Wind Resource Assessment in a Tropical Region with Complex Terrain using SODAR and a Meteorological Tower Network to Measure Low Level Jets and Boundary Layer Conditions

The accurate wind resource assessment in Colima, Mexico, is key in developing resources to meet the energy demands of the region. The present study is conducted in the Colima valley, central western Mexico, using SODAR technology combined with a specific meteorological tower network between 2010 and the present. The measurements, ranging from 30 to 750 m above the valley floor for the SODAR instrument and from 10 to 50 m for the meteorological tower network, are used to investigate the characteristics and evolution of the planetary boundary layer. This valley is limited to the West and South by the “Sierra Madre del Sur”, reaching 2400 m to the west, and to the North by the western end of the Mexican transverse volcanic axis with the Colima volcanic complex reaching 4262 m. Valley gaps between these main features surrounding the Colima valley play an important role as well as sloping terrain on the boundary layer phenomena observed in Colima valley. This valley is located 30 km. from the Pacific Ocean; therefore land-sea contrasts and low level baroclinicity play an important role. The afternoon is characterised by a sea breeze coming from the south west and late night to early morning low level jets from the north, which are typically stronger than the sea breeze flow reaching wind speed maximum around sunrise. These Low Level Jets (LLJs) are in most cases supergeostrophic with the height of the wind speed maximum ranging from 100 m to 700 m and LLJ categories ranging from sub-zero category up to category 3 (following published classifications of LLJs), with lower level zero- to one-category jets dominating with a consistent northerly direction from the gaps in the topography. These phenomena well captured by the SODAR are typically not detected by the classical meteorological towers with the difference of the afternoon sea breeze obviously being due, respectively, to the presence of the night inversion and afternoon turbulent mixing. Considering hub heights of modern wind turbines around 100 m, these mesoscale phenomena are relevant for wind energy assessment, as they increase drastically the potential for wind power generation in the region. Prior studies have revealed the importance to wind power generation of mesoscale phenomena such as LLJs (for example over the North American Great Plains in CASES-99, Lamar Project and ABLE studies, as well as in Northern Europe). The present results are consistent with these studies. Also, these show that logarithmic extrapolation from measurements at lower heights typically used in wind assessments grossly underestimates the early morning wind speeds at hub height, thus making this classical wind assessment technique invalid, especially in complex terrain. The results also support the necessity to use a methodology for wind resource assessment in complex terrain that includes SODAR or LIDAR technology. Additionally, it is interesting to observe that at the difference of the great plains LLJs or other regions where jets form and intensify during the evening transition due to radiative cooling and decoupling from the surface layer, the LLJs observed in this tropical region show a totally different behaviour with a long quiet transition period between the time when the sea breeze dies out after sunset (19:00 LST) and the time LLJs develop and intensify (03:00 LST) to reach their peak around sunrise (07:00 LST); they are later destroyed by morning convection due to strong solar heating. The region under study resides in the tropics (19.23°N), and most of the year the horizontal pressure gradient is typically small implying a relatively weak geostrophic back ground state in the wind field making the observed jets strongly supergeostrophic. Also, the skies are typically clear, implying strong radiative forcing. This being the case most of the time, it is important to mention that in some more rare cases the LLJ is weaker or when solar heating is limited due to the presence of high cloud cover, the sea breeze development is delayed or does not develop at all and the LLJ is stronger and can stay active as long as 38 hours and reach category 3. Considering these extreme cases, regional and large scale forcing are investigated for their influence on the variability of Colima valley LLJs. Finally, LLJs are not only relevant to wind energy; being an important mesoscale phenomena of the boundary layer involved in many processes from dust and contaminant dispersion to mesoscale convective storm formation; the mountainous complex terrain surrounding Colima valley with the presence of an active strato-volcano and North American monsoon active phase in boreal summer months make this phenomena particularly interesting and relevant to study from many perspectives.