Observational Characteristics of Low-Level Jets in Central Western Mexico

摘要

During field experiments using surface weather stations and a Sodar-RASS (radio acoustic sounding system) combination, low-level jets were observed in the Colima Valley of central western Mexico. These low-level jet events have not been previously observed in this part of the tropics. The present study focuses on events from one specific field period from December 2011 to April 2013 performed at a specific location. From this dataset, it was found that these phenomena typically develop at night at around 0300 local time and they reach their maximum speed 20 min after sunrise and end at 1100 local time. For the considered period, low-level jet events occurred during 88 % of nights, and most of these events were fully characterized; thus, 73 % of the nights presented with categorized low-level jets following the classical literature on this mesoscale phenomenon in other parts of the world. The low-level wind maxima occurred with events ranging from category sub-zero to category 3. The strongest category 3 events are around 25 m s and the longest event was 57.5 h, but they are more typical of category 1. Their average speed is 11.7 m s and their height distribution is bimodal, with a group of events occurring at lower levels, with an average height of 190 m; higher levels have a mean height of 480 m. The higher cases are more typical of the rainy season. During all seasons, their direction is predominantly from the north-north-west to north-north-east with some dispersion occurring during the rainy season. The region of study is typically considered to have low wind speeds in the atmospheric boundary layer, making these events super-geostrophic.