Posted by Nal Kalchbrenner and Lasse Espeholt, Google Research
Deep learning has successfully been applied to a wide range of important challenges, such as cancer prevention and increasing accessibility. The application of deep learning models to weather forecasts can be relevant to people on a day-to-day basis, from helping people plan their day to managing food production, transportation systems, or the energy grid. Weather forecasts typically rely on traditional physics-based techniques powered by the world’s largest supercomputers. Such methods are constrained by high computational requirements and are sensitive to approximations of the physical laws on which they are based.
Deep learning offers a new approach to computing forecasts. Rather than incorporating explicit physical laws, deep learning models learn to predict weather patterns directly from observed data and are able to compute predictions faster than physics-based techniques. These approaches also have the potential to increase the frequency, scope, and accuracy of the predicted forecasts.
 |
| Illustration of the computation through MetNet-2. As the computation progresses, the network processes an ever larger context from the input and makes a probabilistic forecast of the likely future weather conditions. |
Within weather forecasting, deep learning techniques have shown particular promise for nowcasting — i.e., predicting weather up to 2-6 hours ahead. Previous work has focused on using direct neural network models for weather data, extending neural forecasts from 0 to 8 hours with the MetNet architecture, generating continuations of radar data for up to 90 minutes ahead, and interpreting the weather information learned by these neural networks. Still, there is an opportunity for deep learning to extend improvements to longer-range forecasts.
To that end, in “Skillful Twelve Hour Precipitation Forecasts Using Large Context Neural Networks”, we push the forecasting boundaries of our neural precipitation model to 12 hour predictions while keeping a spatial resolution of 1 km and a time resolution of 2 minutes. By quadrupling the input context, adopting a richer weather input state, and extending the architecture to capture longer-range spatial dependencies, MetNet-2 substantially improves on the performance of its predecessor, MetNet. Compared to physics-based models, MetNet-2 outperforms the state-of-the-art HREF ensemble model for weather forecasts up to 12 hours ahead.
MetNet-2 Features and Architecture
Neural weather models like MetNet-2 map observations of the Earth to the probability of weather events, such as the likelihood of rain over a city in the afternoon, of wind gusts reaching 20 knots, or of a sunny day ahead. End-to-end deep learning has the potential to both streamline and increase quality by directly connecting a system’s inputs and outputs. With this in mind, MetNet-2 aims to minimize both the complexity and the total number of steps involved in creating a forecast.
The inputs to MetNet-2 include the radar and satellite images also used in MetNet. To capture a more comprehensive snapshot of the atmosphere with information such as temperature, humidity, and wind direction — critical for longer forecasts of up to 12 hours — MetNet-2 also uses the pre-processed starting state used in physical models as a proxy for this additional weather information. The radar-based measures of precipitation (MRMS) serve as the ground truth (i.e., what we are trying to predict) that we use in training to optimize MetNet-2’s parameters.
 |
| Example ground truth image: Instantaneous precipitation (mm/hr) based on radar (MRMS) capturing a 12 hours-long progression. |
MetNet-2’s probabilistic forecasts can be viewed as averaging all possible future weather conditions weighted by how likely they are. Due to its probabilistic nature, MetNet-2 can be likened to physics-based ensemble models, which average some number of future weather conditions predicted by a variety of physics-based models. One notable difference between these two approaches is the duration of the core part of the computation: ensemble models take ~1 hour, whereas MetNet-2 takes ~1 second.
 |
| Steps in a MetNet-2 forecast and in a physics-based ensemble. |
One of the main challenges that MetNet-2 must overcome to make 12 hour long forecasts is capturing a sufficient amount of spatial context in the input images. For each additional forecast hour we include 64 km of context in every direction at the input. This results in an input context of size 20482 km2 — four times that used in MetNet. In order to process such a large context, MetNet-2 employs model parallelism whereby the model is distributed across 128 cores of a Cloud TPU v3-128. Due to the size of the input context, MetNet-2 replaces the attentional layers of MetNet with computationally more efficient convolutional layers. But standard convolutional layers have local receptive fields that may fail to capture large spatial contexts, so MetNet-2 uses dilated receptive fields, whose size doubles layer after layer, in order to connect points in the input that are far apart one from the other.
 |
| Example of input spatial context and target area for MetNet-2. |
Results
Because MetNet-2’s predictions are probabilistic, the model’s output is naturally compared with the output of similarly probabilistic ensemble or post-processing models. HREF is one such state-of-the-art ensemble model for precipitation in the United States, which aggregates ten predictions from five different models, twice a day. We evaluate the forecasts using established metrics, such as the Continuous Ranked Probability Score, which captures the magnitude of the probabilistic error of a model’s forecasts relative to the ground truth observations. Despite not performing any physics-based calculations, MetNet-2 is able to outperform HREF up to 12 hours into the future for both low and high levels of precipitation.
 |
| Continuous Ranked Probability Score (CRPS; lower is better) for MetNet-2 vs HREF aggregated over a large number of test patches randomly located in the Continental United States. |
Examples of Forecasts
The following figures provide a selection of forecasts from MetNet-2 compared with the physics-based ensemble HREF and the ground truth MRMS.
 |
| Probability maps for the cumulative precipitation rate of 1 mm/hr on January 3, 2019 over the Pacific NorthWest. The maps are shown for each hour of lead time from 1 to 12. Left: Ground truth, source MRMS. Center: Probability map as predicted by MetNet-2 . Right: Probability map as predicted by HREF. |
 |
| Comparison of 0.2 mm/hr precipitation on March 30, 2020 over Denver, Colorado. Left: Ground truth, source MRMS. Center: Probability map as predicted by MetNet-2 . Right: Probability map as predicted by HREF.MetNet-2 is able to predict the onset of the storm (called convective initiation) earlier in the forecast than HREF as well as the storm’s starting location, whereas HREF misses the initiation location, but captures its growth phase well. |
 |
| Comparison of 2 mm/hr precipitation stemming from Hurricane Isaias, an extreme weather event that occurred on August 4, 2020 over the North East coast of the US. Left: Ground truth, source MRMS. Center: Probability map as predicted by MetNet-2. Right: Probability map as predicted by HREF. |
Interpreting What MetNet-2 Learns About Weather
Because MetNet-2 does not use hand-crafted physical equations, its performance inspires a natural question: What kind of physical relations about the weather does it learn from the data during training? Using advanced interpretability tools, we further trace the impact of various input features on MetNet-2’s performance at different forecast timelines. Perhaps the most surprising finding is that MetNet-2 appears to emulate the physics described by Quasi-Geostrophic Theory, which is used as an effective approximation of large-scale weather phenomena. MetNet-2 was able to pick up on changes in the atmospheric forces, at the scale of a typical high- or low-pressure system (i.e., the synoptic scale), that bring about favorable conditions for precipitation, a key tenet of the theory.
Conclusion
MetNet-2 represents a step toward enabling a new modeling paradigm for weather forecasting that does not rely on hand-coding the physics of weather phenomena, but rather embraces end-to-end learning from observations to weather targets and parallel forecasting on low-precision hardware. Yet many challenges remain on the path to fully achieving this goal, including incorporating more raw data about the atmosphere directly (rather than using the pre-processed starting state from physical models), broadening the set of weather phenomena, increasing the lead time horizon to days and weeks, and widening the geographic coverage beyond the United States.
Acknowledgements
Shreya Agrawal, Casper Sønderby, Manoj Kumar, Jonathan Heek, Carla Bromberg, Cenk Gazen, Jason Hickey, Aaron Bell, Marcin Andrychowicz, Amy McGovern, Rob Carver, Stephan Hoyer, Zack Ontiveros, Lak Lakshmanan, David McPeek, Ian Gonzalez, Claudio Martella, Samier Merchant, Fred Zyda, Daniel Furrer and Tom Small.
