Evaluating pySTEPS optical flow algorithms for convection nowcasting over the Maritime Continent using satellite data

<p<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 sho...
Ausführliche Beschreibung

<p<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 (<span class="inline-formula"<∼</span< 0–6 <span class="inline-formula"<h</span<). 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 <span class="inline-formula"<h</span< on spatial scales of 10 <span class="inline-formula"<km</span< and coarser and outperforms a persistence nowcast 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 <span class="inline-formula"<h</span< lead time nowcasts, initialised in the morning and afternoon, produces reliable ensembles but with an under-dispersive distribution and produces 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 <span class="inline-formula"<≤</span< 60 <span class="inline-formula"<km</span< spatial scales, whereas the deterministic nowcast shows greater skill at larger spatial scales <span class="inline-formula"<∼</span< 200 <span class="inline-formula"<km</span<. Overall, the results show promise for the use of pySTEPS and satellite retrievals as an operational nowcasting tool over the MC.</p< Ausführliche Beschreibung