Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 139

resolution (Fig. 6.10 ). Effects of dust devils and PBL turbulence are usually not
considered in dust models that use grid point winds to drive emissions (see Chap. 9 ) .
Some authors, however, have used simple gust parameterisations to represent effects
of dry and moist convection on the probability to exceed dust emission thresholds in
a given grid box, which leads to model improvement (Cakmur et al. 2004 ; Morcrette
et al. 2008 ; Ridley et al. 2013 ).

6.9 Conclusion

The spatial and temporal scales of the meteorological processes involved in
dust storms span several orders of magnitude from microscale dust devils to
almost continental-scale dust outbreaks associated with trailing cold fronts. Despite
large differences in topography and climate between the world's most important
dust source regions, some common meteorological features can be identified.
Synoptic-scale dust storms cover a spectrum of types, reaching from cyclonic desert
depressions with weak frontal features to well-developed cyclones with marked
post-frontal ridges to strong winds around the fringes of subtropical highs or
cold continental highs at higher latitudes, often locally modified by topographic
channelling. These types of storms are usually most active in spring and well
represented in regional dust models. At low latitudes dust emission and transport
is affected by synoptic-scale features like AEWs or through interactions with
extratropical weather systems.
On the mesoscale, convective dust storms are found in many dust regions,
particularly during the pre-monsoon season in West Africa and India/Pakistan and
in the vicinity of orographic triggers (e.g. Atlas and Rocky Mountains). An accurate
representation of these storms requires explicitly resolving moist convection and
its organisation into squall lines, which is beyond the computational limits for
most current dust models, creating a fundamental bias over convectively active
regions like summertime West Africa. On even smaller scales dry convection in
the daytime PBL can create dusty plumes or dust devils, whose contribution to
the global dust cycle is unknown and which are typically not represented in dust
models other than through a simple gust parameterisation. Daytime mixing in the
PBL creates a distinct diurnal cycle in many dust regions. At low latitudes, nighttime
acceleration in the stable PBL lead to NLLJs throughout the year whose breakdown
causes distinct emission peaks in the morning, while midlatitude areas are often
characterised by a dust maximum in the afternoon when the PBL is deepest and
taps into momentum from deep baroclinic jets. Improvements to the understanding
and model representation of moist convective and PBL processes in the future will
further enhance our capabilities of simulating dust storms and their interactions with
weather and climate.