Ground-based remote sensing

2010

Diurnal cycle of the inter-tropical discontinuity over West Africa analysed by remote sensing and mesoscale modelling, B. Pospichal, D. Bou Karam, S. Crewell, C. Flamant, A. Hünerbein, O. Bock, and F. Said, Quarterly Journal of the Royal Meteorological Society, 136, 92-106, doi: 10.1002/qj.435.

The diurnal cycle of the Intertropical discontinuity (ITD), i.e. the interface at the ground between moist monsoon air and dry Harmattan air, is an important factor in the West African monsoon system. During the whole of 2006, high-resolution ground-based remote-sensing measurements were performed in the area of Djougou, Benin, which made it possible to observe the ITD and the associated sharp gradient of temperature and humidity in detail. In order to extend the point measurements to a three-dimensional view of the ITD and to enhance the knowledge of the related processes, the mesoscale atmospheric model Meso-NH was run for a case study (84 h period) in April 2006. In addition Meteosat infrared observations were used to determine the ITD position and its movement. From these observations a northward propagation of the moist air front (ITD) of 8–12 m s-1 was calculated. The propagation speed of the front evaluated from the satellite observations was replicated by the model and the time of the front arrival over Djougou was simulated with a maximum error of 1 hour. With respect to the ground-based observations, the model was able to reproduce the dynamics and thermodynamics of the front as well as the diurnal cycle of the planetary boundary layer over the study area. This agreement suggests the use of the model to further describe processes in the lower atmosphere at high resolution.

link:       Quarterly Journal of the Royal Meteorological Society


Observations of the lower atmosphere over West Africa using ground-based remote sensing instruments, B. Pospichal, S. Crewell. In: Integrated ground-based observing systems, D. Cimini, G. Visconti, F. S. Marzano, Eds., Springer Berlin Heidelberg (2010), 279-293

This chapter presents results from a 1-year field experiment in West Africa deploying a suite of ground-based instrumentation. Weather and climate over tropical West Africa are determined by the annual cycle of the West African monsoon. The relationships between the monsoon season and the displacements of the inter-tropical convergence zone and the inter-tropical discontinuity are briefly remarked, as well as the importance of atmospheric humidity in determining the strength of the monsoon. The scarce and discontinuous observations in the West Africa region cause lack of measurements with high temporal resolution and diurnal-to-annual coverage; conversely, long-term and continuous monitoring is essential for deepening the understanding of the mechanisms that are responsible for inter-annual variability of the West African monsoon. Within the African Monsoon Multidisciplinary Analysis project a wealth of observations from ground-based instrumentation, such as microwave radiometer, ceilometer, and microrain radar, were possible for a period of one year or more. The complete annual cycle of cloud cover, integrated water and liquid contents, and temperature profiles are reported. Furthermore, a distinct diurnal cycle, connected with the move of the inter-tropical discontinuity before the onset of the wet season, was observed and used to evaluate a mesoscale model.

link:       Springer Verlag


Meteorological events affecting cold-air pools in a small basin, M. Dorninger, C. D. Whiteman, B. Bica, S. Eisenbach, B. Pospichal, and R.Steinacker, J. Appl. Meteor. Climatol. (2010), in press.



link:       Journal of Applied Meteorology anc Climatology


2008

Microlidar observations of biomass burning aerosol over Djougou (Benin) during African Monsoon Multidisciplinary Analysis Special Observation Period 0: Dust and Biomass-Burning Experiment, J. Pelon, M. Mallet, A. Mariscal, P. Goloub, D. Tanré, D. Bou Karam, C. Flamant, J. Haywood, B. Pospichal, and S. Victori, J. Geophys. Res. 113, D00C18, doi: 10.1029/2008JD009976.

The purpose of this work is to investigate the direct radiative forcing of aerosols over the supersite of Djougou (northern Benin) during the African Monsoon Multidisciplinary Analyses dry season experiment. We focus our simulations on the top of atmosphere, bottom of atmosphere, and atmosphere radiative forcings. During the dry season period, Sun photometer measurements indicate a rather turbid atmosphere with a mean aerosol optical depth for the overall period of 0.78 ± 0.24 (at 440 nm). The aerosol absorption coefficient estimated at the surface ranged between 2.3 and 37.3 Mm-1 (mean value 15.2 ± 10.6 Mm-1 at 520 nm) and the scattering coefficient between 44.5 and 232.3 Mm-1 (mean 145 ± 59 Mm-1 at 520 nm). This leads to a single scattering albedo of between 0.81 and 0.98 (at 520 nm) with a mean (and standard deviation) value of 0.91 ± 0.05, indicating moderately absorbing aerosols. In parallel, micropulse lidar measurements indicate the presence of two distinct aerosol layers, with a first one located between the surface and 1 km and a second one located above 1.5–4.0 km. On the basis of surface and aircraft observations, sunphotometer measurements, lidar profiles, and Moderate Resolution Imagaing Spectroradiometer sensor an estimation of the daily clear sky direct radiative forcing has been estimated for the 17–24 January 2006 period. Simulations indicate that aerosols reduce significantly the solar energy reaching the surface (mean ΔF BOA = -61.5 W/m2) by reflection to space (mean ΔF TOA = -18.4 W/m2) but predominantly by absorption of the solar radiation into the atmosphere (mean ΔF ATM = +43.1 W/m2). The mean heating rate at the surface and within the elevated biomass burning layer is considerably enhanced by 1.50 and 1.90 K day-1, respectively.

link:       Journal of Geophysical Research


2007

Boundary Layer Observations in West Africa using a novel microwave radiometer, B. Pospichal, and S. Crewell, Meteorologische Zeitschrift (2007), 16, 513-523

Boundary layer measurements in Nangatchori, Benin were performed over the period of one full year (2006) using a novel ground-based microwave profiler and additional remote-sensing instruments. In this paper, the diurnal cycle of the ITD (Inter-Tropical Discontinuity) in the transition period between dry and wet season during the month of April is described in detail. Dry air masses from the north (Sahel) and moist air from the south (tropical Atlantic Ocean) cause very sharp temperature and humidity gradients in the low troposphere over West Africa. Continuous observations of these phenomena in terms of temperature and humidity profiles have been achieved for the first time with a high temporal resolution of less than 15 minutes. Especially the ability of the microwave radiometer to provide temperature profiles with high vertical resolution through multi-angle measurements gives detailed boundary layer information in conjunction with meteorological tower observations. Together with additional lidar ceilometer observations of aerosol backscatter the change of air masses can be seen very well. The high data availability of > 85 % allows a statistical analysis of the full month of April in which Nangatchori comes increasingly under the influence of tropical air. Thus the data set is well suited for an improved process understanding, model evaluation in a data sparse area, and possibly together with additional observations the development of improved boundary layer parameterizations for atmospheric models.

link:       Meteorologische Zeitschrift


2004

Inversion breakup in small Rocky Mountain and Alpine basins , C. D. Whiteman, B. Pospichal, S. Eisenbach, P. Weihs., C. B. Clements, R. Steinacker, E. Mursch-Radlgruber, and M. Dorninger, J. Appl. Meteor. (2004), 43, 1069-1082

Comparisons are made between the postsunrise breakup of temperature inversions in two similar closed basins in very different climate settings, one in the eastern Alps and one in the Rocky Mountains. The small, high-altitude, limestone sinkholes have both experienced extreme temperature minima below -50°C and both develop strong nighttime inversions. On undisturbed clear nights, temperature inversions reach to 120-m heights in both sinkholes but are much stronger in the drier Rocky Mountain basin (24 vs 13 K). Inversion destruction takes place 2.6–3 h after sunrise in these basins and is accomplished primarily by subsidence warming associated with the removal of air from the base of the inversion by the upslope flows that develop over heated sidewalls. A conceptual model of this destruction is presented, emphasizing the asymmetry of the boundary layer development around the basin and the effects of solar shading by the surrounding ridgeline. Differences in inversion strengths and postsunrise heating rates between the two basins are caused by differences in the surface energy budget, with drier soil and a higher sensible heat flux in the Rocky Mountain sinkhole. Inversions in the small basins break up more quickly following sunrise than for previously studied valleys. The pattern of inversion breakup in the non-snow-covered basins is the same as that reported in snow-covered Colorado valleys. The similar breakup patterns in valleys and basins suggest that along-valley wind systems play no role in the breakups, since the small basins have no along-valley wind system.

link:       Journal of Applied Meteorology


Minimum temperatures, diurnal temperature ranges, and temperature inversions in limestone sinkholes of different sizes and shapes , C. D. Whiteman, T. Haiden, B. Pospichal, S. Eisenbach, and R. Steinacker, J. Appl. Meteor. (2004), 43, 1224-1236

Air temperature data from five enclosed limestone sinkholes of various sizes and shapes on the Hetzkogel Plateau near Lunz, Austria (1300 m MSL), have been analyzed to determine the effect of sinkhole geometry on temperature minima, diurnal temperature ranges, temperature inversion strengths, and vertical temperature gradients. Data were analyzed for a non-snow-covered October night and for a snow-covered December night when the temperature fell as low as -28.5°C. A surprising finding is that temperatures were similar in two sinkholes with very different drainage areas and depths. A three-layer model was used to show that the sky-view factor is the most important topographic parameter controlling cooling for basins in this size range in near-calm, clear-sky conditions and that the cooling slows when net longwave radiation at the floor of the sinkhole is nearly balanced by the ground heat flux.

link:       Journal of Applied Meteorology


Comparison of vertical soundings and sidewall air temperature measurements in a small Alpine basin , C. D. Whiteman, S. Eisenbach, B. Pospichal, and R. Steinacker, J. Appl. Meteor. (2004), 43, 1635-1647

Tethered balloon soundings from two sites on the floor of a 1-km-diameter limestone sinkhole in the eastern Alps are compared with pseudovertical temperature “soundings” from three lines of temperature dataloggers on the basin's northwest, southwest, and southeast sidewalls. Under stable nighttime conditions with low background winds, the pseudovertical profiles from all three lines were good proxies for free air temperature soundings over the basin center, with a mean nighttime cold temperature bias of about 0.4°C and a standard deviation of 0.4°C. Cold biases were highest in the upper basin where relatively warm air subsides to replace air that spills out of the basin through the lowest-altitude saddle. On a windy night, standard deviations increased to 1°–2°C. After sunrise, the varying exposures of the dataloggers to sunlight made the pseudovertical profiles less useful as proxies for free air soundings. The good correspondence between sidewall and free air temperatures during high-static-stability conditions suggests that sidewall soundings can be used to monitor temperatures, temperature gradients, and temperature inversion evolution in the sinkhole. Sidewall soundings can produce more frequent profiles at lower cost than can tethersondes or rawinsondes, and extension of these findings to other enclosed or semienclosed topographies may enhance future basic meteorological research or support applications studies in agriculture, forestry, air pollution, and land use planning.

link:       Journal of Applied Meteorology