HIGH RESOLUTION SATELLITE IMAGERY OF THE NEW ZEALAND AREA: A VIEW OF LEE WAVES*

Transcription

1 Weather and Climate (1982) 2: HIGH RESOLUTION SATELLITE IMAGERY OF THE NEW ZEALAND AREA: A VIEW OF LEE WAVES* C. G. Revell New Zealand Meteorological Service, Wellington ABSTRACT Examples of cloud distribution and patterns in the New Zealand area seen in high resolution satellite images are presented with a brief discussion. Emphasis is placed on orographically produced cloud structures. INTRO Ill 3CTION Low resolution cloud imagery from satellites has been in routine use by weather forecasters in New Zealand since However, the present generation of satellites with more advanced sensing capability, in conjunction with more complex receiving and processing equipment on the ground, provides a wider range of quantitative measurements having improved time and space resolution. From mid-1981, data have been acquired from both the NOAA 6 and NOAA 7 polar orbiting satellites. These spacecraft each make two sets of passes over the New Zealand area each day at an altitude of about 840 km. The products obtained include high resolution imagery in both visible and infra-red wavebands. The imagery in digital form is processed by computer to remove distortion introduced by the radiometer scanning process, to add a map grid and coastlines, and to scale the image to match a standard map projection. Computer processing can also enhance the appearance of particular features by emphasising small differences in apparent brightness or temperature of different cloud layers. One regularly produced computer rectified * Editor's Footnote: It is hoped to publish further papers discussing satellite imagery in future issues of Weather and Climate. cloud photograph covers the immediate vicinity of New Zealand on a scale of one in five million. Examples of images on this scale are discussed in the following sections. Places named in the text are shown in Fig. 1. ' I - e w a r t I s invereargit t Ang tern NORTH ISLAND t ag BuO Buckle Canonic i,, udei Ma aku Figs AORotoraa R elu g sn_ he is taker& Banks Peninsula SOUTH ISLAND E ARms 9C" Fig. 1: Location of places named in text. oat Cape

2 24 SATELLITE -When viewed from satellite altitude the OBSERVED arrangement of clouds into distinct patterns can C be L clearly O U seen. These patterns fall into several D well defined groups which are signatures of Patmospheric A T processes. The main patterns are T banded, E Rcellular, and spiral structures which occur N Son a variety of scales. Through their effects on cloud formations, fundamental interactions between the earth's surface and the atmosphere are also revealed in the images. Banding is a feature of cloud organisation which was well known before the days al satellite imagery. Cloud bands range in scale from frontal cloud systems, which sometimes extend for thousands of kilometres, to billows (narrow bands associated with wind shear) which can be seen in their entirety by an observer on the ground. LEE WAVE CLOUDS Among the most spectacular cloud patterns appearing in satellite images are those connected with trains of waves formed by air flowing over hills and mountains. An account of lee waves in southwesterly airflow over Banks Peninsula, New Zealand, was given by Sturman (1980) who claimed that the phenomenon was unusual because southwesterly airflow is normally unstable and such clear examples are rare in any case. However. this is believed to be incorrect for the following reason. When the air behind a cold front is subsiding, the instability is confined to a shallow layer capped by a subsidence inversion or stable layer (in which temperature is constant or increasing with height). Above this layer the stability decreases. This stratification, which is not unusual, is favourable for wave generation. Trains of lee wave clouds are observed throughout New Zealand, including Stewart Island. They are most frequently associated with westerly winds but satellite imagery confirms that lee wave clouds are not uncommon in airflow from any direction. Recent examples are given below. 19 July, Fig. 2 shows the cloud distribution in a south-southeast flow over the North Island. (Note that the computer-drawn coastline is misplaced slightly). There is a High Resolution Satellite imagery system of lee wave clouds over the Rotorua- Coromandel region. These are not generated by the Coromandel-Kaimai ranges, to which the flow is nearly parallel, but by the Mamaku Ranges which present a substantial obstruction at right angles to the flow. In addition Mt. Te Aroha, acting as a solitary peak, is probably responsible for the prominent ship-wave or herringbone pattern. There is also a weak wave system over the hills of Northland. Several features of this situation are of interest: (1) To the east of the North Island there was a trough of low pressure with deep convection marked by the cumulonimbus clouds whose tops appear white in this infra-red image. (2) As the trough moved eastwards, this unstable air was gradually being replaced by drier air with slowly increasing stability, associated with a ridge of high pressure to the west. (3) The upper wind profile at Auckland displayed little vertical shear, both direction and speed being nearly constant. The latter two features give rise to favourable conditions for lee wave development. This event shows how the occurrence of lee wave clouds depends quite critically on a favourable combination of moisture and stability factors, as discussed by Sturman (1980), and how the properties of an airstream can change rapidly both in space and in time. 21 June, A commonly occurring wintertime cloud pattern in the New Zealand region is seen in Fig. 3 where a south-westerly airstream is impinging onto the South Island. This blocking of the low-level airflow, along with extensive katabatic drainage and other factors, leads to large parts of the country and coastal waters being cloud-free. To the east, where the windflow has cyclonic curvature, there are shower clouds but to the west, where the air is subsiding, the cellular clouds have lower tops. This subsidence is taking place as an anticyclone intensifies over the Tasman Sea and the centre approaches the south of the South Island. The critical conditions for lee wave development are being met as the stability progressively changes. The subsidence is shown by the presence of an inversion at 780 mb (about 2,000 m) over Christchurch which had

3 High Resolution Satellite Imagery 2 5 ti-a-9116 Uk811, lu(ud,w7l Z 19/07/8i, im u4nepp Fig. 2: Lee waves over the north of the North Island in a outh-southeasterly airflow. About 2000 NZST on 19 July, 1981.

4 26 H i g h Resolution Satellite Imagery Ab ORBITS 10296, Z 20/06/81 CHAN 5 TR LINEAR Cloud distribution in a southwesterly airflow. Lee wave clouds over the South Island. About 0700 NZST on 21 '

5 High Resolution Satellite Imagery AA6 OROIT /06/01 CHAN 5 IR Fig. 4: A cold front advancing from the south over central New Zealand. To the north of the f ront convection d and e e lee p waves in close proximity. About 0700 NZST on 5 June, 1981.

6 28 H i g h Resolution Satellite Imagery Ok.H7 OREIT Fig, 5 : Extensive lee wave activity i n a deep southwesterly airflow. A b o u t 1600 N Z S T on 2 November, 198L

7 High Resolution Satellite Imagery already lowered to 850 mb (about 1,500 m) over Invercargill. At both stations the air below the inversion was close to saturation. A further interesting feature is the ribbon of higher cloud on the western side of the Southern Alps. At levels above about 5 km the flow was nearly southerly and this cloud is being generated in a standing wave on the lee side a "southerly arch" would have been visible in Buller and North Westland. 5 June, This example again illustrates the fine distinction between suitable and unsuitable conditions for lee wave occurrence and is difficult to analyse with the available data. In Fig. 4 a sharply-defined cold front, advancing northwards, lies to the north of Kapiti Island. The area to the south is occupied by a cloudy southerly flow. The westerly flow to the north of the front contains some deep convective clouds especially over the north of the North Island. However, in the lee of the main ranges from East Cape to northern Wairarapa there is a well-formed system of waves. The wind profile at Ohakea showed only weak vertical shear from near the surface to the high troposphere, speeds being about 40 kt. The Auckland radiosonde showed a weak, shallow, stable layer near 2,000 m at midday and the airflow over the southern North Island is likely to have had a similar stratification earlier in the morning when the satellite picture was received. Conditions therefore may have been only marginally favourable for lee waves to occur. 2 November, Lee wave clouds are commonly seen in the spring months when westerly winds predominate and spring 1981 was no exception. A deep southwesterly airstream covered New Zealand on 2 November and the air was close to saturation at levels up to about 4 km in the south of the South Island. A great variety of cloud forms including numerous wave formations can be seen over and downwind from New Zealand in Fig. 5. Cloud layers at several levels give the image a slightly fuzzy appearance especially over the South Island and it is marred by some electrical interference. 29 Interesting features include: (1) The prominent ship-wave pattern apparently generated by Stewart Island (Mt. Anglem) which extends far offshore to the east of Otago and Canterbury. It is probably reinforced by wave systems in the lee of many other ranges of the South Island. (2) The tine-scale wave pattern in the lee of the Northland-Coromandel region. Here moisture was confined to the layer below a subsidence inversion at 820 mb (about 1,800 m). Upper wind soundings showed that apart from some anticlockwise turning of the wind with height at Invercargill and Christchurch there was little vertical shear in deep layers at the time of wave activity. REFERENCE Sturman, A. P., 1980: A case study of lee waves in southwesterly airflow over Bank Peninsula, New Zealand. Weather, 35: