Determining the Environmental Parameters that Govern the Power Law Distribution of Convective System Lifetimes in the Tropical Western Pacific (TWP)

Tropical western Pacific convective systems are identified and tracked in space/time through use of the CMORPH high-resolution rainfall product. Multiple satellite retrieval products (GHRSST for SST, AQUA AIRS for temperature/moisture, QuikSCAT/ASCAT for the mesoscale vector wind field, TRMM PR for convective system structure) and NCEP Reanalysis (for large-scale vertical wind shear) are merged to create a more complete Lagrangian depiction of the thermodynamic and dynamic environment co-located with propagating convection.

The distribution of system lifetimes follows a power law relationship consistent with that found in prior studies. For each convective system-type, categorized by lifetime, the thermodynamic and dynamic environment surrounding the system is analyzed to determine the parameters that dictate system growth (from a rainfall perspective) and system decay. Mesoscale surface wind convergence, divergence, mid-tropospheric relative humidity, boundary layer relative humidity, shear and atmospheric temperature perturbations at various levels are among the parameters analyzed. Among all parameters investigated, it is found that system growth, decay and longevity is tied to boundary layer temperature perturbations. The extent to which these temperature perturbations are associated with SST variations (i.e. an external, large-scale control) versus convective feedback (through downdrafts/cold pools) is discussed within the context of determining if large-scale environments or small-scale features and internal controls (leading to heterogeneity in the environment) are largely responsible for the observed power law distribution of system lifetimes. Such knowledge has implications for future convective parameterization efforts.