Feature

Looking for Rain

It's not just the farmers who worry about whether it's going to rain or not...

David Wratt and Howard Larsen

We are probably the only people who have ever spent sleepless nights worrying that it might not rain in the Southern Alps. Yes, we know there are 11 metres of rain annually in some parts of the mountains. But when you have persuaded scientists from around the world to bring an aircraft, radars, sounding balloons and automatic weather stations to the South Island for just three weeks to study mountain rainfall processes, you do start to wonder whether the nor'wester will be as reliable as usual.

As moist air blows onto the South Island off the Tasman Sea, it is pushed up over the mountains. Because atmospheric pressure decreases with height, this rising air expands and cools. As cold air cannot hold as much water vapour as warm air, moisture condenses on small particles (aerosols) to form clouds or to make existing clouds grow larger. Eventually it falls out of these clouds as snow or rain.

On the downwind side of the mountains, the air moves downwards again and warms due to the resulting increase in pressure, suppressing cloud growth and rain formation. The moisture which condenses on the upwind side of the mountains and falls out as rain releases latent heat into the atmosphere, so the air is warmer when it descends again east of the mountains (partly explaining the hot Canterbury nor'wester).

Rain does not fall out of clouds immediately when condensation occurs on aerosols. It may take anything from 15 minutes to several hours, depending on how fast the moist air is rising, for the tiny cloud droplets to grow big enough to start to drift downwards under gravity. And only then do they collide with each other frequently enough to combine to form raindrops.

A fast growth rate is also favoured if there are relatively few aerosol particle "cloud condensation nuclei" present to compete for the available cloud water. (In clean maritime air these nuclei are often small sea-salt particles).

The growth process speeds up if the cloud is so cold that some of the droplets freeze. Ice particles grow much faster from the water vapour in the atmosphere than do liquid water cloud droplets, so that snow is formed relatively quickly. Since snow falls through the air more slowly than rain drops, wind may carry snowflakes formed on the western side of the Alps across the Main Divide, so they fall in the eastern hydro catchment areas. The snow often melts as it falls, reaching the ground as raindrops.

The Australian Fokker Friendship research aircraft operated by Australian Flight Test Services and instrumented by CSIRO Atmospheric Research.

Good Flying Weather

In the SALPEX (Southern Alps Experiment) field campaign we used an Australian research aircraft to measure how atmospheric properties (wind, humidity, temperature, numbers and sizes of aerosols, cloud droplets and ice crystals) change as moist air approaches the Southern Alps across the Tasman Sea.

In a typical flight we monitored a northwesterly flow a few hours ahead of a cold front. We flew from Christchurch across the mountains and 200km or so out into the Tasman Sea, and released dropsonde probes en route from 5,000m in altitude to measure winds, temperature and moisture. Auckland University and Salford University (Manchester, UK) ran mobile weather radars near Lake Moana and Otira to obtain continuous information on intensity of rainfall, the shape of the raining area, and the height at which any snow was melting to form rain.

We flew sounding balloons regularly from Hokitika and Christchurch, measured rainfall from recording rain gauges located on transects across the Alps, and monitored flow in rivers fed by Southern Alps catchments. A student from Otago University measured wind, temperature, snow depth and solar radiation at Mueller Hut (1770m in altitude in Mt Cook National Park).

Luckily, our fears of three weeks of dry weather proved groundless -- there were three rainstorms during the SALPEX special observing period.

In the northwesterly flow approaching the mountains well ahead of a cold front, the precipitation was often generated from relatively shallow clouds. The aerosol concentrations in this incoming air were relatively low so that once conditions were suitable for condensation, cloud droplets were able to grow quite quickly -- there were not too many nuclei competing for the available water. At this stage a relatively shallow region of upward motion extended well out into the Tasman upwind of the mountains.

Dry Models, Wet Reality

Computer models suggest droplets can grow large enough as they pass through this upwind region to generate the observed rainfall over the West Coast and alpine foothills. As the front moved closer to the mountains, the aircraft found deeper clouds, with ice crystals present at upper levels.

On the radar we can see ice particles falling from aloft, and then gathering further water as they fall through warm cloud below the freezing level; this is the stage at which the rain is most continuous and widespread.

Strong cross-mountain winds, very moist incoming air, and "unstable" conditions (when air rises easily over the mountains rather than wanting to flow around them) all favour heavy rainfall. High winds and well-lifted air favour spillover across the mountains, as do conditions when ice crystals and snowflakes are present through a deep layer.

The RAMS weather forecasting model operated by NIWA made reasonable predictions of the rainfall, provided the horizontal grid points at which calculations are made were set no more than 10- 20km apart. Heavy rain was better simulated than light rain, and predictions were quite sensitive to how the physics of ice crystals and cloud droplets were simulated.

NIWA scientists are now working on ways to feed more detailed local information on the current weather based on satellite and radar observations into such models, so their "starting point" is as accurate as possible. We are also linking river flow models to weather prediction models, to produce quantitative flood warnings.

We expect high resolution weather models to become important tools for weather, river flow and flood forecasting in New Zealand.

Vertically pointing radar measurements are taken through a storm front near Otira. The vertical axis is height (m) above Otira; the horizontal axis is time (hour:min:sec); the key on the right shows the strength of the radar echo.
In the measurements above, the storm is just beginning, with a relatively constricted band of clouds and sporadic showers. Later, following the same storm front, the clouds have become much deeper and rain heavier and more consistent. A clear line can be seen at 1800m, marking where melting snow can be detected, with melting ice crystals sending back a strong reflection.

Courtesy of Telford Institute, University of Salford

Howard Larsen is with NIWA in Wellington
David Wratt is with NIWA in Wellington.