2.5 Upper Tropospheric Cyclonic Vortices
- Some characteristics of the upper tropospheric cyclonic vortices are:
- (a). The upper tropospheric vortices usually do not extend below 20,000 feet (about 6 km).
- (b). A weak inverted wave in the easterlies is usually found beneath these vortices. They may be associated with broad areas of high clouds.
- (c). Downward growth of an upper-level vortex results in the appearance of a surface vortex and an increase in convective clouds.
- (d). On rare occasions the vortices become warm-core and develop into TCs.
- (e). Vortex movement is usually slowly west-southwestward.
Sadler (1964; 1967a,b; 1976a,b) documented the TCs of the eastern North Pacific as revealed by satellite observations, and proposed that the upper tropospheric circulation is a controlling factor in the development as well as the life history of TCs. Using charts of mean 200-hPa circulation for July and August to indicate the locations of circumpolar troughs and ridges, Sadler pointed out that there is one trough line extending over the eastern and central North Pacific (see Fig. 2.14 for July chart) and another trough line extending over the North Atlantic (see Fig. 2.15 for July chart). The upper tropospheric cyclonic vortices can form in the divergence regions associated with the trough and occasionally expand to the lower troposphere. This kind of trough is now referred to as tropical upper tropospheric trough or TUTT.
- Bohan (1974a,b) performed case studies of upper tropospheric cyclones in the Atlantic and Pacific by using geostationary satellite cloud imageries, cloud tracked winds (lower and upper tropospheric winds), airplane reports (winds, temperatures and heights) and radio-sounding data. Bohan identified two classes of upper tropospheric cyclonic vortices; they are an upper tropospheric cold-core low and a cut-off low. The tropical upper tropospheric cold-core low has the following four characteristics:
- (a). An in-phase relationship between the upper tropospheric cold- core low and lower tropospheric easterly wave trough can enhance the convection. An out-of-phase relationship between them can suppress the convection.
- (b). The upper tropospheric troughs are frequently associated with troughs in the sutropical westerlies. When the subtropical disturbances in the northern hemisphere actively move southward, the area between the upper tropospheric anticyclone (or ridge) to its west and cold-core low (or trough) to its east usually has strong northeasterly flows and fast development of active convection.
- (c). The cloud bands associated with the upper tropospheric cyclonic vortices are typically aligned with the vertical wind shear.
- (d). The animated satellite cloud imagery is a better tool for early dection and tracking of the upper tropospheric cyclonic vortices.
- The cut-off has following two characteristics:
- (a). The low-level convergence associated with a cut-off low can initiate squall lines and heavy seas.
- (b). The low-level spiral cloud bands associated with a cut-off low are parallel with the low-level wind direction.
- The upper tropospheric cyclonic vortices have regional differences. Atkinson (1971) reviewed the characteristics of upper tropospheric cyclones in the following four regions:
- (a). North Pacific. In the western North Pacific, there are high correlations between the locations of formative TCs and that of the lower tropospheric monsoon troughs and the TUTT. The TUTT usually occurs from May to November, and is most active between July and September. Sadler (1976a) proposed a revised synoptic model for the TUTT during the early typhoon season (i.e., June-August) in the western Pacific. Sadler (1976a) suggested that the TUTT cyclonic cells originate from the dynamic coupling between the TUTT and transitory mid-latitude disturbances when the subtropical high between these systems is unusually weak.
- (b). North Atlantic. The upper tropospheric trough is used to describe the major semi-permanent circulation pattern that develops in the North Atlantic during the period from August to November. Carlson (1967) analyzed synoptic data over the eastern Caribbean for October 1965 and identified the presence of an upper tropospheric cold core cyclone. These cold core cyclones usually originate near the Azores and move westward along the latitude 20°N. The systems extend over an area of about 20 degrees of latitude and 40 degrees of longitude. The lowest level of a closed circulation associated with the upper cold core cyclone is commonly from the 700 to 500-hPa level. This feature in the North Atlantic is different from that in the North Pacific. Most of the systems are detectable in the low tropospheric temperature field as cold troughs in the easterlies. The cyclones tend to tilt vertically toward the northeast. Convective clouds and rainfall occur in the southeast quadrant, about 5 degrees latitude from the upper cyclone center. Large variations of cloudiness can exist from one system to the next.
- Sadler (1967a) pointed out that the summer TUTT is a dominant feature over the trade wind regions of the North Atlantic Ocean, the Gulf of Mexico and the Caribbean Sea. Sadler also noted that lower tropospheric responses to the TUTT in the North Atlantic are different than those in the North Pacific.
- (c). South Pacific. Tropical cyclones develop during the warm season in the areas below the tropical upper tropospheric trough over the South Pacific. The mean location of the trough is in the area from the equator to 30°S and from 180°E to 120°E. Sadler (1976a) stated that the summer TUTT in the Southern Hemisphere lies over the trade wind region of the east central Pacific and can induce TCs.
- (d). Central America. A rainy, southwest wind, resulting from a deflection of the southeast trades of the eastern South Pacific onto the Pacific coasts of Central America during the period from June to November, is called the "temporale" (Huschke, 1959; Atkinson, 1971). Temporales are most frequent in July and August, when they can reach gale force wind speed and raise a heavy sea and swell. The area of heavy rain is usually located in the northeast quadrant about 5 degrees of latitude from the cyclone center.
Section 2.4 Section 2.6