Applying pySTEPS optical flow algorithms to improve convection nowcasting over the Maritime Continent

Abstract

Abstract. The Maritime Continent (MC) regularly experiences powerful convective storms that produce intense rainfall, flooding and landslides, which numerical weather prediction models struggle to forecast. Nowcasting uses observations to make more accurate predictions of convective activity over short timescales (~0–6 hours). Optical flow algorithms are effective nowcasting methods as they are able to accurately track clouds across observed image series and predict forward trajectories. Optical flow is generally applied to weather radar observations, however, the radar coverage network over the MC is not complete and the signal cannot penetrate the high mountainous regions. In this research, we apply optical flow algorithms from the pySTEPS nowcasting library to satellite imagery to generate both deterministic and probabilistic nowcasts over the MC. The deterministic algorithm shows skill up to 4 hours on spatial scales of 10 km and coarser, and outperforms a persistence forecast for all lead times. Lowest skill is observed over the mountainous regions during the early afternoon and highest skill is seen during the night over the sea. A key feature of the probabilistic algorithm is its attempt to reduce uncertainty in the lifetime of small scale convection. Composite analysis of 3-hour lead time nowcasts, initialised in the morning and afternoon, show it produces reliable ensembles but with an under-dispersive distribution, and produced area under the curve scores (i.e. ratio of hit rate to false alarm rate across all probability thresholds) of 0.80 and 0.71 over the sea and land, respectively. When directly comparing the two approaches, the probabilistic nowcast shows greater skill at ≤ 60 km spatial scales, whereas the deterministic nowcast shows greater skill at larger spatial scales ~200 km. Overall, the results show promise for the use of pySTEPS and satellite retrievals as an operational nowcasting tool over the MC.

Mini-review of Pysteps: An open-source python library for precipitation nowcasting

“…Pysteps library has several modules for optical flow methods to estimate motion fields. The library provides many techniques for optical flow analysis, such as the Lukas-Kanade [12] method, which use a pyramid-based approach to handle motion [19]. In addition, the Pysteps framework benefits from the use of global variational echo-tracking (VET) and spectral techniques, such as DARTS, which improve the precision and reliability of motion field estimates.…”

“…Smith [19] subsequently employed a comparable approach, using brightness temperature (BT) data obtained from the Himawari-8/9 satellites over the maritime continent. The author employed optical flow algorithms from the Pysteps nowcasting library to produce deterministic and probabilistic nowcasts over the maritime continent.…”

“…The study conducted by Smith in 2023 [19] also compared the LK optical flow technique with the STEPS ensemble nowcast over the maritime continent. The study concluded that the probabilistic STEPS algorithm is capable of generating ensemble forecasts that are both accurate and reliable for 3-hour lead periods.…”