Cloud Forecasts for Australian Astronomers

Cloud Forecast Loop

Clouds wouldn't be so bad for astronomers, if they weren't so hard to forecast. But with new high resolution model output now available from the Australian Bureau of Meteorology, things are looking up a little at least. The animation above is not a real satellite picture, but rather a 'synthetic satellite loop' - a forecast of what the weather model thought the clouds would look like over a 48 hour period.

Background

Weather models and forecasts have improved a great deal with the growth of satellite observations and massive computing power. But most weather agencies keep their model output in house and only publicly release a certain number of forecast products. The exception has always been the United States. Thanks to their constitution, what the taxpayer funds the taxpayer gets to see, and so does the rest of the world. And so raw output from their Global Forecast System has always been available on the internet for other companies to use (and make money from).

So while you may have tried different weather web sites like Weather Underground, The Weather Channel, AccuWeather and many more, the reality is they are all presenting the same data, just in different ways. And while you may have been able to get a forecast for your own suburb, cells in the model are spaced tens of kilometres apart and the values are interpolated in between these grid points. The precision implied by getting a weather forecast tailored to your street address is simply not there.

Nevertheless, these weather websites are understandably popular, but very few provided what astronomers needed. It was and still is possible to generate your own cloud forecast charts directly from Global Forecast System data but that is a tedious interface to say the least.

Andrew Cool in South Australia stepped in and delivered Skippy Sky for Australian (and other) astronomers with very simple to use visualisations of clouds and other GFS data. Skippy Sky has since been my primary cloud forecasting tool. Astronomy Panel from 7-Timer also presents GFS cloud data for any location but you're better off looking at the Skippy Sky maps to see the 'bigger picture' rather than a point sample which is often misleading.

The Australian Bureau of Meteorology - Access Model

As the name implies, the Global Forecast System is a global model. While it is among the best, it doesn't match the higher resolution regional models run by the Australian Bureau of Meteorology (BoM). The BoM has recently made a significant change to their weather models - the new model is called 'Access' - the Australian Community Climate and Earth-System Simulator. The Access model is derived from a UK Met Office model, with a technical review of its performance available in this online BoM paper.

Luckily for Australian astronomers, Lawrie Rikus at the Centre for Australian Weather and Climate Research has made this model output available online, on his research page 'Synthetic Satellite Picture Forecast Loops'. You might like to submit feedback to the BoM asking them to officially publish cloud forecasts (for example, using their new graphical 'Forecast Explorer'). The more they know people are interested the more likely they are to provide such a service.

Comparisons

Cloud Forecast Comparison Australia
Click for full size

Shown above is a real infra-red satellite image and the Skippy Sky and Access A forecasts for the same time. In Skippy Sky the false colour represents percentage of total cloud cover. On the Skippy Sky site you can select to show only low, mid or high cloud cover which is very useful. On the synthetic forecast image, as on a real infra-red image, high cloud is bright white (colder temperatures) while low cloud is darker grey. At night, the ground cools down and it can become very difficult to tell the difference between cold ground and low cloud. This is a downside to this research product which is aiming to 'reproduce' a satellite image, rather than presenting the model data in the clearest possible way (like Skippy Sky).

Cloud Forecast Comparison South-East Australia
Click for full size

The second example is of the highest resolution 'state scale' model. But here it is clear that just because a model runs at 5km resolution does not mean it is accurate to that level! In this case, the Astronomical Society of Victoria was running a Spring Star Party at the Leon Mow Dark Sky Site in central Victoria. The Access model gave us hope that the skies would clear during the night but we got a bit unlucky and the low cloud got stuck around central Victoria all night. The models are good but they're not magic!

Products Available

Synthetic Satellite Picture Forecast Loops

The table below lists the various model outputs listed on the Synthetic Satellite Picture Forecast Loops page. As is always the case, the higher resolution models can only be run over smaller areas for shorter periods of time. If you are just looking at the night ahead, you can try the high resolution product for your local region. Generally though, the 'Access A' loop covers all of Australia out to two nights ahead and is likely to be your best bet. The accuracy implied by the beautifully detailed cloud images in the local scale models isn't really there in practice (and the starting time for the small scale models appears to be 12 hours behind the others).

If you want to see a little further ahead, the lower resolution 'Access R' runs out to three nights ahead over the whole Australian region. The global model 'Access G' looks nearly 10 days ahead but with much lower resolution. Don't get caught up in the details, but it can give you a useful idea of what large scale frontal systems are coming 3-6 days out. Anything beyond that gets very speculative.

All of these forecasts are updated twice a day, roughly around 3am and 3pm.

Product Geographic Area Resolution Forecast Period
Access-A Australia 0.12 degrees/13km 48 hours in 1 hour steps
Access-A Water Vapour Australia 0.12 degrees/13km 48 hours in 1 hour steps
Access-G Global 0.833 degrees/92km 120 hours in 6 hour steps, then 228 hours (9.5 days) in 12 hour steps
Access-R Australian Region 0.375 degrees/42km 72 hours in 3 hour steps
Access-T Tropical/Equatorial Region 0.375 degrees/42km 30 hours in 3 hour steps
VicTas, Sydney, SEQld, SA, Perth High resolution over local regions 0.05 degrees/5km 36 hours in 1 hour steps

The national 'Access A' and especially the local area models are a revolutionary step forward in the resolution of weather models available to Australian amateur astronomers. In numerical resolution at least, they are 5-10 times better than what we previously had access to.

By way of comparison, until July 2010 the US GFS model ran at 0.5 degree (55km) resolution. Since July 2010, GFS now runs at 0.25 degree (27km) resolution in 3 hour steps out to 192 hours (8 days) and 12 hour steps up to 16 days ahead (but pretty sketchy in its usefulness beyond ~7 days). The enhanced resolution since July is pretty remarkable for a global model but for dynamic, low level cloud the Australian 'Access' models should have the edge.

For longer range forecasts (say beyond 3 days) the GFS model is likely to be at least as accurate as the Australian Access G product. GFS is also likely to perform well for large scale frontal systems and high level jet stream cloud even in short range forecasts. In short, both GFS and the BoM's Access model are as good as it gets - what's great about having access to the Australian data is that we have two independent sources.

Previously we had one dataset (US Global Forecast System) published under many different names, but really all the same data. Now, by looking at Access and Skippy Sky - two completely different models - you can get an idea of the confidence of the forecast. If there is a close match between Skippy Sky and the BoM Access models, then you can be reasonably confident in the forecast. But if they differ substantially then you know to hedge your bets.

If everything is working right, the image below should be the cloud that is headed your way in the next few hours according to the Access A model:

I'll finish with the caveat that weather in general and clouds in particular are dynamic, chaotic and inherently hard to predict. This page gives you some idea of the complexity of numerically modelling weather. While the model output now available is better than ever, the reality could still look a whole lot different on the ground at your observatory or dark sky site!

Comments

Lunar Eclipse test

Hi Phil. Thanks for the article. I'm gonna give these a shot for the upcoming lunar eclipse on Saturday. Conditions look iffy for most of the southern half of WA...so hopefully I find a gap.