with time and penetrated into band II. This caused strong low-level convergence and the enhancement of updrafts in band II. Consequently, stronger radar-echoes were formed in band II and band II rapidly developed. Ice/snow particles were transported from band I into band II by the low-level outflows. It was considered that the rapid growth of these particles in the enhanced updrafts in band II would have contributed to the rapid development of band II.