Cold Pool Outflows Research Articles

Uplift of Saharan dust south of the intertropical discontinuity

In situ observations from a flight made during the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation (GERBILS) field campaign (June 2007) show significant dust uplift into the monsoon flow immediately south of the intertropical discontinuity in the western Sahara. Dust loadings were highest in the moist monsoon air and the observations are consistent with dust uplift by the nocturnal monsoon winds. There is some evidence that cold pools within the monsoon flow contributed to the dust uplift: regions of elevated dust, water vapor, and ozone within the monsoon air are consistent with precipitation cooling and moistening air from upper levels and the resultant dusty cold pools propagating northward. However, only southward propagating cold pool outflows could be observed in satellite imagery. Using European Centre for Medium‐Range Weather Forecasts analyses and satellite data, it is shown that the asymmetry in the seasonal dust cycle is closely related to the downdraft convective available potential energy (DCAPE) from convective storms. There is both more dust and more DCAPE during monsoon onset than during retreat. The larger DCAPE values during monsoon onset, as well as the stronger nocturnal monsoon flow and the stronger heat trough circulation, are expected to contribute to the higher dust loadings at this time. Both the monsoon flow and cold pool outflows within it result in dust uplift in the western Sahara during the monsoon onset, which is when the maximum dust uplift occurs. For dust modeling, this shows the importance of accurately modeling not only the monsoon flow itself but also deep convection and cold pools.

Aerosol Effects on Clouds, Precipitation, and the Organization of Shallow Cumulus Convection

Abstract This study investigates the effects of aerosol on clouds, precipitation, and the organization of trade wind cumuli using large eddy simulations (LES). Results show that for this shallow-cumulus-under-stratocumulus case, cloud fraction increases with increasing aerosol as the aerosol number mixing ratio increases from 25 (domain-averaged surface precipitation rate ∼0.65 mm day−1) to 100 mg−1 (negligible surface precipitation). Further increases in aerosol result in a reduction in cloud fraction. It is suggested that opposing influences of aerosol-induced suppression of precipitation and aerosol-induced enhancement of evaporation are responsible for this nonmonotonic behavior. Under clean conditions (25 mg−1), drizzle is shown to initiate and maintain mesoscale organization of cumulus convection. Precipitation induces downdrafts and cold pool outflow as the cumulus cell develops. At the surface, the center of the cell is characterized by a divergence field, while the edges of the cell are zones of convergence. Convergence drives the formation and development of new cloud cells, leading to a mesoscale open cellular structure. These zones of new cloud formation generate new precipitation zones that continue to reinforce the cellular structure. For simulations with an aerosol concentration of 100 mg−1 the cloud fields do not show any cellular organization. On average, no evidence is found for aerosol effects on the lifetime of these clouds, suggesting that cloud fraction response to changes in aerosol is tied to the frequency of convection and/or cloud size.

Organization of mesoscale convective systems: 1. Numerical experiments

The shear‐parallel and shear‐perpendicular linear mesoscale convective systems (MCSs) are studied by a three‐dimensional nonhydrostatic cloud resolving model. On the basis of previous observational studies, a unique “reference layer” can be defined for both MCSs. A shear‐parallel MCS can be distinguished from a shear‐perpendicular MCS by the shear and moist instability in the reference layer only regardless of the shear at the other levels. This drastically simplifies the problem at hand. The reference layer is located below 400 mbar. It must be thicker than 200 mbar, and the mean shear of the layer must be larger than 2 m s−1 per 100 mbar. Its stratification is unstable or near neutral. The shear‐parallel MCS is produced when the reference layer is near neutral or less unstable. In its first phase the new cells are produced by the basic wave propagation mechanism, modified by the shear. The growth rate is relatively small, and there is no cold pool in this phase. In the second phase the updrafts become strong and expand. A cold pool is formed along the shear‐parallel direction, and the new cells are formed along the edge of the cold pool. The interaction between the shear and the updraft plays a major role in the formation of the shear‐parallel MCSs. The cold pool outflow convergence and the interaction between the cold pool and the shear are also important in the second phase. On the other hand, the shear‐perpendicular MCS is produced when the reference layer is highly unstable. The cold pool is always strong, and the interaction between the shear and cold pool plays a major role in the formation of the linear structure in agreement with the theory proposed by Rotunno et al. (1988). Sensitivity experiments indicate that both types of MCS organization can exist without precipitation or a cold pool and in the dry system. This finally provides a foundation for the theoretical analysis presented in the companion paper.

Shear-Parallel Moist Convection over the Tropical Ocean: A Case Study from 18 February 1993 TOGA COARE

Abstract This paper describes a newly observed form of shear-parallel rainbands that were observed on 18 February 1993 in Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment using the NCAR ELDORA airborne Doppler radar. Interesting features of these rainbands included their rapidly changing and highly cellular nature, their complicated and layered kinematic structure, and the effects of the rainbands on the tropical environment. In their lower levels, the rainbands consisted of east–west elongated clusters of small convective cells, aligned with the z = 0–5 km shear vector. Strong low-level convergence, inflow from the north, and enhanced westerly winds extended from the surface up to 5 km. Much of this air was vented out of the storm at midlevels, exiting to the north in the 0°C stable layer at z = 5–7 km. This midlevel outflow fed an area of widespread, stratiform precipitation lying to the north of the rainband. The cold pool outflow emanating from this area of precipitation t...