Boundary layer observations in West Africa using a ground-based 14-channel microwave radiometer Bernhard Pospichal and Susanne Crewell University of Cologne

AMMA-African Monsoon Multidisciplinary Analyses  AMMA: International project for research on West African monsoon  Large number of remote sensing instruments were operated for the first time in West Africa  A HATPRO microwave radiometer was deployed by the Universities of Bonn and Cologne for one year (2006) in Nangatchori (Ouémé, Benin). Aims of AMMA: Bernhard Pospichal, MicroRad 2008

 Measurement of atmospheric emission in two frequency bands:  A: –31.4 GHz, 7 frrequencies on the upper wing of the water vapour absorption line and in the atmospheric window (IWV,LWP)  B: – 58.0 GHz, 7 frequencies at the oxygen absorption complex (-> temperature profiles) HATPRO microwave profiler

Vertical measurements high temporal resolution (1 sec) Simultaneous observation of all 14 channels Products: Retrieval of LWP (liquid water path) and IWV (integrated water vapor) Temperature profiles up to 10 km altitude Humidity profiles up to 5 km Retrievals: Statistical retrieval algorithms are developed on the basis of a large set of atmospheric profiles observed by radiosondes in Northern Australia HATPRO: two different operation modes Bernhard Pospichal, MicroRad 2008  Under assumption of horizontal homogeneity, measurements under different elevation angles (90 to 5 degrees) provide additional information on temperature profiles in the atmospheric boundary layer Boundary layer scans

Overview of 2006 Djougou observations JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Daily percentage of clouds cloudy (< 7500 m) clear sky no measurements JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC IWV kgm Rain mm/day Rain rate mm/d Bernhard Pospichal, MicroRad 2008

IWV statistics The high number of observations allowed to make monthly statistics of IWV Main characteristics: January: several moist air outbreaks (IWV up to 47 kg/m 2 in dry season) April: transition to wet season (IWV from 10 to 52 kg/m 2 ) July: wet season, low variability (standard deviation only 3 kg/m 2 ) December: dry season, no moist air masses JULY DECEMBER JANUARYAPRIL

IWV statistics

JANUARYAPRIL AUGUSTDECEMBER LWP statistics Statistics for LWP for different months of 2006 (note: y-axis in logarithmic scale) Main characteristics: January: moist air outbreaks caused some cloudiness April: transition to wet season (still quite few clouds) August: peak of wet season, large number of cloudy periods, some values exceeding 2000 g/m 2 December: no clouds present!

Temperature profile statistics

Temperature, diurnal cycle Diurnal cycle of potential temperature in 50 m above ground Diurnal cycle of potential temperature in 700 m above ground Bernhard Pospichal, MicroRad 2008

Temperature, diurnal cycle Monthly potential temperature difference: m above ground Gradient of potential temperature is positive during negative during night and slightly posivite during daytime Differences between wet and dry season: Nighttime inversions much stronger during dry season (maximum in december), Daytime superadiabatic layer close to the ground in dry season, neutral layering in wet season Bernhard Pospichal, MicroRad 2008

Example 28 June 2006 Day in wet season no rain shallow convection during daytime Bernhard Pospichal, MicroRad 2008

Example 28 June 2006 HATPRO IWV and LWP Bernhard Pospichal, MicroRad 2008

Example 28 June 2006 HATPRO relative humidity Bernhard Pospichal, MicroRad 2008

Example 6 October 2006 Day towards the end of wet season One short rainfall event (13 UTC) Early morning fog, rising PBL, some higher clouds Bernhard Pospichal, MicroRad 2008

Temperature, diurnal cycle Bernhard Pospichal, MicroRad 2008

Close view on April 2006  Transition from dry to wet season in Djougou (9.7°N) occurs during April IWV ranges from 10 to 50 kgm -2  Transition is not gradual – Djougou is periodically under the influence of drier (from the north) and moister air (from the south) relative position to inter-tropical discontinuity (ITD)  Diurnal cycle is evident in high frequency oscillations of IWV time series IWV kgm -2 1 Apr 10 Apr 20 Apr 30 Apr diurnal event on April 10/11 Bernhard Pospichal, MicroRad 2008

Surface observations 10/11 April 2006 strong inversion weak winds well mixed wind from S Temperature in Nangatchori: solid line: 1.2 m agl dashed: 2.5 m agl dash-dotted: 4 m agl Wind direction: o Djougou x Nangatchori Wind speed at Djougou All data are 15-min averages Bernhard Pospichal, MicroRad 2008

Ceilometer obs 10/11 April 2006 Bernhard Pospichal, MicroRad 2008

RH Time (UTC) T ΘeΘe Θ

Summary: HATPRO microwave radiometer was deployed in West Africa during 2006 Despite the rather bad infrastructure, data availability is > 80% with a temporal resolution of 2 seconds (vertical mode) and 15 minutes (elevation scanning mode) Monthly averaged hourly boundary layer profiles of temperature and humidity show the evolution of the atmosphere throughout the year or in the course of the day Low level jet during March/April has been observed with high temporal and vertical resolution Outlook: 3D-Scanning of the atmosphere with HATPRO provides a look on water vapour and temperature distribution in the surroundings of the station. This feature has not been available in Africa yet. Bernhard Pospichal, MicroRad 2008

IR LWPIWV Total Sky Imager IR, LWP and IWV in polar plots. Note: Only the anomaly of the values is shown here! Data from COPS campaign in Black Forest 2007, Courtesy to Stefan Kneifel (negative axes represent positive values!) Recent developments: Azimuth scanning capabilities Bernhard Pospichal, MicroRad 2008

Until 12 UTC asymmetric humidity field after 12 UTC strong decrease in humi- dity by about 10 kgm -2 13:00-14:30 convection in NW direction after 15 UTC: humidity increase For clear sky cases: Strong correlation between IR- und IWV structures Case study 14 July 2007 Bernhard Pospichal, MicroRad 2008

Thank you for your attention! AMMA - African Monsoon Multidisciplinary Analysis Bernhard Pospichal, MicroRad 2008