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 that in the large-scale limit of weak vertical velocity are considered insignificant almost everywhere. Here, we ask whether equatorial mesoscale clouds that fall outside the large-scale limit are affected by the NCT. A simple scaling indicates that a Lagrangian parcel convecting at 10 m s21 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 that describes an azimuthally symmetric convective circulation with an analytical expression for an incompressible poloidal flow. Because the model incorporates the full three-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 m s21 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 m s21. Together, the numerical and analytic results suggest the NCT consequentially alter equatorial mesoscale convective circulations and should be considered in conceptual models.

Climate change is expected to alter the statistical properties of precipitation. There are two related but consequentially distinct theories for changes to precipitation that have received some consensus: (1) the time-and-space integrated global total precipitation should increase with longwave cooling as the surface warms, (2) the most intense precipitation rates should increase at a faster rate related to the increase in vapor saturation. Herein, these two expectations are combined with an analytic integration of three conceptually independent properties of the tropical hydrological cycle, the intensity, probability, and frequency of precipitation. The total precipitation in both a cloud-resolving model and tropical Global Precipitation Measurement mission data is decomposed and reconstructed with the analytic integral. By applying (1) and (2) to the precipitation characteristics from the model and observations to form a warming proxy model, it is suggested that a wide range of future distributions of precipitation intensity, probability, and frequency are possible.

As hydrometeors fall within or from a cloud, they reach a terminal velocity because of friction with the air through which they settle. This friction has previously been shown to result in significant vertically integrated dissipation of energy, but the nature and vertical profile of this dissipation warrant further investigation. Here, its energetic origin is discussed. It is confirmed explicitly that the dissipated energy originates from the conversion of hydrometeor potential energy during settling as suggested in an earlier study by Pauluis and Held. The magnitude of this heating is then analyzed in a cloud-resolving model simulation of tropical, aggregated convection. Maximum heating from hydrometeor friction reaches ~10 K h-1. The simulation is compared to one without hydrometeor frictional heating. For the case simulated, hydrometeor frictional heating results in a drier mean state, greater cloud cover, lessened convective mass flux, and a warmer atmosphere throughout much of the troposphere. It is suggested that the heating imparted to the atmosphere by dissipation allows the air to recover most of the energy previously expended in lofting hydrometeors.

The use of bin or bulk microphysics schemes in model simulations frequently produces large changes in the simulated storm and precipitation characteristics, but it is still unclear which aspects of these schemes give rise to these changes. In this study, supercell simulations using either a bin or a double-moment bulk microphysics scheme are conducted with the Regional Atmospheric Modeling System (RAMS). The two simulations produce very different storm morphologies. An additional simulation is run for each scheme in which the diameter-fall speed relationships for ice hydrometeors are modified to be similar to those used by the other scheme. When fall speed relationships are homogenized, the two parameterization schemes simulate similar storm morphology. Therefore, despite the use of largely dissimilar approaches to parameterizing microphysics, the difference in storm morphology is found to be related to the choice of diameter-fall speed relationships for ice hydrometeors. This result is investigated further to understand why. Higher fall speeds lead to higher mixing ratios of hydrometeors at low levels and thus more melting. Consequently, stronger downdrafts and cold pools exist in the high fall speed storms, and these stronger cold pools lead to storm splitting and the intensification of a left mover. The results point to the importance of hydrometeor fall speed on the evolution of supercells. It is also suggested that caution be used when comparing the response of a cloud model to different classes of microphysics schemes since the assumptions made by the schemes may be more important than the scheme class itself.

The North American monsoon is frequently affected by transient, propagating upper tropospheric vorticity anomalies. Sometimes called Tropical Upper-Tropospheric Troughs (TUTTs), these features have been claimed to episodically enhance monsoon rainfall. Here, we track long-lived TUTTs in 40 years of reanalysis data, producing composites and case studies from 340 TUTTs which last, on average, 7 days as they move westward across the North American monsoon region. TUTTs are thought to form from midlatitude Rossby wave breaking; case studies from our dataset support this theory. TUTTs move westward within the easterly upper-level flow in which they are embedded. In vortex-centered composites along the full tracks of long-lived TUTTs, we find no detectable increase in rainfall within the main TUTT circulation. Instead, negative precipitation anomalies lie within about 500 km of the TUTT center. Quasi-geostrophic ascent occurs in the southeast quadrant of TUTTs but is confined to the upper troposphere and does not appear to interact with precipitation. Positive anomalies of ascent and rainfall occur south and southeast of TUTTs but lie outside the main TUTT vortex, perhaps indicating concurrent variations in nearby climatological precipitation maxima. In contrast with previous case studies and subjective analyses that showed TUTTs enhance precipitation in parts of northwestern Mexico, our composites along the tracks of long-lived TUTTs portray these systems, to first order, as strong vorticity anomalies trapped in the upper troposphere that interact only weakly and indirectly with precipitation.

Clouds are crucial for Earth's climate and radiation budget. Great attention has been paid to low, high and vertically thick tropospheric clouds such as stratus, cirrus and deep convective clouds. However, much less is known about tropospheric mid-level clouds as these clouds are challenging to observe in situ and difficult to detect by remote sensing techniques. Here we use Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite observations to show that thin mid-level clouds (TMLCs) are ubiquitous in the tropics. Supported by high-resolution regional model simulations, we find that TMLCs are formed by detrainment from convective clouds near the zero-degree isotherm. Calculations using a radiative transfer model indicate that tropical TMLCs have a cooling effect on climate that could be as large in magnitude as the warming effect of cirrus. We conclude that more effort has to be made to understand TMLCs, as their influence on cloud feedbacks, heat and moisture transport, and climate sensitivity could be substantial.

Single-moment microphysics schemes have long enjoyed popularity for their simplicity and efficiency. However, in this article it is argued through theoretical considerations, idealized thunderstorm simulations, and radiative-convective equilibrium (RCE) simulations that the assumptions inherent in these parameterizations can induce large errors in the proper representation of clouds and their feedbacks to the atmosphere. For example, precipitation is shown to increase by 200% through changes to fixed parameters in a singlemoment scheme and low-cloud fraction in the RCE simulations drops from about 15% in double-moment simulations to about 2% in single-moment simulations. This study adds to the large body of work that has shown that double-moment schemes generally outperform single-moment schemes. Therefore, it is recommended that future studies, regardless of their focus and especially those employing cloud-resolving models to simulate a realistic atmosphere, strongly consider moving to the exclusive use of multimoment microphysics schemes.