Precipitation Associated with Convergence Lines

Abstract

Precipitation is often organized along coherent lines of low-level convergence, which at longer time and space scales form well-known convergence zones over the world’s oceans. Here, an automated, objective method is used to identify instantaneous low-level convergence lines in reanalysis data and calculate their frequency for the period 1979–2013. Identified convergence lines are combined with precipitation observations to assess the extent to which precipitation around the globe is associated with convergence lines in the lower troposphere. It is shown that a large percentage of precipitation (between 65% and 90%) over the tropical oceans is associated with such convergence lines, with large regional variations of up to 30% throughout the year, especially in the eastern Pacific and Atlantic Oceans. Over land, the annual-mean proportion of precipitation associated with convergence lines ranges between 30% and 60%, and the lowest proportions (less than 15%) associated with convergence lines occur on the eastern flank of the subtropical highs. Overall, much greater precipitation is associated with long coherent lines (greater than 300 km in length) than with shorter fragmented lines (less than 300 km), and the majority of precipitation associated with shorter lines occurs over land. The proportion of precipitation not associated with any convergence line primarily occurs where both precipitation and frequency of convergence lines are low. The high temporal and spatial resolution of the climatology constructed also enables an examination of the diurnal cycle in the relationship between convergence lines and precipitation. Here an example is provided over the tropical Maritime Continent region.

Understanding the Dynamic Contribution to Future Changes in Tropical Precipitation From Low‐Level Convergence Lines

“…We then apply the regression model using the occurrence and strength of the convergence lines to both observations and GCMs to estimate the precipitation at each point. The slight overestimation of the reconstructed precipitation (Figures 2c and 2d) is partly because some convergence lines are dry Weller, Shelton, et al, 2017). We find that the proportion of the precipitation associated with convergence lines can be faithfully reconstructed (Figures 2a and 2b) with large errors confined to regions away from the major convergence zones where the mean precipitation is small.…”

“…Once the convergence lines are identified, they are associated with the National Oceanic and Atmospheric Administration/Climate Prediction Center morphing technique (CMORPH; Joyce et al, 2004) 6-hourly accumulated precipitation when a convergence line is found sufficiently close (i.e., adjacent grid points) to the precipitation grid point (see Weller, Shelton, et al (2017) for details). It is noted that ERA-Interim winds are often based on relatively few observations over the tropics, and therefore, the degree to which they represent reality is uncertain.…”

“…The convergence line identification code used in the study was that published by Weller, Shelton, et al (2017). This study was supported by the Australian Research Council Centre of Excellence for Climate System Science (CE110001028).…”

“…It is not the intention of the present study to address this debate and assign causality; instead, it is to simply exploit the close relationship between low-level convergence lines and precipitation. Convergence lines can be formed by weather features such as the equatorward extension of fronts, gravity waves, boundary layer rolls, evaporatively driven cold pools, and topographically generated weather systems such as mountain waves and sea and land breezes (Weller, Shelton, et al, 2017). However, when averaged over longer time and space scales, these short-lived convergence lines form the well-known tropical convergence zones (Berry & Reeder, 2014;Hastenrath, 1995;Widlansky et al, 2013;Wodzicki & Rapp, 2016), such as the Intertropical Convergence Zone and South-Pacific Convergence Zone that dominate the larger-scale, longer-term rainfall variability (Borlace et al, 2014;Cai et al, 2012;Vincent et al, 2011;Weller et al, 2014).…”

Projected Response of Low‐Level Convergence and Associated Precipitation to Greenhouse Warming

“…In a second important step, precipitation is associated with the objectively identified convergence lines when they are found to be close to each other for each 6‐hourly field (see Weller et al, for details). In the current study, adjacent grid points (±1 grid point in size) are searched for an identified convergence line for each rain grid point to account for all the precipitation that may be related to the broader system associated with a convergence line.…”