The Non-Traditional Coriolis Terms and Tropical Convective Clouds

Abstract

The full, three-dimensional Coriolis force includes the familiar sine-of-latitude terms as well as frequently dropped cosine-of-latitude terms (Nontraditional Coriolis Terms [NCT]). The latter are often ignored because they couple the zonal and vertical momentum equations which in the large-scale limit of weak vertical velocity is considered insignificant almost everywhere. Here, we ask whether equatorial mesoscale clouds which fall outside the large-scale limit are affected by the NCT. A simple scaling indicates that a Lagrangian parcel convecting at 10 ms−1 through the depth of the troposphere should be deflected over 2 km to the west. To understand the real impact of NCT, we develop a mathematical framework which describes an azimuthally symmetric convective circulation with an analytical expression for an incompressible poloidal flow. Because the model incorporates the full 3-dimensional flow associated with convection, it uniquely predicts not only the westward tilt of clouds but also a meridional diffluence of western cloud outflow. To test these predictions, we perform a set of cloud-resolving simulations whose results show preferential lifting of surface parcels with positive zonal momentum and zonal asymmetry in convective strength. RCE simulations show changes to the organization of coherent precipitation regions and a decrease in mean convective intensity of approximately 2 ms−1 above the freezing level. An additional pair of dry cloud-resolving simulations designed to mimic the steady state flow of the model show maximum perturbations to the upper level zonal flow of 8 ms−1. Together, the numerical and analytic results suggest the NCT consequentially alter equatorial mesoscale convective circulations and should be considered in conceptual models.