Rain in New Zealand from convergence zones induced in the low-level winds by the interaction of the land with the synoptic scale wind flow

Non-frontal low-level convergence induced in the atmosphere by the influence of the land on the low-level wind flow is responsible for many significant rainfall events in New Zealand and presents a variety of forecasting problems on an almost daily basis.

Rainfall heavy enough to cause flooding over New Zealand’s largest city, Auckland, has resulted from convergence of sea-breezes from opposite coasts of a peninsula. Heavy rainfall has also occurred in association with convergence between two branches of an airstream, splitting to go around New Zealand’s South Island, then colliding in the lee of the mountains.

Lighter rainfall events frequently occur in convergence lines created in the presence of an anticyclonic inversion. The inversion inhibits the formation of cumulus clouds tall enough to produce showers but also inhibits the low-level air from rising over the barrier of the land. This increases the tendency for the low-level airstream to split into two branches, which converge on the lee side of the land, producing a line of showers despite the stable conditions.

Frequently these lines last for less than 24 hours but have, on occasion, lasted for several days, and moved hundreds of kilometres downstream from their place of origin. Typically the rainfall produced by these showers is less than 5mm in most locations but it can be associated with poor visibility and low cloud that is significant for aviation. Also, if unforecast, the showers undermine public confidence in weather forecasts.

In cold southerly outbreaks with widespread showers, the shower activity typically decreases as the subsidence inversion descends from aloft with the approach of the next anticyclone from the west. However, as the inversion lowers below the height of the South Island mountains a meso-scale ridge develops upstream of the mountains as cold air dams up against them. This induces an offshore component in the surface wind, which changes from southerly to southwest along the coast, thereby driving the showers away from the land. However, this also gives rise to a convergence zone offshore between southwest and southeast winds, causing a line of enhanced showers. This line of showers is frequently advected over the city of Wellington bringing several hours of continuous rain there when the previous showers had cleared as much as ten hours before. Although not heavy, this rain brings poor visibility to one of the most important airports in New Zealand.

In stable conditions the air blocked by the land flows around the end of the mountain ranges creating zones of enhanced wind that can be as strong as 20 m/s which is more than double the synoptic-scale wind. Known locally as rivers of wind, these zones are typically tens of kilometres wide and extend hundreds of kilometres downstream, and frequently have a line of enhanced showers along one boundary.

As meso-scale models become ever more sophisticated, weather forecasters in New Zealand will look to them to reproduce these convergence lines, and other associated phenomena.