Scientists are concerned Florida's unprecedented water temperature could lead to bleaching this year. Pileus are often very short lived because the rising cloud tower that caused it often keeps rising through the pileus and obliterates it.Struggling or dead corals in the mass bleaching event in the Florida Keys in 2014. If humid layers of environmental air exist above rapidly rising cumulus towers, then pileus clouds can form as the environmental air is pushed up and out of the way of the rising cumulus clouds. The flanking line forms along and above the gust front, which marks the leading edge of colder outflow air from the rear-flank downdraft ( RFD). Cumulus congestus are also informally called towering cumulus ( TCu). The flanking line is a band of cumuliform clouds that increase from the medium-size cumulus mediocris ( Cu med) furthest from the storm to the taller cumulus congestus ( Cu con) close to the main updraft. Storms that have overshooting tops are often more violent and turbulent. This is caused by the inertia of the upward moving air in the main updraft, which overshoots above its neutrally buoyant equilibrium level ( EL). 14.4) is a smooth, flat, narrow, low-altitude cloud that extends along the boundary between the inflow of warm moist air to the thunderstorm and the cold air from the rain-induced forward flank downdraft (FFD).Ī dome of overshooting clouds sometimes forms above the anvil top, above the region of strongest updraft. Not all gust fronts have arc clouds, particularly if the displaced air is dry. Often the undersides of arc clouds are dark and turbulent-looking, while their tops are smooth. 14.4), usually associated with the flanking line. These cloud bands mark the leading edge of gust-front outflow from the rear-flank downdraft (Fig. 14.2b) or shelf clouds form near the ground in boundarylayer air that is forced upward by undercutting cold air flowing out from the thunderstorm. Not all thunderstorms have all these associated clouds.Īrc clouds (official name arcus, Fig. Sometimes you can see other clouds attached to thunderstorms, such as a funnel, wall, mammatus, arc, shelf, flanking line, scud, pileus, dome, and beaver tail (Fig. Figure 14.3 Photo of supercell thunderstorm.ġ4.1.2. The most severe, long-lasting, less-frequent thunderstorms are supercell thunderstorms (Figs. More complex thunderstorms can have one or more updraft and downdraft regions. However, in this book we will use the word thunderstorm to mean any cumulonimbus cloud, regardless of whether it has lightning. Such storms are technically not thunderstorms. Not all cumulonimbus clouds have lightning and thunder. When viewed from the ground under the storm, the main updraft often has a darker cloud base, while the rainy region often looks not as dark and does not have a well-defined cloud base. When the downdraft air hits the ground it spreads out, the leading edge of which is called the gust front. Also in the stem is a downdraft with precipitation. When this rising air hits the tropopause, it spreads to make the anvil. Within the stem of a mature thunderstorm is the cloudy main updraft tower topped by an updraft bubble (Fig. (c) Horizontal composite, showing the anvil at storm top (as viewed from above by satellite), the precipitation in the low-to-middle levels (as viewed by radar), and the gust front of spreading winds at the surface. Light shading indicates clouds, green and red shadings are moderate and heavy precipitation, and arrows show air motion. Figure 14.2 (a) Sketch of a basic (airmass) thunderstorm in its mature stage. For a storm with a larger anvil that looks strongly glaciated (i.e., has a fibrous appearance associated with ice-crystal clouds), then you would call the cloud a cumulonimbus capillatus. If the thunderstorm top is just starting to spread out into an anvil and does not yet have a fibrous or streaky appearance, then you identify the cloud as cumulonimbus calvus (see the Clouds Chapter). Figure 14.1 Airmass thunderstorm having a single mature cell. The anvil extends furthest in a direction as blown by the upper-tropospheric winds. The large top is called the anvil, anvil cloud, or thunderhead, and has the official name incus (Latin for anvil). 14.1 & 14.2) has a nearly vertical stem of diameter roughly equal to its depth (of order 10 to 15 km). \)Ī mature thunderstorm cloud looks like a mushroom or anvil with a relatively large-diameter flat top.
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