Tornadoes do not have a weekly anthropogenic cycle and by their nature supercells have minimal susceptibility to aerosol influences

Description

A recent paper (Rosenfeld and Bell 2011: “Why do tornadoes and hailstorms rest on weekends,” Journal of Geophysical Research) has purported to show that high concentrations of aerosol modulate the frequency of tornadoes yielding an anthropogenic weekly cycle in tornado occurrence. The physical explanation proposed by Rosenfeld and Bell for how increased aerosol concentrations would cause increased frequency and severity of tornadoes and hail in supercells is inconsistent with actual supercell storm structures and their environments. Supercell updrafts, especially the main updraft which often has a characteristic bounded weak echo region, already have limited growth of drops below the 0°C level, muting any of the purported aerosol influences. Rosenfeld and Bell propose that aerosol-enhanced clouds have fewer and larger raindrops, which would lessen the potential for evaporative cooling and thus yield weaker (warmer) outflow from the storm. However, the distribution of precipitation particles and formation pathways varies markedly from sector to sector within supercell storms, making the aerosol linkage to outflow temperature near the tornado indirect at best. Meteorological parameters such as boundary-layer relative humidity and lifted condensation level (LCL) height have proven to be skillful at discriminating tornadic supercells from non-tornadic supercells without taking into account the aerosol content or the size distribution of hydrometeors.

Further, investigation of spring and summer tornadoes within the U.S. east of 100°W longitude indicates that there is no robust weekly cycle or mid-week maximum in tornado occurrence or tornado days. We examine 19,825 tornado cases over the months May–August for the years 1965–2009. We distinguish between all tornadoes of intensity F0 and greater (F0+) and F1 and greater (F1+). This distinction is made since F1+ tornadoes are more consistently reported than F0s. There are many more F0s than F1s—typically 30–70% depending on the period examined—since frequency of tornadoes falls off as the intensity increases. The days of the week for which the maximum and minimum in tornado counts occur varies depending on the specific period of time examined and whether F0 tornadoes are included or not (Figure 1).