  The Indian monsoon brings around 80% of the annual rainfall over the summer
months June--September to the Indian subcontinent. The timing of the monsoon
onset and the associated rainfall has a large impact on agriculture, thus
impacting the livelihoods of over one billion people. To improve forecasting
the monsoon on sub-seasonal timescales, global climate models are in continual
development. One of the key issues is the representation of convection, which
is typically parametrised. Different convection schemes offer varying degrees
of performance, depending on the model and scenario. Here, we propose a method
to compute a convective timescale, which could be used as a metric for
comparison across different models and convection schemes. The method involves
the determination of a vertical convective flux between the lower and upper
troposphere through moisture budget analysis, and then relating this to the
total column moisture content. The method is applied to a WRF model simulation
of the 2016 Indian monsoon, giving convective timescales that are reduced by a
factor of 2 when the onset of the monsoon occurs. The convective timescale can
also be used as an indicator of monsoon transitions from pre-onset to full
phase of the monsoon, and to assess changes in monsoon phases under future
climate scenarios.
