Temperature Lapse Rate and Freezing Level Variability in the Peruvian Andes

Abstract:

Wet and dry seasons and strong diurnal variability of the outer Tropics and steep terrain dictate weather patterns in the northern Peruvian Andes, 9°S. We analyze more than a decade of hourly observations from an embedded sensor network (ESN) consisting of two weather stations anchoring six embedded iButton Thermochron® temperature loggers and Lascar El-USB2 dataloggers between the elevations 3,500 and 4,700 m a.s.l. of the pro-glacial Llanganuco and Quilcayhuanca Valleys, revealing temporal and spatial variability in freezing-level height (FLH), strong diurnal and seasonal impacts on near-surface lapse rate (LRs) and greater warming at higher altitudes suggestive of valley fluxes that impact mass balance of glaciers. The FLH is consistently higher during the afternoon and exhibits an average diurnal range between 150 and 420 meters during the wet and dry seasons. Warm phase El Niño Southern Oscillation events are attributed to higher mean freezing levels while cold ENSO events are tied to lower FLHs. There was no discernable impact of ENSO on LRs and only slight seasonal variability. The wet season LR was on average 6.6 °C/km with a standard deviation of 0.40 and the average LR for the dry season was 6.8°C/km with a standard deviation of 0.63. Results of two-km resolution Weather Research and Forecasting (WRF) modelling for wet and dry periods in 2005 suggests a diurnally oscillating valley wind and strongly stratified LRs. Near-surface LRs for the ESN and WRF were steeper (greater) during the dry season, ESN = 9.05 and WRF = 7.45 °C/km, compared to the wet season, ESN = 6.03 and WRF = 5.81 °C/km. The WRF LRs were comparable to the average of near-surface LRs calculated from ESN temperatures for the wet period, but the ESN LR is significantly larger than the WRF LR for the dry period. WRF near-surface LRs were lower than those of the ESN during times of solar heating. WRF significantly underestimates LR from 09 to 15 hrs local and overestimates from 18 to 00 hrs. The FL calculated from averaging the composite 3-hourly values were lower during the dry season, ESN = 4,817 and WRF = 4,766, compared to the wet season, ESN = 4,913 and WRF = 4,792 m. The wet season diurnal range of the FL was 734 m for the ESN and 196 m for WRF projections. Results could be applied to validation for climate models and satellite derived variables.