Intercomparison of Weather Stations: A Case Study of a Complex Topography Valley in Tropical Andes.

Monitoring, measuring, and estimating environmental variables are key tools for understanding hydrometeorological phenomena at different spatial and temporal scales, particularly in tropical regions with complex topography, due to the limited amount of available information and the monitoring networks' difficulties for the installation and maintenance of time. To increase knowledge about specific phenomena related to rainfall occurrence and changes in variables such as winds, temperature, and humidity, some projects have proposed Intensive Observation Periods (IOP) in specific regions. Although many of these initiatives have good spatial and temporal data resolution, most need more sustainability over time. Added to this is the inherent uncertainty in the monitoring networks design according to the study regions' specificities and the selection of proper sensors based on their physical principles and operation to represent the realistic behavior and phenomenology variation in high tropical mountain regions.

SIATA is a funded project with public resources that aims to monitor and model hydrometeorological variables in a tropical narrow valley in the Colombia Andes, seeking to understand environmental threats in support of disaster risk management. The project has a remote and in-situ monitoring network of atmospheric variables, such as precipitation, temperature, humidity, pressure, and direction and velocity winds. The localization of the stations had been established to cover the territory's needs. Some stations have a registration period of more than ten years, with a temporal resolution of one minute. Existing extended networks, such as rain gauges, have more than 150 stations in an area of 1100 km². The elevation changes vary around 1500 m, from the valley base to 1400 m.a.s.l to 2900 m.a.s.l at the highest point, which is characterized by high slopes and highly urbanized; this presents a challenge for the densification and maintenance of networks.

That is how, for 12 years, the SIATA project has been building different monitoring networks, becoming a sustainable intensive observation over time. In order to ensure truthful data, appropriate estimation, and adequate surface measurements, two multi-sensor points are placed inside the valley in an area no larger than 10 m². Each monitoring station includes different compact weather sensor technologies such as Vaisala, Thies, Atmos41, and Lufft; which have different measuring methods according to every sensor type. In addition, each station carries a rain gauge with a tipping, weight-based and disdrometer mechanism. This work uses these conditions to realize intercomparison activities with the monitoring variables using different statistical techniques. The results showed significant differences in some variables, with relative humidity and precipitation being critical. Even though the weather stations are very close at each multi-sensor point, the differences and associated uncertainty are caused by the sensor type, station manufacture, and associated physical principle. In the case of precipitation, this uncertainty is also due to the high spatiotemporal variability of the Droplet Size Distribution (DSD), the physical process intrinsic to the precipitation phenomenon.

Due to the segmentation in the intercomparison results and to obtain realistic data, we conducted a comparison campaign between the compact weather station types and a station with certified calibration. As a result, the hydrometeorological variables measurements for each station type and their respective uncertainty are obtained. This result is extrapolated to all weather stations' networks, and we observe the magnitudes with their variability depending on the diurnal cycle and seasonality, given the modulation of the annual insolation cycle.