TOPOGRAPHICALLY INDUCED CONVECTIVE CLOUD PATTERNS

TOPOGRAPHICALLY INDUCED CONVECTIVE CLOUD PATTERNS The conditions that a forecaster must monitor to determine the ability of a convergence zone to trigger deep convection are: Boundary layer wind direction and speed with respect to the convergence zone Amount of buoyant energy available for convection Depth and magnitude of lifting required to initiate convection Vertical wind shear.

Convection associated with sea-breezes Conceptual model of idealized sea and land-breeze circulations Fig 1: Land is on the right and Sea on the left. P0 to P3 are pressure surfaces (from Pielke, 1981)

TOPOGRAPHICALLY INDUCED CONVECTIVE CLOUD PATTERNS Fig 2: GOES-7 1 km resolution VIS Image over central Florida at 1200 local time on 18 Sep 87. The sea-breeze front can be seen at A and B; cloud clears downwind of a Lake at C.

Convection associated with sea-breezes Fig. 3. NOAA-7 AVHRR images over the British Isles at 1437 UTC on 7 July 1983: (a) VIS. (b) IR. The sea-breeze front can be seen at A and B. with deeper convection at C and D, and at E where breezes from opposite coasts meet.

Convection associated with sea-breezes The position and subsequent movement of convergence zones in which convection develops are influenced by: coastline shape low-level wind direction and speed temperature difference between land and sea surface, which can be affected by the presence of cloud cover and the diurnal cycle The depth of convection within these bands is often too shallow for precipitation. However, given a suitably unstable environment, these zones can become the focus for vertical motion and deep convection with shower or thunderstorm development. Examples can be seen at C and D in Fig. 3.

Effects of the coastline shape: light winds Locally, the shape of the coastline plays an important role in the development of convection along sea-breezes. In Fig. 4, the satellite image shows stronger convective development where the sea-breezes converge from opposite directions, at B, as well as heavier convective cells associated with peninsulas, at C and D. The preference for strong convection where sea-breezes merge in unstable environments is also evident in Fig.3 at E.

Effects of the coastline shape: light winds Fig. 4. GOES-7 2 km resolution VIS image over Florida at 1445 local time on 11 June 1989. Cloud has cleared downwind of a lake at A, and coastline shape has helped convection to develop on the sea-breeze front at B. C and D.

Effects of the coastline shape: light winds Fig. 5. Conceptual model of the effect of coastline shape on convergence zones associated with sea-breezes.

Effects of the coastline shape: light winds Fig 6. Cloud averages for the southeast USA for June 1986: (a) cloud in VIS images at 1200 local time, (b) cloud colder than —32 °C in IR images at 1400 local time. A is associated with the sea-breeze front and B with the Appalachian Mountains.

Effects of the coastline shape: light winds High frequency of cloudiness over the entire land mass of the southeastern USA, while clear skies are preferred offshore. A similar IR composite for cloud tops colder than —32 °C, Fig. 6(b), shows that a large proportion of the stronger convection takes place near the location of the sea-breeze front in coastal areas, especially over peninsulas (e.g. at A). (In this composite, there is also an area of preferred convective activity located over the Appalachian Mountains near B.) Such climatologies are helpful for understanding phenomena such as the sea-breeze, and they become especially useful as forecasting aids when classified by boundary layer wind flow.

Effects of the coastline shape: moderate winds Cloud lines and bands of showers often form over peninsulas and extend downwind inland; this is especially true with moderate low-level winds parallel to the peninsula’s axis and small directional wind shear with height. If these bands form in an unstable air stream, they can be the focus of frequent and prolonged shower or thunderstorm activity. Adjacent cloud-free zones usually extend a considerable distance downwind of the peninsula’s associated bays and estuaries (see also Fig. 6.3.11). This allows inferences to be made regarding likely areas for shower activity. Examples of such bands can be seen in Fig. 7(a) along the lines AA and BB, from southwest England and southwest Wales. The UK weather radar network image in Fig. 6.3.7(b) shows the precipitation distribution associated with the bands. Although mostly light, there were some heavier cells within the bands. On this occasion the British Isles were affected by a fairly unstable southwesterly airstream of moderate speed, Fig. 6.3.7(c). The streamline chart, Fig. 7(d), shows the zone of convergence associated with band AA in Fig. 6.3.7(a).

Effects of the coastline shape: moderate winds Fig. 7. (a) NOAA-9 VIS image over the UK at 1406 UTC on 13 May 1986; AA and BB are cloud bands downwind of peninsulas. (b) UK network radar image, (c) surface chart (isobars in hPa), and (d) surface streamlines (arrows) and convergence zone (dashed line) over southwest England at 1400 UTC on the same day. ln(b), rainfall rates (mm/hr) shown are: dark blue 0.3—I, green 1—4, yellow 4—8,

Effects of the coastline shape: moderate winds A simple conceptual model of the formation of a peninsula band Fig. 8

Effects of the coastline shape: moderate winds A simple conceptual model of the formation of a peninsula band is shown in Fig. 8. The circulation develops following heating of the land and the convergence of sea-breezes from either side of the peninsula. The position of the band depends on the ambient wind direction. Convective cloudiness is associated with the upward branch of the circulation, and clear skies (lack of Cu cloudiness) are generally associated with its subsiding branches. Favourable conditions for peninsula bands are: • moderate low-level winds parallel to the peninsula’s axis, • small directional wind shear with height, • an unstable air stream.

Convection associated with land-breezes Lines or bands of convective cloud, which may produce showers or thunderstorms, may develop at night offshore along the land-breeze front. In Fig. 9, the narrow band of convection BE off Florida’s west coast locates the land-breeze front. This low-level convergence zone, the night-time converse of the sea-breeze, develops because air over the land becomes cooler than over the sea due to stronger night-time radiational cooling over land In comparison to the sea-breeze, land-breeze circulations and convection are generally weaker, due to the potential for greater convective instability over land with solar heating. As with sea-breezes, night-time cloud cover and its effect on radiational cooling over land will influence land-breeze development. Strong convective events may occur with the land-breeze when the shape of the coastline, or drainage flows from nearby higher terrain, accentuate the convergence along the land-breeze front Coastline shape effects are shown at A in Fig. 9 and at B in Fig. 10(b). Neumann (1951) has discussed the impact of drainage flows near the Mediterranean coast.

Convection associated with land-breezes Fig. 9. GOES 1 km resolution VIS image over Florida at 0700 local time

Convection associated with land-breezes Fig.10. Cloud averages for the southeast USA for June 1986 at 0800 local time: (a) cloud in VIS images, (b) cloud colder than —32 °C in IR images. AA marks the land-breeze front, and at B the coastline Shape favours deeper convection.

Convection associated with land-breezes The early morning GOES-East VIS composite, Fig. 10(a), shows clouds located offshore along the Gulf coast of the southeastern USA (along AA). The corresponding IR composite, Fig. 6.3.10(b), shows that the largest area of deep convection (B) off the Florida Gulf coast is located in a region where convergence along the night-time land-breeze should be enhanced. The main peninsula of Florida is not a favoured location for early morning cloudiness during the summer, as is reflected in these early morning cloud climatologies. The land-breeze front may appear as a narrow cloud line offshore of a coastline. Deeper convection is favoured in regions where convergence along the land-breeze front is accentuated due to local topography.