West African Heat Low Climatology and Connection to Dry Tropical Cyclones

Heat lows are low pressure systems formed from localized intense sensible heat emitted from the Earth’s surface. Their structure has previously been qualitatively compared to that of a dry tropical cyclone, as both are warm core vortices which exhibit low level cyclonic circulation and anticyclonic circulation aloft – though heat lows are given energy through surface sensible heat flux rather than latent heat flux. This work examines the climatology western Saharan heat lows and draws a quantitative comparison between their dynamics and tropical cyclones by extending the concept of tropical cyclone potential intensity over land. Using parameters from the ERA5 reanalysis, we develop a method calculating potential intensity over land in assuming surface sensible heat flux as the main energy source for heat lows in the Sahara. We filter a dataset of low pressure systems in ERA5 from 1979-2019 to extract idealized heat low cases occurring over land in West Africa, and observe that tracks are most frequent in summer months and in regions with relatively smooth surface orography. Applying the potential intensity calculation method to monthly averaged ERA5 parameters shows a distribution of heat low potential intensity that correlates well with the spatial density of heat low tracks for JJA. Seasonal averaged potential intensity by hour reveals a strong dependence on the changes in buoyancy and surface heating over the diurnal cycle. Potential intensity calculated around the center point of composite heat low track cases captures the magnitude but not the hourly variability of surface winds observed in ERA5. Overall, our results suggest that potential intensity can capture the spatial distribution of heat low tracks in West Africa, but not the diurnal time series of their intensity.