'Rope' cloud with a 'Southerly buster'         Cliff Revell

 

Satellite pictures on the afternoon of the 21st October 2007 showed a well-defined 'rope' cloud, a long narrow line of cumulus, marking the leading edge of a frontal cloudband over the Southern Ocean. (Fig.1) This feature retained its identity over the next 24 to 36 hours and on the morning of the 22nd extended southeastward from about Cape Turnagain, having moved at an average speed of 40kt.  The cloud band as a whole had become distorted by the landmass of New Zealand into a typical gull-wing shape. (Fig.2).  By mid-afternoon the rope cloud had advanced to a position north of Gisborne and was weakening while becoming separated from the main cloud band, which itself was disintegrating in the New Zealand section (Fig.3).

Cloud lines of this type were first identified in satellite pictures in the late 1960s.  An example was described by Seitter and Muench 1985.

 

Fig.1 Visible picture from satellite MTSAT at 1400NZST 21 October 2007.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 2 Visible picture from satellite MTSAT at 0900 NZST 22 October 2007.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 3 Visible picture from satellite MTSATat 1400NZST on 22 October 2007.

 

 

 

 

 

 

 

 

 

During its passage along the east coast the cloud system was accompanied by a 'southerly buster', a rapid change to a strong southerly wind and associated large rise in barometric pressure.  Southerly buster events were intensively studied during SOUCHEX in the late 1980s (Smith et al 1991, Ridley 1990).   They found evidence that the occurrence and movement of southerly changes along the east coast are controlled largely by synoptic-scale processes through changes in the upper-level winds as a trough approaches in the mean westerly flow.  Most, though not all, are associated with a cold front that originates over the Tasman Sea or Southern Ocean.  In the present case a short-wave trough propagated rapidly through the pattern of very strong west to southwest flow at upper levels over New Zealand

 

This event is represented in Fig.4, a time section from Invercargill to Hicks Bay, showing plots of surface wind at routine reporting times and isallobars of 3-hour pressure change (not corrected for diurnal variation). At stations where changes were masked by Fohn influences (the prefrontal air was particularly dry in the Canterbury region), there was, at least initially, a rise in dew point as air from a new source arrived.  For instance the dew point sequence at Le Bons Bay, and also at Castlepoint, displayed this characteristic.  On the other hand the change at Cape Campbell was more typical of that associated with a cold front passage with dew point falling from the time of the wind change.  The change appears to have been particularly strong at exposed coastal locations from Canterbury to Wairarapa.  Sustained wind speeds of 45kt were reported at Le Bons Bay and 35kt at Kaikoura while even along the east coast of the North Island sustained winds of 25kt were experienced.  It is likely that higher wind speeds occurred at intermediate times.  These speeds are comparable with the speed of movement of the change itself.

 

From the coarse sampling of observations available it is not possible to determine the precise relationships among the various features involved but in this instance the rope cloud appears to have been closely associated with the surface wind-shift and pressure-jump line, the surface cold front.   The nature and behaviour of surface cold fronts continues to be a subject of study, e.g. Schultz 2005.

 

Fig. 4 Time cross section from Invercargill (NV) to Hicks Bay (HB) during passage of the rope cloud (see text)    GS – Gisborne

NR – Napier

CT – Castlepoint

NG – Ngawi

CC – Cape Campbell

KI – Kainkoura

LB – Le Bons Bay

TU – Timaru

DN – Dunedin

 

 

References:

Smith, R. K., R.N. Ridley, M.A. Page, J. T. Steiner, and A.P.Sturman,1991: Southerly Changes on the East Coast of New Zealand. Monthly Weather Review 119, 5, 1259-1282.

 

Ridley, R.N., 1990: Southerly buster events in New Zealand. Weather and Climate 10, 2, 35-54.      

 

Schultz, D. M., 2005: A review of cold fronts with Prefrontal T roughs and Wind Shifts.  Monthly Weather Review 133, 8, 2449-2472.

 

Seitter, K.L. and H.S. Muench,  1985: Observation of a Cold Front With Rope Cloud.     Monthly Weather Review 113, 5, 840-848.