2.1. Radiation Measurements
 The RADAGAST field experiment consisted of two primary measurement locations: the ARM Mobile Facility (AMF) [Miller and Slingo, 2007] and the Geostationary Earth Radiation Budget instrument aboard Meteosat-8. The AMF was located at Niamey airport (2.18°E, 13.48°N) and was present between mid-December 2005 and early January 2007: data relevant to this work range from 6 January to 31 December. The GERB instrument (GERB-2) was located above 3.3°W, 0.0°N. The AMF was active for each day of 2006, with short instrumental dropouts, whilst GERB suffered from only two outages longer than a day. (However, the GERB instrument was taken offline into sun avoidance mode during 4 hours about midnight for 110 days.)
 The measurements of the downwelling surface radiation are from two sources: broadband fluxes from a pyrgeometer (using one of the two instruments), and spectral zenith radiances from the Atmosphere Emitted Radiance Interferometer (AERI) instrument [Knuteson et al., 2004]. A distinct advantage of the AERI is its calibration sequence, scheduled between measurements of sky radiation, which results in known measurement characteristics for all times. The rawinsonde ascents from the AMF were made four times daily at times described as 0600, 1100, 1700, and 2300 UT. (Restricting sonde data to those launched within 1 hr of these times removed 4% of the launches.) The more physically accurate MWRRET [Turner et al., 2007] measurements of CWV replaced the canonical (MWRLOS) data stream. These data were all retrieved from the ARM archive and are summarized in Table 1. In addition, using AMF observations alone a surface-based cloud mask was created, analogous to other approaches [Henderson, 2006; Long and Ackerman, 2000].
Table 1. Surface Data Streams Used in This Worka
|Data Stream||Variables Measured||Temporal Resolution|
|Aerisummary||Averaged longwave zenith-pointing spectrally resolved radiance||400 s|
|Skyrad60s||Downwelling longwave flux, sky-looking infrared thermometer (down_long_hemisp_shaded2 variable is used for flux)||60 s|
|Gndrad60s||Upwelling longwave flux, surface-looking infrared thermometer||60 s|
|Mwrret||Improved microwave radiometer column water vapor retrievals||60 s|
|Sondewnpn||Rawinsonde, Vaisala RS-92, ascents||4 daily|
|Met||2 m height temperature, pressure||60 s|
 The GERB [Harries et al., 2005] instruments measure the total and shortwave radiance from geostationary orbit, and their difference returns the longwave radiance. Also aboard the Meteosat platform is the Spinning Enhanced Visible and Infrared Imager (SEVIRI) spectral imager [Schmetz et al., 2002]. A regression, on the basis of a database of radiative transfer calculations, utilizes the contemporaneous SEVIRI spectral measurements to derive the GERB radiance-to-flux conversion. This is used in turn to produce the operational GERB flux products [Clerbaux et al., 2003] from the radiance measurements; the Edition 1 averaged, rectified, and geolocated (ARG) products [Dewitte et al., 2008] are used in this work. The SEVIRI radiances themselves are used by the Nowcasting Satellite Application Facility (NWCSAF) [Slingo et al., 2009; Derrien and Le Gleau, 2005] to produce a TOA cloud mask which is used here. These products are summarized in Table 2.
Table 2. Satellite Data Streams Used in This Work
|GERB ARG||Edition 1 radiances and fluxes||∼46 × 46 km|
|NWCSAF cloud mask||SEVIRI-based cloud mask||∼3 × 3 km|
|MOD11A2/MYD11A2||Surface temperature from MODIS Terra/Aqua||1 × 1 km|
|CIMMS IR emissivity||Surface LW emissivity via MODIS measurements||0.05° × 0.05°|
 The errors from radiative measurements are provided in two different fashions: for the surface measurements, a standard deviation is made from the 60 samples per minute, whilst for the GERB ARG values, the quoted error for the radiances is a relative value and there is an additional error for the fluxes. These are summarized [Slingo et al., 2006] as one standard deviations: ±5Wm−2 for the surface fluxes, 1% for the GERB radiances, and ∼5 Wm−2 for the GERB fluxes. The systematic errors of the instruments are analyzed in this work in the context of calculating the atmospheric divergence.
2.2. Tools and Other Measurements
 The radiative transfer simulations were made using the Edwards-Slingo code [Edwards and Slingo, 1996] (hereafter referred to as ES96). Despite being designed for computational efficiency, it remains flexible in several areas, e.g., two-stream and spherical harmonic solvers are available and the latter was used for calculation of fluxes and radiances. The optical parameters of gases and aerosols are specified via spectral files, and here the file is as used in the MetOffice HadGEM1 model [Martin et al., 2006]. This is a nine-band model which operates between 3 μm and 100 μm and whose aerosol specification is from the WMO [World Meteorological Organization, 1990] with a modified size distribution [d'Almeida et al., 1991]. Using the times of sonde launches restricts the calculations to 4 per day, but this is sufficient to resolve the diurnal cycle for this work.
 A (binary) cloud mask is used to remove the times in the measurements when cloud affects the radiation. The SEVIRI-NWCSAF cloud mask results in ∼270 values per GERB ARG pixel, whose average when greater than 0.1 is interpreted as cloud. This mask results in evident false negatives from the surface perspective therefore the surface-based cloud mask replaces it for surface comparisons. However, when combining TOA and surface fluxes for divergence estimates, a joint mask is produced by assuming independence of each mask and combining them via a Boolean OR operator. Using the joint mask the percentage of available observations is decreased by ∼20% during the dry season and ∼66% during the wet (monsoon) season. (These seasons are of approximately the same length, and the monsoon period is defined for 2006 [Slingo et al., 2008] as being between 5 May and 29 October inclusive.)
 The sonde data are used to produce the temperature and humidity profiles for the radiative transfer code. The humidity is then scaled [Soden et al., 2004; Turner et al., 2004] such that it produces a CWV which is equal to the value from MWRRET retrievals. The profiles have ∼150 pressure levels with a fixed ozone prescription [Ellingson et al., 1991] (250 DU), and the long-lived gas species (CO2, CH4, O2, N2O) have vertically constant mixing ratios consistent with values from 2005 while halocarbons are neglected. These assumptions cause (acceptable) biases at the surface of −0.5 Wm−2, and at TOA of 0.7 Wm−2.
 These profiles can be used to simulate pristine sky (aerosol- and cloud-free) radiation fields and then, by comparisons with surface downwelling measurements, an estimate can be made of the direct radiative effect of aerosols. A companion paper in this special section concerning infrared aerosol retrievals from AERI radiances [Turner, 2008] shows good agreement of 11 micron optical depths from Niamey with 1 micron Aerosol Robotic Network (AERONET) retrievals from Banizoumbou [Holben et al., 2001], ∼60 km east of Niamey. (The AERONET site at Niamey itself only operated from August 2006 onward.) The analysis showed that a reasonable assumption is for the Ångstrom exponent between 11 microns and 1 micron to be a constant (∼0.33). The Banizoumbou AERONET retrievals are used for the analysis of differences between observations and calculations, but not directly as inputs into the RT profiles.
 The airport location means the area underneath the AMF radiation instruments is unrepresentative of the larger GERB ARG footprint. In particular, the wider area includes the River Niger and the urban area of Niamey proper. Hence, the profile surface properties, emissivity and surface temperature (LST), use satellite retrievals to supplement AMF measurements. The retrievals are from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments: LST via the MOD11A2 and MYD11A2 (the LST with 8 day time resolution and 1 km spatial resolution from the Terra/Aqua MODIS instrument) products [Wan and Li, 1997] and emissivity via the Cooperative Institute of Mesoscale Meteorological Studies (CIMMS) IR emissivity database [Seemann et al., 2008].
 The CIMMS emissivity retrievals are monthlong averages for several narrow bands over the longwave spectrum, from which the broadband emissivity was obtained using two separate approaches: by a bandwidth-weighted average and by using predetermined weightings [Wang et al., 2005]. Over the GERB ARG footprint, both methods resulted in emissivities with a difference of 0.01 RMS. The yearlong (1 January 2006 to 30 November 2006) average of these area average emissivities is 0.93 ± 0.01 so the RT profile emissivity, ε, is fixed to this value.
 To specify the LST in the RT profiles, TARG, an estimate of the temperature local to the AMF, TAMF, is required. This LST is transformed to TARG by using MODIS retrievals as described below. The measurement of LST, TIRT(t), from the AMF datastream uses a IR thermometer (IRT) which derives temperature from measurements of surface-emitted radiances between 9.8 μm and 11.5 μm. Alternately, a second measurement of upwelling radiation is from the upwelling pyrgeometer which observed the broadband flux, FULR;AMF(t). This flux can be turned into a temperature by assuming black body and Lambertian emission
Here we assume εAMF = 1, and so there is some unknown scaling factor. This is accounted for by scaling the diurnal mean of Tpyrg such that it is then equal to the diurnal mean of TIRT
where the overbar represents an average over the day (24 hrs) that relates to t.
 The IRT assumes a constant emissivity relation between the IR window and LW broadband, and if this were valid at the AMF site then the difference between TIRT and the derived Tflux would be expected to be proportional, i.e., a time-independent constant scaling factor. Figure 1 shows the difference between TIRT and Tflux for the four sonde launch times. It is evident that there is time dependence, particularly at 1100 UT, but verifying whether TIRT or Tflux is the appropriate value for TAMF requires comparisons of TOA radiances (section 4.1).
 Given TAMF, a transformation is needed to produce the temperature representing the wider area of the GERB ARG pixel, TARG. The MODIS LST retrievals are used for this purpose. They are (essentially) 8-day averages to compromise between data availability and time resolution. The retrievals are used to estimate the relationship between the LST for the MODIS footprint nearest the AMF, TMOD;AMF, and the LST over the ARG footprint, TMOD;ARG. The MODIS LST area transform was calculated over all 8-day periods between May and December, and was found to be approximately linear
To use this transform, the assumption made is that the MODIS footprint LST about the AMF is equivalent to the AMF LST: TAMF and TARG replace their MODIS equivalents in the equation. The MODIS footprint resolution (∼500 m) is sufficiently small to negate effects of the river or its adjacent paddy fields (≥3.5 km SW of the airport location). High-resolution imagery and local cartography also suggest that there are no obvious changes in land cover which could invalidate the assumption.