Cloud structures over NZ on 26 July

On Thursday 26 July 2012 a cold southeasterly airstream flowed onto the North Island, around an anticyclone centred just east of the South Island. In this blog post we’ll look at some interesting small-scale cloud structures around the country on this day.

Below is the weather map at midday on Thursday 26 July. The red arrows show the sense of the broad-scale rotation around the anticyclone.

While the North Island was experiencing a southeasterly flow, the isobars were widely spaced over the South Island, indicating little wind there. Take a look at the animation below, based on visible light as received by the MtSat-2 geostationary satellite.

MTSAT-2 visible satellite images, each an hour apart, from 10am to 3pm NZST Thursday 26 July 2012. Images courtesy Japan Meteorological Agency.

As explained in the post on the effect of resolution, the visible satellite image shows all cloud as white or light grey, regardless of how high or low the cloud is. Most of the cloud over New Zealand is on the east coast of the North Island. Just off the Manawatu coast there is a plume-shaped area of cloud that extends northwestwards parallel to the flow. This is low-level cloud in a region where the air flow near the Earth’s surface is coming together, or converging. As the air convergences it is forced to rise and, with enough moisture, cloud forms.

There is a similar process occurring off eastern Bay of Plenty. In this case there is a zone of more concentrated convergence that shapes the clouds into a rope-like appearance, but the orientation of it is still towards the northwest and parallel to the flow.

Most of the South Island is cloud-free, but there is a patch of grey-looking cloud around Lakes Tekapo and Pukaki. This is also low cloud, but it has formed during the night in the valleys and basins. In the afternoon there’s been just enough heat from the weak winter sun to break up and disperse the cloud. The cloud base at Pukaki during the morning was reported as being about 600 metres above the ground - the air temperature had risen from minus 4 overnight to plus 3 by lunchtime.

The Terra polar-orbiting satellite has a very high resolution sensor. Terra passed over New Zealand within the period of the previous images, and I’ve reproduced the image below, split into two colour images.

Very high resolution satellite image within the period of the previous images. Image courtesy of MODIS Rapid Response Project, NASA/GSFC.

The features discussed above are very apparent. The area of convergence off Manawatu evidently has a double structure, and it is striking how this high resolution image shows individual cumulus clouds as white dots (as discussed in the previous post). There are cloud streets over the central North Island from the Kaweka to the Raukumara ranges.

Over the Pacific ocean there is a lot of cloud having a cellular structure. This is typical of a cold body of air that moves onto relatively warmer water. The air bubbles up into cumulus clouds that tend to clump together into ring-shaped clusters.

The low cloud over Lakes Tekapo and Pukaki (below) has a flat appearance typical of layered stratus cloud. It extends its fingers into the valleys between the peaks of the surrounding ranges.

As previous image, but for South Island.

The cloud over the ocean east of Canterbury is stratocumulus, a combination of the lumpy texture of cumulus cloud and the layering of stratus cloud. In many respects there was nothing particularly unusual about our weather on 26 July, but the satellite images were still able to reveal some fascinating and beautiful cloud structures.


Layers and streets of cloud

Clouds come in many different types and are characterised and named according to both their shape and height in the atmosphere.  While a single snapshot in time at a given location may only contain one type of cloud, there are many days when multiple cloud types can be observed in the sky at once. On Saturday, 3 July 2010, we had a nice example in satellite imagery of different cloud types sitting at different levels above Waikato.

Here’s what the satellite image looked like Saturday afternoon for southwestern parts of Waikato near Kawhia Harbour:

(Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Looking at that image you can probably spot three different cloud types quite easily. The broad, flat-looking cloud near the top right of the image is an area of fog sitting on the ground. Above this, and casting shadows on the fog layer, are lines or “streets” of puffy looking cumulus cloud. Higher still are thin wisps of cirrus both over the sea and extending onto the land, some of which you can see through to the cumulus and coastline below.

To see how these clouds changed during the afternoon, here’s a short animation of hourly images covering the period from midday to 3pm:

You should be able to spot the area of fog slowly shrinking, while cumulus forms near the coast in warm updrafts over the land eventually spreading inland across Waikato as streets of cloud.  The lower quality of this animation makes the small wisps of cirrus hard to spot.

So how do these cloud streets form?
One might be tempted to compare each individual cloud street to a smoking chimney, and that is perhaps partially true. The cloud continuously forms at a stationary point, over a coastal hill for example (acting like a chimney), and is then blown away by the wind – a southwesterly in this case. However, this doesn’t completely explain the situation as you might expect the cloud to form all the way along the coast, and then move inland as a solid layer, not as lines of cloud with gaps between them. The answer is clearly a little more complicated and is best illustrated by the following image from Wikipedia:

(Image courtesy Daniel Tyndall, Department of Meteorology, University of Utah, via Wikipedia)

Clouds form when rising air cools and reaches its saturation point, causing the moisture in the air to condense into water drops. Here we see lines of rising air or updrafts between counter-rotating tubes or rolls of air that are aligned with the flow. In between the updrafts are lines of sinking air in which cloud can’t form. The result is narrow lines of cloud, separated by narrow lines of cloud-free air.

Note that this is quite a different process to lines of cloud that are often observed downstream and parallel to a mountain range and perpendicular to the wind flow, but that story can wait for another blog post …