Strong Convective Instability Research Articles

Tower mast of precipitation over the central Tibetan Plateau summer

Over the Tibetan Plateau, solar heating often produces strong convective instability in the atmosphere. Using 3 years (1998–2000) of Tropical Rainfall Measuring Mission (TRMM) precipitation radar data, our investigations revealed a tower mast shape of precipitation over the Plateau in both height‐longitude and height‐latitude cross‐sections. High rain rate center over the Plateau is located above 6km as a tower penetrating into the mid‐troposphere against the nearby background, implying a unique latent heating source injecting directly to the middle atmosphere. Results indicate that there are more isolated rain cells over the Plateau than its nearby regions, and the strongest diurnal cycle of rainfall existing over the Plateau with a peak around 1600 and a valley around 0500 local time, indicating the dominance of convective clouds caused by solar heating. It is also found that the TRMM algorithm might have misclassified weak convections as stratiform rains.

Radiative Heat Transfer and Hydrostatic Stability in Nocturnal Fog

We have performed a one-dimensional and transient radiative heat transfer analysis in order to investigate interaction between atmospheric radiation and convective instability within a nocturnal fog. The radiation element method using the Ray Emission Model (REM2), which is a generalized numerical method, in conjunction with a line-by-line (LBL) method, is employed to attain high spectral resolution calculations for anisotropically scattering fog. The results show that the convective instability has a strong dependence on radiative properties of the fog. For the condition of a 20-μm droplet diameter and liquid water content of 0.1 × 10−3 kg m−3;, the temperature profile within the fog becomes “S” shaped, and a convective instability layer forms in the middle or lower level of the fog. However, for the same water content and a 40-μm diameter droplet, no strong convective instability layer forms, whereas for a 10-μm diameter droplet a strong convective instability is observed.

Formation and Maintenance Processes of a Stationary Band-shaped Heavy Rainfall Observed in Niigata on 4 August 1998.

On 4 August 1998, a torrential rainfall was observed in the Niigata area, in central Japan. The rainfall was induced by a quasi-stationary rainband aligned in the east-southeast direction from the west of Sado Island. The rainband lasted for several hours with the characteristic features of a back-building formation, in which new convective cells were successively generated in an upstream direction. The rainfall area and intensity simulated by a nonhydrostatic mesoscale model with a 5-km horizontal grid agreed well with the observation. The rainband with a back-building nature was reproduced successfully by a nonhydrostatic model with a 2-km grid. The simulation results showed that the heavy rainfall formed as a result of strong convective instability enhanced by low-level humid and middle-level dry inflows from the southwest into the weak convergence region associated with the Baiu frontal zone. Sensitivity experiments indicated that neither the terrain of Sado Island, nor the evaporative cooling of raindrops affected the formation of the rainband. Although moved to the downstream when the low-level wind convergence was not enhanced enough, the simulated rainband stagnated after the convergence became so strong that the whole low-level air could be lifted upward while crossing the convergence zone. Therefore, the quasi-stationary rainband was maintained by the convection-enhanced low-level wind convergence existing upstream of the rainband. Over the convergence zone associated with the quasi-stationary rainband, new convective cells successively formed and separated from the zone after having developed higher. This successive formation of new cells could maintain a back-building formation. Such a back-building formation was found at several points over the wind convergence zone. These meso-β-scale convective systems with a back-building nature produced the precipitation region with a length over 200 km.

Dynamical Influence of Microphysics in Tropical Squall Lines: A Numerical Study

Based on environmental conditions of the 22 and 23 June squall lines during the Convection Profonde Tropicale in 1981 (COPT81) experiment in West Africa, a series of numerical simulations are performed with a two-dimensional nonhydrostatic cloud model to examine the dynamical effect of microphysics in tropical squall lines. The role of ice-phase microphysics strongly depends on ambient conditions. For the environment of strong convective instability, the ice phase is important regarding the system-scale structure but is not important to the convective-scale dynamics. On the other hand, the ice influence is crucial to the squall-line convective system if the environment has a weak convective instability and is almost saturated at low levels.

Chemico-physical processes influencing acidic precipitation; the case study of Milan urban area

The aim of the model presented in this work is to describe the acidic contents of cloud droplets' temporal evolution, both during their formation process and the following stage of precipitation. The attention is mainly focused on precipitations in which the contribution of local polluted air masses, typical of big urban areas, is predominant. Some phenomena are particularly taken into consideration: the exchange processes of those pollutants mostly influencing thepH, from the gas to the liquid phase; the process of sulphur oxidation in the liquid phase and finally the phenomenon of inclusion of the atmospheric particulate through interception processes. The model has been useful in the analysis of experimental data about the precipitations in Milan urban area from 1st January 1991 to 30th June 1992. For istance, it has been possible to explain both a rapid decrease ofpH, happening with strong convective instability, and the significant washout of basic particulate, taking place during the first minutes in most of the events. The sensitivity analysis has also shown thatpH depends on ammonia concentration and on temperature in a significant way.

中国東部における積乱雲群出現の日変化と総観場の解析(1979年6～8月)

The appearance frequency of Cb-clusters and its diurnal variation were examined in relation to large-scale conditions based on the observational data from June to August 1979. The analysis was applied to the three stages in the seasonal march in that year, i.e., Pre-Meiyu (1-17 June), Meiyu (20 June-22 July) and Mid-summer (23 July-17 August). The main results are as follows. (1) The Cb-clusters appeared frequently not only in the Baiu frontal zone but also in North to Northeast China (Area N1) in Meiyu, and in Central China (Area C2) in Mid-summer. A remarkable diurnal variation of appearance frequency of the Cb-clusters with an evening maximum was observed in Area N1 in Meiyu and Area C2 in Mid-summer. The daily maximum of ground surface temperature in these two cases was very high. (2) In Area N1 in Meiyu, the Cb-clusters with diurnal variation were mainly embedded in large-scale cloud systems corresponding to slow-moving upper-level lows or troughs. The specific humidity in the lower layer increased from Pre-Meiyu to Meiyu. These two factors, as well as the strong surface heating, would provide a favorable condition for the frequent appearance of Cb-clusters due to the increase of convective instability. The large-scale horizontal convergence in the lower layer was stronger at 12 UTC (20 BST) than at 00 UTC (08 BST), which would lead to the development of Cb-clusters in the evening. (3) Area C2 was covered by the subtropical high, and the low-level divergence was generally stronger at 12 UTC than at 00 UTC. In spite of these conditions, the isolated Cb-clusters developed very frequently in the evening. The very strong surface heating in the daytime could contribute to release of the strong convective instability by generating meso-scale upward motion as a trigger.

Alfén waves in a thermally stratified fluid

The properties of Alfven waves propagating along a uniform horizontal field in a highly conducting incompressible medium in the presence of strong convective instability are examined in the Boussinesq approximation. In particular, it is sought to determine whether there are exact solutions to the dynamical equations in the presence of convective forces. It is shown that a class of exact solutions of arbitrary amplitude, but of limited form, which may be of some physical interest, does exist. For large amplitudes, any mixtures of polarization states are shown to cause scattering into new modes.

A new interpretation of luminous blue stars

A major revision of current theoretical ideas about the brightest blue stars must be made if Carson's (1976) radiative opacities are adopted in stellar models. Unlike earlier opacities, these exhibit a large 'bump' due to CNO ionization, which leads to very strong central condensation, convective instability, and pulsational instability in hot diffuse stellar envelopes. Despite a number of theoretical uncertainties, the new picture of the structure of very luminous stars is reasonably successful in accounting for a variety of previously unexplained observations. The stellar models for the phase of core hydrogen burning predict large radii and rather cool effective temperatures for O stars and a spreading out of the main-sequence band in the H-R diagram toward luminous cool supergiants for masses higher than about 20 solar masses. In massive X-ray binary systems, circular orbits and supergiant-like visual companions are expected to be quite common. Long-period variability is predicted to exist for massive blue supergiants of luminosity class Ia. The models for helium stars predict large radii and rather cool effective temperatures for Wolf-Rayet stars, as well as multimodal pulsational instability and, possibly, surface turbulence for these stars.