## A naive approach to rain prediction

02.10.2016 19:49

Ever since the Slovenian environment agency started publishing Doppler weather radar imagery I've been a regular visitor on their web site. For the last few years I try to use my bicycle for my daily commute as much as possible. However, I'm not such a hard-core fan of biking that I would do it in any weather. The animated map with the recent rainfall estimate history helps very much with that: it's quite easy to judge by eye whether there will be rain in the next 30 minutes or so and hence whether to seek alternative modes of transportation.

Some time ago I had a more serious (or should I say scientific) use for weather data, related to one of the projects at the Institute. Gašper helpfully provided me with some historical archives then and I also started collecting images myself straight from ARSO website. That project came and went, but the amount of data on my disk kept growing. I've been continuously tempted to do something interesting with it. I've previously written what the data seems to reveal about the radar itself.

Most of all, I've been playing with the idea of doing that should-I-take-the-bus prediction automatically. Obviously I'm not a weather expert, so my experiments were very naive from that perspective. For instance, a while ago I tried estimating an optical flow field from the apparent movement of regions with rain and then using that to predict their movement in the near future. That didn't really work. Another idea I had was to simply dump the data into a naive Bayesian model. While that also didn't work to any useful degree as far as prediction is concerned, it did produce some interesting results worth sharing.

What I did was model rain at any point in the radar image (x, y) and time t as a random event:

R_{x,y,t}

To determine whether the event happened or not from the historical data, I simply checked whether the corresponding pixel was clear or not - I ignored the various rates of rainfall reported. To calculate the prior probability of rain on any point on the map, I did a maximum-likelihood estimation:

P(R_{x,y}) = \frac{n_{x,y}}{N}

Here, nx,y is number of images where the point shows rain and N is the total number of images in the dataset.

I was interested in predicting rain at one specific target point (x0, y0) based on recent history of images. Hence I estimated the following conditional probability:

P(R_{x_0,y_0,t+\Delta t}|R_{x,y,t})

This can be estimated from historical data in a similar way as the prior probability. In other words, I was interested in how strongly is rain at some arbitrary point on the map x,y at time t related to the fact that it will rain at the target point at some future time tt. Are there any points on the map that, for instance, always show rain 30 minutes before it rains in Ljubljana?

The video below shows this conditional probability for a few chosen target points around Slovenia (marked by small white X). Brighter colors show higher conditional probability and hence stronger relation. The prior probability is shown in lower-right corner. In the video, the time difference runs in reverse, from 4 hours to 0 in 10 minute steps. For each point, the animation is repeated 4 times before moving to the next point.

The estimates shown are calculated from 62573 radar images recorded between July 2015 and September 2016. Since the format of the images has been changing over time it's hard to integrate older data.

As you might expect, when looking several hours into the future, there is very little relation between points on the map. All points are dull red. If it rains now in the east, it might rain later in Ljubljana. Similarly, if it rains in the west. There's no real difference - the only information you get is that it's generally rainy weather in the whole region.

When you decrease the time difference, you can see that nearby points are starting to matter more than those far away. Brighter colors concentrate around the target. Obviously, if it rains somewhere around Ljubljana, there's a higher chance it will shortly rain in Ljubljana as well. If you note the color scale though, it's not a particularly strong relation unless you're well below one hour.

What's interesting is that for some cities you can see that rain more often comes from a certain direction. Around the coast and in Notranjska region, the rain clouds seem to mostly move in from the Adriatic sea (lower-left corner of the map). This seems to fit the local saying you can sometimes hear, that the "weather comes in from the west". In the mountains (top-left), it seems to come from the north. All this is just based on one year of historical data though, so it might not be generally true over longer periods.

Of course, such simple Bayesian models are horribly out-of-fashion these days. A deep learning convolutional neural network might work better (or not), but alas, I'm more or less just playing with data on a rainy weekend and trying to remember machine learning topics I used to know. There's also the fact that ARSO themselves now provide a short-term rain prediction through an application on their website. It's not the easiest thing to find (Parameter Selection - Precipitation in the menu and then Forecast on the slider below). I'm sure their models are way better than anything an amateur like me can come up with, so I doubt I'll spend any more time on this. I might try to add the forecast to ARSO API at one point though.

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