ALLS: American Low-Level jetS

Summary

The ALLS program is an internationally coordinated effort to monitor, quantify, and analyze low-level circulations that modulate regional rainfall. These circulations commonly assume a jet-like structure in the lower troposphere, referred to as the Low-Level Jet (LLJ). American Low-Level Jets (ALLS) are characterized by mesoscale cross-stream structure, and synoptic to continental scale along-stream dimension.

Review of observed climate variability, regional hydrology, and high impact weather over the Americas points to a prominent gap in past monitoring of low-level atmospheric jets. These circulations promote exchange of atmospheric water vapor from low to mid-latitudes and its subsequent condensation. They modulate spring and summer rainfall events over the Missis- sippi and La Plata river basins and exert controlling influences for droughts, floods, and severe weather. Broad, agriculturally productive river basins of North and South America are natu- rally irrigated by moisture that is transported by low level jets (LLJ) and precipitated downwind of their speed maxima. Organization of American droughts, floods, and severe weather over these American bread-baskets is often modulated by these narrow LLJs charac- terized by cross-stream scale of several hundred km and synoptic to continental streamwise dimensions. LLJ variations occur on all time scales, with regular diurnal fluctuations featuring nocturnal maxima.