Initiation Of Moist Convection Research Articles

Impact of upstream flow conditions on the initiation of moist convection over the island of Corsica

The influence of upstream flow on the initiation of moist convection over Corsica is investigated using semi-idealized simulations. The simulations, performed with the Consortium for Small-scale Modeling (COSMO) model, are initialized with one vertical profile at all grid points and under constant boundary conditions. The upstream wind velocity and wind direction, as well as the saturation deficit and stability, are systematically varied to estimate the impact of the individual factors on the initiation and strength of moist convection. Also, the influence of radiation is evaluated. It is found that deep convection can only develop with high solar radiation. With lower radiation, thermal wind systems are weak, and only shallow convection is triggered. Results show that with strong solar radiation, a strong upstream wind velocity is less efficient in triggering deep convection than a weaker one. This is due to the dynamic effects on the flow field. A low upstream wind velocity enables thermal wind systems to become dominant and the preconvective conditions for deep convection are more favorable. This leads to more events with deep convection and precipitation. The wind direction can be the determining factor for the initiation of deep convection and also controls the location of precipitation. A change of just 15° can decide on whether deep convection occurs or not. With a high upstream wind velocity, the location is always on the lee side of the island. With a lower upstream wind velocity, two hot spots for convection initiation, one in the northern central part and one in the western part of the island, are identified. It is further found that the impact of saturation deficit is higher than the impact of stability. With increased instability, the atmospheric conditions are more favorable for the development of convection, but the high saturation deficit in the middle troposphere prevents the formation of deep convection. Through reduction of the saturation deficit, multiple convective cells can develop in the more humid atmosphere. Additionally, the integrated water vapor is larger, which means that more precipitable water is available allowing larger amounts of precipitation.

Formation of the Convective Lines off the Mountainous Coast of Southeastern Taiwan: A Case Study of 3 January 2004

Abstract Convective lines frequently occurring off the mountainous coast of southeastern Taiwan under weakly synoptically forced weather conditions are one of the most well-known mesoscale phenomena in Taiwan. These lines usually develop close to the coast but frequently can also be observed well offshore. While nearshore lines are better understood, the formative processes of the offshore lines remain unclear. The main objective of this study is to use various available observations (e.g., Doppler radar, surface and sounding observations, QuikSCAT, and NCEP data) collected over southeastern Taiwan to investigate a specially chosen case on 3 January 2004, in an attempt to identify possible mechanisms leading to the formation of the offshore lines. The studied line was formed ∼50–60 km away from the southeastern coast of Taiwan and oriented south-southwest to north-northeast and approximately parallel to the coast. Detailed analyses of the event indicated that the formation of the line did not appear directly relevant to coastal land/sea breezes or offshore flow. Instead, orographic forcing was observed to play a crucial role in the initiation of the line. Particularly, an orographically forced northerly flow due to upstream blocking was evident to the west of the line. The convergence generated as the prevailing northeasterly onshore flow encountered the nearshore blocked flow was found to be an important low-level forcing conducive to the initiation of moist convection associated with the line. In addition, the persistence of the coastally blocked flow and the degree of ambient convective available potential energy (CAPE) were also shown to be closely related to the maintenance and overall intensity of the observed line’s convection. The orographically induced convective forcing identified in this study is distinctly different from the conceptual model of line formation proposed by previous studies of the nearshore lines. It is thus strongly suggested that multiple mechanisms may exist upstream of coastal mountains in southeastern Taiwan, which act to initiate the lines with diverse formative locations relative to the coastline.

The Initiation of Moist Convection at the Dryline: Forecasting Issues from aCase Study Perspective

Abstract The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 field projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression. The results of the present study confirm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three ...