Douglas J. Parker

African Monsoon Multidisciplinary Analysis: An International Research Project and Field Campaign

Abstract African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve our knowledge and understanding of the West African monsoon (WAM) and its variability with an emphasis on daily-to-interannual time scales. AMMA is motivated by an interest in fundamental scientific issues and by the societal need for improved prediction of the WAM and its impacts on West African nations. Recognizing the societal need to develop strategies that reduce the socioeconomic impacts of the variability of the WAM, AMMA will facilitate the multidisciplinary research required to provide improved predictions of the WAM and its impacts. This will be achieved and coordinated through the following five international working groups: i) West African monsoon and global climate, ii) water cycle, iii) surface–atmosphere feedbacks, iv) prediction of climate impacts, and v) high-impact weather prediction and predictability. AMMA promotes the international coordination of ongoing activities, basic research, and a...

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.

THE JET2000 PROJECT Aircraft Observations of the African Easterly Jet and African Easterly Waves

Abstract Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight C130 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ. Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale ...

The Convective Storm Initiation Project

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety ofground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

THE AMMA RADIOSONDE PROGRAM AND ITS IMPLICATIONS FOR THE FUTURE OF ATMOSPHERIC MONITORING OVER AFRICA

In the face of long-term decline, the AMMA research program has reactivated the radiosonde network over West Africa. The lessons learned in AMMA have significance for the upper-air network throughout the continent.

Conditional Convective Heating in a Baroclinic Atmosphere: A Model of Convective Frontogenesis

Abstract It is shown here that there exists a regime of balanced frontogenesis that is forced almost entirely by the diabatic hating due to convection at a front. This theory is explored in the context of the two-dimensional semigeostrophic equations with an Eady basic state: convection is parameterized to be dependent on the low-level moisture convergence of the cross-frontal ageostrophic flow, in accordance with recent diagnostic studies. The significant result is that the growth rate of the convective frontal system becomes independent of the total wavelength of the domain once the diabatic heating exceeds a relatively large threshold magnitude. In this regime the frontal zone has a width and structure dependent on the heating magnitude but not on the wavelength. The system is described as “solitary” or “isolated” since the dynamics are self-contained and independent of the far field. The energetics of the system have a diabatic conversion that is an order of magnitude greater than that due to the larg...

A seamless assessment of the role of convection in the water cycle of the west African Monsoon

A suite of 40 day UK Met Office Unified Model simulations over West Africa during summer 2006 are analyzed to investigate the causes of biases in the position of the rainbelt and to understand the role of convection in the regional water budget. The simulations include climate, global operational, and limited area runs (grid spacings from 1.5 to 40 km), including two 12 km runs, one with parameterized and one with explicit convection. The most significant errors in the water cycle terms occur in the simulations with parameterized convection, associated with the diurnal cycle and the location of the convection. Errors in the diurnal cycle increase the northward advection of moisture out of the Sahel toward the Sahara but decrease the advection of moisture into the Sahel from further south, which limits the availability of moisture for Sahelian rainfall. These biases occur within the first 24 h, showing that they originate from the representation of fast physical processes, specifically, the convection scheme. Once these rainfall regimes have been established, the terms of the water budgets act to reinforce the biases, effectively locking the rainbelts latitude. One of the simulations with parameterized convection does, however, produce a better latitudinal distribution of rainfall because on the first day it is better able to trigger convection in the Sahel. Accurate representation of the diurnal cycle of convection and the ability to trigger convection in a high convective inhibition environment is key to capturing the water cycle of the region and will improve the representation of the West African Monsoon.

Modeling soil moisture-precipitation feedback in the Sahel: Importance of spatial scale versus convective parameterization

Feedback between soil moisture and precipitation influence climate variability in semiarid regions. However, serious concerns exist about the ability of coarse-scale global atmospheric models to depict one key aspect of the feedback loop, namely the sensitivity of daytime convection to soil moisture. Here we compare regional simulations using a single model, run at different spatial resolutions, and with convective parameterizations switched on or off against Sahelian observations. Convection-permitting simulations at 4 and 12 km capture the observed relationships between soil moisture and convective triggering, emphasizing the importance of surface-driven mesoscale dynamics. However, with the inclusion of the convection scheme at 12 km, the behavior of the model fundamentally alters, switching from negative to positive feedback. Similar positive feedback is found in 9 out of 10 Regional Climate Models run at 50 km. These results raise questions about the accuracy of the feedback in regional models based on current convective parameterizations.

What is the Mechanism for the Modification of Convective Cloud Distributions by Land Surface–Induced Flows?

Abstract The aim of this study is to determine the mechanism that modulates the initiation of convection within convergence zones caused by land surface–induced mesoscale flows. An idealized modeling approach linked quantitatively to observations of vegetation breezes over tropical Benin was used. A large-eddy model was used with a prescribed land surface describing heterogeneities between crop and forest over which vegetation breezes have been observed. The total surface fluxes were constant but the Bowen ratio varied with vegetation type. The heterogeneous land surface created temperature differences consistent with observations, which in turn forced mesoscale winds and convection at the convergence zones over the crop boundaries. At these convergence zones optimum conditions for the initiation of convection were found in the afternoon; the equivalent potential temperature was higher in the convergence zones than over anywhere else in the domain, due to reduced entrainment, and the mesoscale convergence...

Impact of mesoscale vegetation heterogeneities on the dynamical and thermodynamic properties of the planetary boundary layer

This study uses aircraft observations over the Republic of Benin from the African Monsoon Multidisciplinary Analyses (AMMA) campaign to investigate the impact of vegetation heterogeneities on the dynamics within the planetary boundary layer, such as convection, transport, and mixing. Isoprene, a biogenic volatile organic compound emitted primarily by woody vegetation, was used as a tracer for transport to link the land surface to the boundary layer properties. Associated to planetary boundary layer (PBL) temperature gradients at vegetation boundaries, a persistent mesoscale organization of the winds which controlled patterns in the formation of cumulus congestus clouds was observed. A strong relationship among PBL temperatures, meridional wind velocity, isoprene concentrations, and fraction of forest or shrub cover was found, corroborating the land surface forcing of the observed dynamics. The observations show that the convergence zones tended to occur on the southern edge of warm surface and atmospheric anomalies. The northerly synoptic wind appears to have increased the coherency of the southerly part of the mesoscale flow and displaced the convergence zones southward. The relationships between the PBL potential temperatures and the meridional wind and isoprene concentrations were spatially coherent down to wavelengths of 10 and 8 km, respectively. A seasonal climatology of visible satellite data shows enhanced cloud cover in the afternoon over cropland, suggesting that the results presented are not limited to this case study but are of climatological significance in the region.