Mesoscale precipitation systems in the Tibetan Plateau region as represented by global reanalysis, regional downscaling and satellite data between 2001 - 2016
Abstract
The Tibetan Plateau (TP) is one of the world’s most important water towers. Convective precipitation systems play an
important role by providing freshwater for the highly densely populated downstream regions, but they can also cause hazardous
floods. Most of the convective processes occur at mesoscale where a spatiotemporal knowledge gap still exists in climate
model simulations in mountainous regions. This report aims to bring new information to this knowledge gap by analyzing how
convective precipitation systems are represented in two Regional Climate Model (RCM) simulations: the Weather Research
and Forecasting simulation from the University of Gothenburg (WRF_GU) and the High Asia Refined analysis version 2
(HAR). The datasets differ in the techniques used to resolve mesoscale convective processes and in how they incorporate largescale
circulations. We have also conducted analyses using a coarser global reanalysis dataset (ERA5) and satellite observations
(GPM) for comparison. We have defined a convective precipitation system as Mesoscale Precipitation Systems (MPSs) to
include all convection on a scale larger than local. These have later been tracked using an object-based approach, which enables
a more detailed temporal analysis of their characteristics and evolvement as we can follow their every timestep during a
lifecycle.
Our results showed that the two RCM simulations detected more MPSs over the TP compared to ERA5 and GPM. In the larger
domain, HAR detected most while WRF_GU the least. We could also see that MPSs contributed with a significant amount of
precipitation, even in drier regions. MPSs tended to peak post-midnight in the larger domain while during the afternoon over
the TP. Due to its coarse resolution, early convection was not detected in ERA5. This was evident in its delayed timing of
MPS convection and low number of detected MPSs over the TP. These findings suggest an added value of using a higher
spatial resolution when analyzing convection in mountainous regions. Furthermore, our results implicate that an RCM
simulation using CP and re-initialization caused excessive convection while a non-CP RCM simulation using spectral nudging
resulted in the opposite. By tracking MPSs at every timestep of their lifecycle, it was also found that longer lived systems were
more prone to produce intense precipitation with distinct peaks and lows during a day compared to smaller ones. We believe
the findings of our research could be valuable in further analyses of mesoscale convection using RCM simulations of different
spatial and temporal resolution, with and without CP. We also believe that more research such as this can help fill the
spatiotemporal knowledge gap in modelling mesoscale convective processes. In the end, this could aid in accurately modelling
convective systems in water scarce regions dependent on them for freshwater and affected by them in terms of hazardous
floods.
Degree
Student essay
Collections
View/ Open
Date
2022-09-01Author
Holmberg, Benjamin
Keywords
Tibetan Plateau
monsoon
westerlies
mesoscale
convection
precipitation
climate modelling
global reanalysis
regional downscaling
spatial resolution
re-initialization
spectral nudging
convective parameterization
Series/Report no.
B1200
Language
eng