West African storm tracks and their relationship to Atlantic tropical cyclones

The automatic tracking technique used by Thorncroft and Hodges (2001) has been used to identify coherent vorticity structures at 850hPa over West Africa and the tropical Atlantic in the ECMWF 40-year reanalysis. The presence of two dominant source regions, north and south of 15ºN over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity. There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2-6-day-filtered meridional wind, which provides a synoptic-scale measure of African Easterly Wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual timescales

Exploring the differences in cloud properties observed by the Terra and Aqua MODIS Sensors

The aerosol-cloud interaction in different parts of the globe is examined here using multi-year statistics of remotely sensed data from two MODIS sensors aboard NASA's <i>Terra</i> (morning) and <i>Aqua</i> (afternoon) satellites. Simultaneous retrievals of aerosol loadings and cloud properties by the MODIS sensor allowed us to explore morning-to-afternoon variation of liquid cloud fraction (CF) and optical thickness (COT) for clean, moderately polluted and heavily polluted clouds in different seasons. Data analysis for seven-years of MODIS retrievals revealed strong temporal and spatial patterns in morning-to-afternoon variation of cloud fraction and optical thickness over different parts of the global oceans and the land. For the vast areas of stratocumulus cloud regions, the data shows that the days with elevated aerosol abundance were also associated with enhanced afternoon reduction of CF and COT pointing to the possible reduction of the indirect climate forcing. A positive correlation between aerosol optical depth and morning-to-afternoon variation of trade wind cumulus cloud cover was also found over the northern Indian Ocean, though no clear relationship between the concentration of Indo-Asian haze and morning-to-afternoon variation of COT was established. Over the Amazon region during wet conditions, aerosols are associated with an enhanced convective process in which morning shallow warm clouds are organized into afternoon deep convection with greater ice cloud coverage. Analysis presented here demonstrates that the new technique for exploring morning-to-afternoon variability in cloud properties by using the differences in data products from the two daily MODIS overpasses is capable of capturing some of the major features of diurnal variations in cloud properties and can be used for better understanding of aerosol radiative effects

African easterly waves in an idealized general circulation model: instability and wave packet diagnostics

We examine the group dynamic of African easterly waves (AEWs) generated in a realistic, spatially non-homogeneous African easterly jet (AEJ) using an idealized general circulation model. Our objective is to investigate whether the limited zonal extent of the AEJ is an impediment to AEW development. We construct a series of basic states using global reanalysis fields and initialize waves via transient heating over West Africa. The dominant response is a localized, near-stationary wave packet that disperses upstream and downstream. The inclusion of a crude representation of boundary layer damping stabilizes the waves in most cases, consistent with other studies in the past. In some basic states, however, exponential growth occurs even in the presence of damping. This shows that AEWs can occasionally emerge spontaneously. The key result is that, whether triggered by an external forcing or generated internally, the wave packet can remain within the AEJ for multiple wave periods instead of being swept away. Drawing from other studies, this also suggests that even the damped waves can grow if coupled with additional sources of energy such as moist convection and dust radiative feedback. The wave packet in the localized AEJ appears to satisfy a condition for absolute instability, a form of spatial hydrodynamic instability. However, this needs to be verified more rigorously. We conclude that the limited zonal extent of the AEJ is not an impediment. Our results also suggest that the intermittent nature of AEWs is mediated, not by transitions between convective and absolute instability, but likely by external sources such as propagating equatorial wave modes.</p