Temporary densification of a rain gauge network to gain knowledge about
orographic rain enhancement on the Island of Tahiti - French Polynesia
Abstract
In tropical islands, interactions between atmospheric circulation and
island topography generate complex patterns of orographic rain. Despite
recent advances, numerical weather models are still challenged by
orographic rain enhancement, in particular because the physical
processes responsible for rain generation over tropical islands vary at
spatial and temporal scales that are not yet fully resolved in these
numerical models. A large part of our knowledge about tropical island
precipitation patterns therefore relies on rain observations. However,
observing strongly variable rain fields is not an easy task. It requires
instruments able to capture rain fluctuations at fine temporal (1 min -
1h) and spatial (100 m - 1 km) scales. To reach these requirements,
state of the art rain observation methods rely on remotely sensed
weather radar images and in-situ rain gauge measurements, or solely on
rain gauge observations in the many areas like Tahiti where radar
observations are not available (mainly due to financial cost). In this
study we investigate to which extent a temporary densification of the
rain monitoring network of Tahiti (lon = 149.5°W, lat = 17.6°S, area =
1042 km2, max altitude = 2241 m) can help us gaining new insights about
orographic rain enhancement on a high altitude tropical island with
complex topography. To this end, 10 low-cost but high resolution rain
gauges have been deployed for one year (Aug 2020 - Aug 2021) in addition
to the long term network of around 20 rain gauges operated by the
Direction de l’Equipement Tahiti (Groupement d’Etudes et de Gestion du
Domaine Public) and the French Weather Service (Météo France Polynésie).
Based on this new dataset, we first characterize the space-time patterns
of orographic rain enhancement over Tahiti at an unprecedented
resolution, and link these emerging patterns to local features of the
atmospheric circulation. Next, we investigate the added value of our
temporary network for purposes of rain mapping (i.e., spatial
interpolation of point observations). Finally, we explore the benefits
and limitations of temporary rain gauge network densification for
orographic rain enhancement observation.