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Title: A One-Year Study of the Diurnal Cycle of Meteorology, Clouds, and Radiation in the West African Sahel Region

Abstract

The diurnal cycles of meteorological and radiation variables are analyzed during the wet and dry seasons over the Sahel region of West Africa during 2006 using surface data collected by the Atmospheric Radiation Measurement (ARM) program’s Mobile Facility, satellite radiation measurements from the Geostationary Earth Radiation Budget (GERB) instrument aboard Meteosat 8, and reanalysis products from the National Center for Environmental Prediction (NCEP). The meteorological analysis builds upon past studies of the diurnal cycle in the region by incorporating diurnal cycles of lower tropospheric wind profiles, thermodynamic profiles, integrated water vapor and liquid water measurements, and cloud radar measurements of frequency and location. These meteorological measurements are complemented by 3-hour measurements of the diurnal cycles of the TOA and surface shortwave (SW) and longwave (LW) radiative fluxes and cloud radiative effects (CREs), and the atmospheric radiative flux divergence (RFD) and atmospheric CREs. Cirrus cloudiness during the dry season is shown to peak in coverage in the afternoon, while convective clouds during the wet season are shown to peak near dawn and have an afternoon minimum related to the rise of the Lifting Condensation Level into the Saharan Air Layer. The LW and SW RFDs and CREs exhibit diurnal cycles duringmore » both seasons, but there is a relatively small difference in the LW cycles during the two seasons (10-30 Wm^(-2) depending on the variable and time of day). Small differences in the TOA CREs during the two seasons are overwhelmed by large differences in the surface SW CREs, which exceed 100 Wm^(-2). A significant surface SW CRE during the wet season combined with a negligible TOA SW CRE produces a diurnal cycle in the atmospheric CRE that is modulated primarily by the SW surface CRE, peaks at midday at ~150 Wm^(-2), and varies widely from day to day.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science - Office of Biological and Environmental Research - Atmospheric Radiation Measurement (ARM) Program
OSTI Identifier:
1249437
DOE Contract Number:
AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Quarterly Journal of the Royal Meteorological Society; Journal Volume: 142; Journal Issue: 694
Country of Publication:
United States
Language:
English

Citation Formats

Marquardt-Collow, Allison, Ghate, Virendra P., Miller, Mark A., and Trabachino, Lynne. A One-Year Study of the Diurnal Cycle of Meteorology, Clouds, and Radiation in the West African Sahel Region. United States: N. p., 2016. Web. doi:10.1002/qj.2623.
Marquardt-Collow, Allison, Ghate, Virendra P., Miller, Mark A., & Trabachino, Lynne. A One-Year Study of the Diurnal Cycle of Meteorology, Clouds, and Radiation in the West African Sahel Region. United States. doi:10.1002/qj.2623.
Marquardt-Collow, Allison, Ghate, Virendra P., Miller, Mark A., and Trabachino, Lynne. 2016. "A One-Year Study of the Diurnal Cycle of Meteorology, Clouds, and Radiation in the West African Sahel Region". United States. doi:10.1002/qj.2623.
@article{osti_1249437,
title = {A One-Year Study of the Diurnal Cycle of Meteorology, Clouds, and Radiation in the West African Sahel Region},
author = {Marquardt-Collow, Allison and Ghate, Virendra P. and Miller, Mark A. and Trabachino, Lynne},
abstractNote = {The diurnal cycles of meteorological and radiation variables are analyzed during the wet and dry seasons over the Sahel region of West Africa during 2006 using surface data collected by the Atmospheric Radiation Measurement (ARM) program’s Mobile Facility, satellite radiation measurements from the Geostationary Earth Radiation Budget (GERB) instrument aboard Meteosat 8, and reanalysis products from the National Center for Environmental Prediction (NCEP). The meteorological analysis builds upon past studies of the diurnal cycle in the region by incorporating diurnal cycles of lower tropospheric wind profiles, thermodynamic profiles, integrated water vapor and liquid water measurements, and cloud radar measurements of frequency and location. These meteorological measurements are complemented by 3-hour measurements of the diurnal cycles of the TOA and surface shortwave (SW) and longwave (LW) radiative fluxes and cloud radiative effects (CREs), and the atmospheric radiative flux divergence (RFD) and atmospheric CREs. Cirrus cloudiness during the dry season is shown to peak in coverage in the afternoon, while convective clouds during the wet season are shown to peak near dawn and have an afternoon minimum related to the rise of the Lifting Condensation Level into the Saharan Air Layer. The LW and SW RFDs and CREs exhibit diurnal cycles during both seasons, but there is a relatively small difference in the LW cycles during the two seasons (10-30 Wm^(-2) depending on the variable and time of day). Small differences in the TOA CREs during the two seasons are overwhelmed by large differences in the surface SW CREs, which exceed 100 Wm^(-2). A significant surface SW CRE during the wet season combined with a negligible TOA SW CRE produces a diurnal cycle in the atmospheric CRE that is modulated primarily by the SW surface CRE, peaks at midday at ~150 Wm^(-2), and varies widely from day to day.},
doi = {10.1002/qj.2623},
journal = {Quarterly Journal of the Royal Meteorological Society},
number = 694,
volume = 142,
place = {United States},
year = 2016,
month = 1
}
  • Here, the diurnal cycles of meteorological and radiation variables are analysed during the wet and dry seasons over the Sahel region of West Africa during 2006 using surface data collected by the Atmospheric Radiation Measurement (ARM) programme's Mobile Facility, satellite radiation measurements from the Geostationary Earth Radiation Budget (GERB) instrument aboard Meteosat 8, and reanalysis products from the National Centers for Environmental Prediction (NCEP). The meteorological analysis builds upon past studies of the diurnal cycle in the region by incorporating diurnal cycles of lower tropospheric wind profiles, thermodynamic profiles, integrated water vapour and liquid water measurements, and cloud radar measurementsmore » of frequency and location. These meteorological measurements are complemented by 3 h measurements of the diurnal cycles of the top-of-atmosphere (TOA) and surface short-wave (SW) and long-wave (LW) radiative fluxes and cloud radiative effects (CREs), and the atmospheric radiative flux divergence (RFD) and atmospheric CREs. Cirrus cloudiness during the dry season is shown to peak in coverage in the afternoon, while convective clouds during the wet season are shown to peak near dawn and have an afternoon minimum related to the rise of the lifting condensation level into the Saharan Air Layer. The LW and SW RFDs and CREs exhibit diurnal cycles during both seasons, but there is a relatively small difference in the LW cycles during the two seasons (10 – 30 W m –2 depending on the variable and time of day). Small differences in the TOA CREs during the two seasons are overwhelmed by large differences in the surface SW CREs, which exceed 100 W m –2. A significant surface SW CRE during the wet season combined with a negligible TOA SW CRE produces a diurnal cycle in the atmospheric CRE that is modulated primarily by the SW surface CRE, peaks at midday at ~150 W m –2, and varies widely from day to day.« less
  • the climate of West Africa, in particular the Sahel, is characterized by multiyear persistence of anomalously wet or dry conditions. Its Southern Hemisphere counterpart, the Kalahari, lacks the persistence that is evident in the Sahel even though both regions are subject to similar large-scale forcing. It has been suggested that land surface-atmosphere feedback contributes to this persistence and to the severity of drought. In this study, surface energy and water balance are quantified for nine stations along a latitudinal transect that extends from the Sahara to the Guinea coast. In the wetter regions of West Africa, the difference between wetmore » and dry years is primarily reflected in the magnitude of runoff. For the Sahel and drier locations, evapotranspiration and soil moisture are more sensitive to rainfall anomalies. The increase in evapotranspiration, and hence latent heating, over the Sahel in wet years alters the thermal structure and gradients of the overlying atmosphere and thus the strength of the African easterly jet (AEJ) at 700 mb. The difference between dry and wet Augusts corresponds to a decrease in magnitude of the AEJ at 15[degrees]N on the order of 2.6 m s[sup [minus]1], which is consistent with previous studies of observed winds. Spatial patterns were also developed for surface water balance parameters for both West Africa and southern Africa. Over southern Africa, the patterns are not as spatially homogeneous as those over West Africa and are lower in magnitude, thus supporting the suggestion that the persistence of rainfall anomalies in the Sahel might be due, at least in part, to land-atmosphere feedback, and that the absence of such persistence in the Kalahari is a consequence of less significant changes in surface water and energy balance. 38 refs., 10 figs. 2 tabs.« less
  • The relationship among clouds, surface radiation flux, and the sea surface temperature (SST) of the tropical western Pacific Ocean over the diurnal cycle is addressed in the context of the Atmospheric Radiation Measurement (ARM) Program scientific objectives for the tropical western Pacific Ocean. An understanding of the relationship between clouds and SST on a variety of time and space scales is needed to understand fully the cloud-radiation feedback in the tropical oceans and the maintenance of the warm pool. Here the diurnal cycle is emphasized. Data from the TOGA COARE Intensive Observation Period is examined and interpreted using an oceanmore » mixed layer model that includes a parameterization of the {open_quotes}skin{close_quotes} temperature, explicit salinity, a surface momentum flux. Using a mix of modeling and observations, three different case studies are examined in detail: clear and calm, clear and windy, and disturbed. For these typical sets of conditions and processes in the tropical ocean warm pool, the upper-ocean structure is clarified so that skin sea surface temperature, the bulk surface temperature (at a depth of 1 cm), and the temperature at 0.5 and 5 m below the surface (which is the level that buoys and ships routinely observe {open_quotes}surface{close_quotes} temperature) can be interpreted. Sensitivity studies are conducted with the model to investigate the roles of wind speed, precipitation, ocean turibity, and ocean initial state in modulating the radiation-induced diurnal cycle in SST. It is found that in high insolation, low wind regimes that the skin temperature may be as much as 30{degrees}C warmer than the 0.5-m buoy temperature. Spatial distribution of the diurnal amplitude of the SST are calculated for the global Tropics, and speculations are made regarding the implication of the SST variability to the tropical climate. 65 refs., 12 figs., 5 tabs.« less
  • The diurnal variation of convection and associated cloud and radiative properties remains a significant issue in global NWP and climate models. This study analyzes observed diurnal variability of convection in a coastal monsoonal environment examining the interaction of convective rain clouds, their associated cloud properties, and the impact on the surface radiation and corresponding boundary layer structure during periods where convection is suppressed or active on the large scale. The analysis uses data from the Tropical Warm Pool International Cloud Experiment (TWP-ICE) as well as routine measurements from the Australian Bureau of Meteorology and the U.S. Department of Energy Atmosphericmore » Radiation Measurement (ARM) program. Both active monsoonal and large-scale suppressed (buildup and break) conditions are examined and demonstrate that the diurnal variation of rainfall is much larger during the break periods and the spatial distribution of rainfall is very different between the monsoon and break regimes. During the active monsoon the total net radiative input to the surface is decreased by more than 3 times the amount than during the break regime - this total radiative cloud forcing is found to be dominated by the shortwave (SW) cloud effects because of the much larger optical thicknesses and persistence of long-lasting anvils and cirrus cloud decks associated with the monsoon regime. These differences in monsoon versus break surface radiative energy contribute to low-level air temperature differences in the boundary layer over the land surfaces.« less
  • The principle climatic characteristics of the region of Niamey, Niger, in the Sahelian belt are illustrated on the basis of long meteorological series and with the aim of understanding the influence of dust on radiation. Empirical formulae are presented which make it possible to determine global radiation on the basis of duration of insolation, and diffuse radiation on the basis of global radiation. These formulae contain monthly constants, and it is shown that these are linked. The formulae are of practical interest for users for solar energy in the region.