Atmospheric water vapor and precipitation processes in central and southern Chile

Water vapor is a key component of the hydrological cycle since it is directly involved in the production of precipitation (rain, snow, hail). The transport of water vapor from the tropics (20ºN-20ºS) is fundamental to produce precipitation in midlatitudes (30ºS-50ºS) were local amounts atmospheric moisture are lower than the water column precipitated during a typical storm. This is especially evident during extreme precipitation events, where precipitation accumulation can surpass 2 or 3 times the local atmospheric water vapor available.

Extreme precipitation events (EPEs) are expected to increase due to the anthropogenic climate change, and therefore studies addressing the dynamics and forcing factors of these events are increasingly important. Current research examining the relationship between water vapor transport and precipitation in central-southern Chile have advanced in this direction. However, there is a lack of research aiming to understand water-vapor-precipitation process at the mesoscale, where changes in the order of hours associated to convection are important. Even more, despite many storms in central-southern Chile show convective characteristics (e.g. precipitation rates of 10 mm/h or larger), studies looking at the mesoscale processes has not been addressed so far, partially due to the lack of ground-based weather radars.

As a result, this research proposal takes the challenge of studying the transport of water vapor and link it with precipitation processes (stratiform and convective) at the mesoscale level in central and southern Chile by using a suit of observations and numerical modeling. To determine the water vapor mechanisms involved in the precipitation processes, the study will employ an atmospheric moisture budget, which involves the balance between a storage term (precipitation in this case) and the linear interaction between local changes, advection, and convergence of water vapor following an air parcel. The budget will be computed using gridded data from a state-of-the-art atmospheric reanalysis (ERA5), numerical simulations with the Weather Research and Forecasting (WRF) model, and mathematical techniques such as finite differences and the trapezoidal integration rule. In addition, a relatively dense network of GPS deployed in central-southern Chile will provide direct estimates of local changes of the column water vapor, allowing us to perform a thorough validation of both ERA5 and WRF.

Precipitation processes will be examined using several sources. The polar orbiting Global Precipitation Measurement (GPM) satellite mission provides global swaths of radar reflectivity using a dual-frequency radar (Ku and Ka bands) in a swath-width of 245 km with 5 km resolution at nadir, and vertical beams spaced at 250 m. Along with radar reflectivity, GPM provides estimates of precipitation rates and a classification of the precipitation type, facilitating the identification of precipitation processes. A vertically pointing precipitation radar (Micro Rain Radar, MRR) is currently installed at Universidad de Concepción and will provide time-height sections of radar reflectivity that will complement GPM observations. In addition, a second MRR is planned to be installed in central Chile to provide further meridional context of precipitation processes. Finally, a couple of optical disdrometers and meteorological stations will deliver surface estimates of precipitation at hourly (and higher) rates. In parallel, ERA5 will provide precipitation estimations and classification (stratiform, convective), while WRF will allow to examine precipitation in detail for selected case studies.

At the end of this project, it will be clear what component(s) of the moisture budget are dominating precipitation during EPE storms, clarify the relative importance of stratiform and convective precipitation during EPEs, and elucidate if EPEs with strong convective precipitation are forced by atmospheric instabilities, advection of moisture being lifted by the complex terrain, or moisture convergence occurring over the ocean and moving inland. These results will provide the basis for future efforts looking to improve precipitation forecasting tools.

This project is funded by the ANID Fondecyt Iniciacion 11230184.