Using Surface Moisture Flux Convergence for Convective Nowcasting

For several years, the Meteorological Service of Catalonia has operated a dense network of automatic weather stations, which the Forecast and Surveillance Team uses to monitor hazardous meteorological conditions in real time. In convective scenarios, wind direction data have proven useful for identifying low-level convergence zones, which are often indicative of potential convective development. However, a more objective parameter was needed to assess whether these convergence areas provide sufficient forcing to initiate or sustain convection.

This need is addressed through the use of Moisture Flux Convergence (MFC), a variable that combines specific humidity advection with wind field convergence to quantify the potential for moisture-driven vertical ascent. MFC is calculated by interpolating wind components and specific humidity from the station network onto a regularly spaced grid.

The resulting surface MFC fields are employed as a nowcasting tool (0–2 hours), capable of identifying areas of moist air convergence and mesoscale boundaries between surface air masses. These zones often coincide with key ingredients for convective initiation or maintenance. Initial applications show that surface MFC performs particularly well in warm-rain scenarios, characterized by high moisture content and low cloud bases. It also shows good performance in deep convection events with similarly low cloud bases. Its effectiveness is more limited in storms with elevated cloud bases and in regions with complex terrain, where wind interpolation becomes less reliable.

Overall, surface MFC offers a valuable complement to traditional observational tools, enhancing spatial and temporal resolution for short-term convective monitoring and operational decision-making processes.