Cloud streets are frequently observed when a cold air mass advects over a warm sea surface in the winter season; thick and long cloud streets can be sometimes seen in the lee of some islands or mountains near a coast. According to the conventional theory, the vertical shear and static instability play an important role in the formation of the thin cloud streets. For the thick cloud streets in the lee of the isolated mountain, however, another mechanism seems to be important in addition to the conventional theory. The effects of the two factors, strong static instability and topographically induced mechanical disturbance, are investigated by use of the CSU RAMS (Regional Atmospheric Modeling System) with high horizontal and vertical resolution. In the calculation, uniform stratification and wind velocity (low Froude number) are assumed at the inflow boundary. To clarify the importance of both the strong static instability and the topographically induced mechanical disturbance on the formation and maintenance of cloud streets, three kinds of numerical experiments with different sea-surface temperature were carried out, including a numerical experiment without a mountain. Cloud streets were successfully simulated in the lee of an isolated mountain near a coast, with the addition of a large sensible heat flux at the sea surface. Well developed cloud streets occur in the simulation between a pair of convective rolls below a height of 1 km over the sea. The following five results were obtained: 1) For the formation of the pair of convective rolls, both strong static instability and a topographically induced mechanical disturbance are required at the same time. 2) Strong sensible heat flux from the sea surface is the main source of the convective rolls (the buoyancy caused by the condensation process in the cloud is negligibly small). 3) The pair of convective rolls contain two sub-rolls. One is the outer roll, which has a large radius but a weak circulation, and the other is the inner roll, which has a small radius but a strong circulation. The former gathers a large amount of moisture by convergence in the lower marine boundary layer, and the latter transfers the convergent moisture to the upper boundary layer by the strong vertical motion between the rolls. 4) The pair of inner rolls form the line-shaped cloud streets, and keep them on a narrow line along the center-line of the domain. 5) Cloud streets can be simulated both by non-hydrostatic and by hydrostatic models, which implies that vertical inertia is not always important for cloud-street formation. The horizontal scale of the topography does not seem to be restricted to within the small scale, where non-hydrostatic effects are important.
A new fast-response ozone photometer which uses ultraviolet (UV)-absorption is described. Its three UV-absorption cells each cycle among three modes: reference, sample and transition modes. The three cells are differentially phased, enabling the ozone concentration to be determined in a cell in one second without interruption. The system is efrective for measurements requiring a fast response, such as measurements on aircraft. We made a trial instrument that adopts this system and confirmed its theoretically predicted operation. The trial instrument showed a significantly faster response than earlier types of ozone photometers. This system may be applicable to other gas measurements that uses UV-absorption.
The JMA global model at T106 horizontal resolution has been integrated for 10 years with observed SST. Intraseasonal variations in the tropics during N.H. summer have been examined using 30-60 day filtered anomalies from the model simulated results. Eastward propagation of velocity potential anomalies on a 30-60 day time scale has been noticed throughout the equatorial region with a period of about 38 days. Composite analyses have been made using the principal components of an EOF analysis of the upper tropospheric velocity potential anomalies. The observed characteristics of the oscillation have been simulated by the model reasonably well. The equatorial oscillation modulates the strength of the Indian monsoon and plays an important role in deftning its intraseasonal variability on a 30-60 day time scale. The model simulates well the northward propagation of the rainfall anomalies from the equatorial Indian Ocean to the Indian region. An EOF analysis has been made using the filtered rainfall anomalies in the Indian longitudes. Composite analyses of rainfall, wind and surface fields have been made using the principal components of the EOF analysis. Circulation features in the different phases of the northward propagation resemble that of active and weak monsoon conditions and agree well with the observed analyses. It it noticed that latent heat flux variations over the sea on a 30-60 day time scale associated with the modulation of large scale monsoon circulation by the eastward propagating equatorial intraseasonal mode play a more important role than the land surface processes in the northward propagation.
The distribution of anomalies of 500 hPa height over the North Pacific is examined with special reference to the occurrence of anomalous southward first Oyashio intrusions east of Japan. EOF analysis of 50O hPa height anomalies over 1967-86 is used to identify the correlation with the occurrence of the anomalous southward Oyashio intrusion. It is shown that the southernmost latitude of the Oyashio has a significant maximum lag correlation of three month to the first EOF component of 500 hPa height anomalies with a southward development of the Aleutian Low. It is also shown that during the period of occurrence of the anomalous southward intrusion of the Oyashio, negative SST anomalies are located in the zonal area over 30°N-50°N in winter. A part of the cooling region is accompanied by the weakened wind stress with the westward direction of the wind stress anomaly, indicating that the cooling can not be attributed to either the sensible/latent heat fluxes or Ekman transport. It is indicated that the southward shift of the subarctic circulation in response to the southward shift of the Aleutian Low results in the negative SST anomalies.