A43E-01 INVITED
Relative roles of land/vegetation, oceans, mountains and biomass burning in determining climate variabilities of the South American monsoon onset
Research in recent decades has greatly improved our understanding of the roles of land surface and vegetation, sea surface temperature in the adjacent oceans, mountains and biomass burning as individual factors that shape climate variabilities of the South American monsoon system. This improvement has set a stage for examining the relative roles of these factors in determining the climate variabilities of this monsoon system from seasonal to decadal scales, and response of the monsoon system to increasing anthropogenic forcing. In this presentation, the author will present an attempt to address the relative roles of the aforementioned processes with focus on climate variabilities of the monsoon onset. The presentation can serve as a straw man to stimulate discussions and further studies on this subject.
A43E-02
The effect of vegetation biophysical processes (VBP) on global as well as West African and south American precipitations
Although the role of individual land surface characteristics, such as soil moisture and albedo, in the climate system has been widely recognized, the effects of vegetation biophysical processes (VBP) are not yet fully understood. In this study, VBP effects are assessed using the coupled GCMs and land models under constrain of observed precipitation and reanalysis data. The Simplified Simple Biosphere Model (SSiB) has been coupled to the UCLA atmospheric general circulation model (UCLA AGCM/SSiB-1) to investigate the role and mechanism of land/atmosphere interactions. The coupled system was run for 6-years using climatological sea surface temperature (SST) and the results compared with those obtained with the same AGCM except for the use of an earlier land surface parameterization (UCLA GCM/CNTL). In this earlier version, climatological surface albedo and ground wetness are prescribed and surface temperature is obtained by using a simple single layer energy balance model. The UCLA AGCM/SSiB-1 results indicate a substantial impact from the explicit representation of VBP. The systematic bias in the CNTL precipitation climatology is reduced, especially over land. The annual mean precipitation bias and rms error were reduced by 60% and 40% over global land, respectively. The improvement is consistent for all continents and all seasons. Improvements are most clear in monsoon climate regions, such as West African monsoon and South American monsoon and areas characterized by large landmass, such as the boreal forest areas over the Eurasian and North American continent. These regions also appeared to be the regions sensitive to climate/VBP interactions with the NCEP GCM in other studies. Over West Africa, both bias and rms error in the UCLA GCM/SSiB were reduced by about 80%. Over Amazon, they were reduced by 50% and 25%, respectively. Among four seasons, VBP has significant contribution in spring and summer for most regions over the world, except West Africa, where VBP has significant effect over all four seasons. These results indicate that under unstable atmospheric conditions, not only low frequency mean forcings from the land surface, such as monthly mean albedo, but also VBP perturbation processes are important to the continental scale precipitation. A realistic representation of VBP in the GCM, therefore, is important for global water and energy studies. It is particular crucial to the monsoon climate regions such as West Africa and South America and areas characterized by large landmass such as the boreal forest zone.
A43E-03
AMMA Land surface Model Intercomparison Project (ALMIP)
Extreme climatic variability has afflicted West Africa over the last half century, which has resulted in significant
socio-economic consequences for the people of this region. There is therefore a need to improve seasonal to
inter-annual
prediction of the West-African monsoon (WAM),
however, difficulties modeling the WAM
arise from both the paucity of observations at sufficient space-time
resolutions, and due to the complex interactions between the
biosphere, atmosphere and hydrosphere over this region.
In particular, there is evidence that the land surface
influences the variability of the WAM over a wide range of
spatio-temporal scales. A critical aspect of this
coupling is the feedback between the regional atmospheric circulation
and the strong meridional surface flux gradients of mass and energy.
One of the main goals of the African Monsoon Multi-disciplinary
Analysis (AMMA) Project is to obtain a better understanding of the
physical processes influencing the West-African Monsoon (WAM) on daily
to inter-annual timescales. An improved comprehension of the relevant
land surface processes is being addressed through the construction of
a multi-scale atmospheric and land surface parameter forcing database
using a variety of sources; numerical weather prediction forecast data,
remote sensing products and local scale observations. The goal of this
database is to drive land surface, vegetation and hydrological models
over a range of spatial scales (local to regional) in order to gain
better insights into the attendant processes. This goal is being met
under the auspices of the AMMA Land surface Model Intercomparison
Project (ALMIP). In the recently completed Phase 1 of this project, an
ensemble of state-of-the-art land surface schemes have been run in
"off-line" mode (i.e. decoupled from an atmospheric model) at a
regional scale over western Africa for four annual cycles (2002-5). In
this talk, intercomparison results will be presented. In addition,
results from a second experiment will be presented which show the
significant impact of including remotely sensed data on the model
simulations.
http:www.cnrm.meteo.fr/amma-moana/amma_surf/almip/index.html
A43E-04
SMART-COMMIT Observations and Deep-Blue Retrievals of Saharan Dust Properties during NAMMA
Monsoon rainfalls sustain the livelihood of more than half of the world's population. The interaction between natural/anthropogenic aerosols, clouds, and precipitation is a critical mechanism that drives the water cycle and fresh water distribution. Analyses of the long-term trend of July-August precipitation anomaly for the last 50 years in the 20th century depict that the largest regional precipitation deficit occurs over the Sahel, where the monsoon water cycle plays an important role. Thus, it is of paramount importance to study how dust aerosols, as well as air pollution and smoke, influence monsoon variability. The NASA African Monsoon Multidisciplinary Activities (NAMMA) was conducted during the international AMMA Special Observation Period (SOP-3) of September 2006 to better comprehend the key attributes of the Saharan Air Layer (SAL) and how they evolve from the source regions to the Atlantic Ocean. The SAL occurs during the late spring through early fall and originates as a result of low-level convergence induced by heat lows over the Sahara that lifts hot, dry, dust laden air aloft into a well mixed layer that extends up to 500mb. This is crucial for understanding the impact of SAL on the key atmospheric processes that determine precipitation over West Africa and tropical cyclogenesis. Results obtained from the synergy of satellite (Deep-Blue) and surface (SMART-COMMIT) observations will be presented and discussed how the physical, optical and radiative properties of the dust in the SAL evolve from the continental to the marine environment.
A43E-05
Shallow Meridional Circulation of the West African Monsoon: A New Conceptual Model
The large-scale meridional circulation of the West African monsoon consists of, according to conventional wisdom, a classic, Hadley-type deep overturning cell. The ascending branch of this overturning cell is collocated with the monsoon rainband, its low-level southerly branch is in the boundary layer, commonly referred to as the monsoon flow, its northerly return flow is in the upper troposphere, and its descending branch spreads over a broad region over the eastern Atlantic Ocean. Recent diagnoses of in situ sounding data and global reanalysis products have revealed a different picture: In addition to this deep overturning cell, there exists a shallow meridional circulation (SMC). This SMC shares the same boundary-layer southerly monsoon flow with the deep overturning cell. But its ascending branch is further inland, north of the monsoon rainband, and is collocated with the Saharan heat low. Its northerly return flow, in the lower troposphere, penetrates through the monsoon rainband in the mean. This dual meridional circulations associated with the West African monsoon forms a contrast not only to the traditional concept of its large-scale circulation patterns, but also to the dual meridional circulations associated with the marine ITCZ. There are substantial discrepancies among existing global reanalyses and climate models in their capability of capturing the SMC of the West African monsoon. It is postulated that the springtime northward "jump" of the West African monsoon rainband is partially related to this shallow meridional circulation.
A43E-06
Onset of the West African Monsoon: Role of Inertial Instability in the Monsoon Jump
The West African monsoon jump is a sudden change in the position of the rainfall maximum over the continent in boreal summer. Through the spring, the precipitation maximum lies over the Guinean coast near 5°N. At this time, the hydrodynamics is closely tied to the ocean environment of the Gulf of Guinea, similar to the Atlantic marine ITCZ. In June or July the dynamics changes over the course of a few days and the precipitation maximum is repositioned much farther inland, near 12°N over the Sahel. Since the presence of the land surface is responsible for the occurrence of the jump, it can also be termed the monsoon onset. The dynamics of the monsoon jump is studied using a regional atmospheric model in two simulation designs. In one, the West African monsoon system is represented in a climatological context, with prescribed time-mean lateral and surface boundary conditions and 90-km resolution. In the second design, which is part of the West African Monsoon Modeling and Evaluation (WAMME) program, both the climatology and 5 individual (2002-2006) years are simulated with higher resolution (50 km) and an interactive land surface. The monsoon jump is captured realistically in the model. The dynamics of the jump is found to be related to the development of inertial instability over West Africa in the summer, when strong meridional geopotential height gradients develop in late spring/early summer. The role of the monsoon jump in the interannual variability of the monsoon is discussed.
A43E-07
Effects of Saharan dust on the diurnal and seasonal variability of the West African Monsoon
The effects of Saharan dust on the West African Monsoon (WAM) are investigated using the NASA finite-volume general circulation model (fvGCM). Seasonally evolving, 3-dimensional global radiative forcing of Saharan dust is derived from the GOddard Chemistry-Aerosol-Radiation Transport (GOCART) and incorporated into the fvGCM. By comparing control experiments with no dust forcing, to identical experiments with dust forcing, we examine the impact of dust forcing on the diurnal and seasonal variability of the WAM. Results show that the amplitude of the diurnal cycles of rainfall and temperature over WAM land is generally reduced by dust forcing, but increased over the adjacent oceans. The diurnal cycles are also found to be dependent on the synoptic scale conditions. On daily to seasonal time scales, dust radiative forcing induces a cloud-precipitation-dynamic feedback that draws moisture and rainfall inland toward the elevated dust layer, analogous to the "Elevated Heat Pump" (EHP) effect found for the South Asian Monsoon (Lau et al. 2006). The net effects are manifested in a northward shift of the WAM rainbelt and an advance of the rainy season. The interaction of land surface and atmospheric processes associated with the EHP are discussed.
A43E-08
West African Monsoon in a 20km-Mesh Atmospheric GCM
West African monsoon climate simulated by a global 20-km mesh atmospheric general circulation model (AGCM) forced by the global sea surface temperature (SST) during the period 1979-2005 is investigated. In comparison with a lower resolution (180-km mesh) model experiment, it is revealed that the 20-km mesh AGCM shows the superiority in simulating orographic rainfall not only its location but also its amount. However, both the models have common biases in northward penetration of summer rainfall into Sahara, which is associated with moister conditions in the model lower troposphere. This bias is exaggerated in the higher resolution model. The association of the interannual variability of the West African monsoon rainfall in JJAS season to global SST distribution, however, is better captured in the 20-km model than that in the 180-km model. Diurnal variation of precipitation is also compared with the Tropical Rainfall Measuring Mission (TRMM) data.