CLOUD STREETS

written by KNMI

UNDER CONSTRUCTION

Definition

A cloud street is a type of organized convection. It has the form of an extended line of cumulus cloud almost parallel to the wind direction, often in an otherwise lightly cloudy sky. Various sources of thermals spaced across the wind may give rise to parallel cloud streets. It appears that such streets may also be produced in an air mass in which the convection layer has a well-marked top and in which the wind direction is almost constant. A cross section normal to the wind direction (see figure below) shows the pattern of vertical movements.

Cross section of cloud streets shows the patterns of vertical circulation.

Requirements for development

Surface winds of 10 - 15 kt.
Wind direction nearly constant with height in the convective layer.
An inversion or stable layer to limit the vertical development of convective currents, usually at a height of 1.5 to 2 km (see image below).
Curvature of the wind profile (see image below). The wind speed should increase with height to a maximum of at least 10 m s^-1 in the middle or upper part of the convection layer. Above these levels the wind may decrease or increase again.

Temperature and wind velocity profile for cloud streets.

Spacing and alignment of cloud streets

The distance between adjacent streets has been observed as approximately three times the height of the inversion or stable layer. Cumulus streets are aligned parallel to, or within a few degrees of, the direction of the wind in the convective layer. Bends in the wind flow are often indicated by bends in the cloud streets. A single line of cumuli often extends to more than 100 km downwind; the entire field may extend over 100 km downwind and has been observed to have a width in excess of 500 km. On very high resolution satellite pictures up to 100 nearly parallel lines of cumuli have been observed.

Synoptic situation

The synoptic conditions for the horizontal pressure and temperature distribution within the convective layer are show in the figure below. The direction of the isotherms compared with the direction of surface isobars indicates cold air advection. This results in a thermal wind component which reduces the pressure gradient aloft. In this way the requirement for a curved wind profile is fulfilled. Cloud streets most frequently occur in strong cold air outbreaks, but they are also observed in a fresh inflow of moist warm air. The convective boundary layer in which they occur may be quite shallow, rarely more than 2 km deep. If there is vertical shear in both wind speed and direction, thermal waves will form over the cloud streets.

Pattern of isobars at the surface and isotherms representing the whole convective layer favourable for cloud street development.

9 March 1998/1258 UTC - RGB image (channels 1, 2 and 4)

Example of RGB image with cloud streets.

References

LILLY D. K. (1966): On the stability of the Ekman boundary flow; J. Atmos. Sci., Vol. 23, p. 481 - 494
BROWN R. A. (1979): A secondary flow model for the planetary boundary layer; J. Atmos. Sci., Vol. 27, p. 742 - 757
ETLING D. (1971): The stability of the Ekman boundary layer flow as influenced by thermal stratification; Contr. Atmos. Phys., Vol. 44, p. 168 - 186
WMO, Technical Note No. 158. WMO-No. 495 (1993); Handbook of meteorological forecasting for soaring flight

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