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Process-oriented analysis of organized convection and synoptic disturbances in the tropics

In recent years, global models have improved such that they exhibit some skill in sea-
sonal tropical cyclone (TC) forecasting and are a useful source of both near-term TC activity
forecast guidance and long-term climate projections. However, many challenges remain, par-
ticularly with regards to TC genesis prediction. Models exhibit a wide spread in their ability
to reproduce the observed TC frequency, with most models typically producing too few TCs
in certain regions, such as the North Atlantic, while producing too many in other regions,
such as the Indian Ocean during monsoon season. These biases contribute to uncertainty
in future projections of TC frequency. While the impacts of mean state biases on the TC
frequency biases have been extensively studied, less attention has been paid to how well
the models represent the synoptic scale disturbances that lead to the formation of TCs. In
particular, it is largely unknown whether and to what extent the degree of synoptic-scale
convective organization affects the probability of synoptic disturbances developing into TCs
and whether the models correctly represent the effect of convective organization. Addressing
this question requires examining how deep convection becomes organized in those distur-
bances and how it interacts with circulation, moisture, and radiation, in both observations
and climate models.

Our project will aim to evaluate climate model simulation of synoptic-scale tropical dis-
turbances, the organization of tropical convection within them, and associated interactions
among convection, moisture, and radiation. We will develop and apply a new set of di-
agnostics for synoptic-scale tropical disturbances and convective organization within them,
filling a gap in the MDTF framework, in which no diagnostic currently exists for tropical
disturbances. After developing them with reanalysis datasets and applying them to high-
resolution CMIP6 historical simulations (from HighResMIP), we will use the new diagnostics
to answer questions such as “Is a greater degree of convective organization associated with
developing disturbances?” and “Do models capture observed differences between developing
and non-developing disturbances?” The new diagnostics for tropical disturbances will be
implemented into the NOAA Model Diagnostics Task Force (MDTF) software package.
The proposed research fits well within the MAPP – Process-Oriented Diagnostics for
Climate Model Improvement and Applications (2864458) competition, as it develops and
applies process-oriented diagnostics to a clearly-identified gap in the existing MDTF soft-
ware package synoptic disturbances and associated organized convection that may serve
as precursors for TCs. The application of the new diagnostics will identify key physical
processes responsible for skillful simulation of synoptic-scale tropical disturbances and TC
formation and target areas for model improvement. This will advance our understanding
of model biases, and could lead to improvement in model simulation of TC genesis and fre-
quency and eventually enhance our ability to evaluate the current and future risk of coastal
storms. It will also improve our understanding of and ability to simulate the disturbances
themselves, which are important sources of tropical weather variability in addition to their
role as TC precursors.

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