Capturing Convergence Lines in the Tropics and Associated Rainfall

Precipitation is often organized along coherent lines of low-level convergence. Mesoscale convergence lines produced by the diurnal cycle of continental heating and cooling are thought to play an important role in the initiation and organisation of clouds and convection in the tropics. Such structures include fronts, sea breezes, gravity waves, dry lines, inhomogeneous surfaces. At longer time and spaces scales, individual convergence lines are organized into larger structures such as the Intertropical Convergence Zone and South Pacific Convergence Zone. However, the connection between convergence and convection is poor in global models, and poor representation of convection is the source of many model biases in the tropics. As such, properly simulating the initiation of convection in both climate and numerical weather prediction models remains a problem.

Here, we examine this long standing problem in meteorology - the role of convergence lines (and convergence more generally) in the initiation of convection and to better understand the deficiencies in the parameterizations necessary for climate and seasonal scale projections. As a first step in diagnosing the root cause of these problems as well as a means of better understanding the organisation of convection, an objective method has been developed for identifying convergence lines in re-analysis and model data. The method used to identify such convergence lines objectively is a variation on an earlier algorithm based technique. The method is applied here to the ERA-Interim reanalysis of difference resolutions with the aim to move to an assessment of climate model output. The frequency and rainfall associated with these convergence lines is quantified, including an attempt to examine the diurnal variation of both convergence lines and the rainfall.