Boundary layer observations in West Africa using a ground-based 14-channel

microwave radiometer

Bernhard Pospichal and Susanne CrewellUniversity of Cologne

pospichal@meteo.uni-koeln.de

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

Bernhard Pospichal, MicroRad 2008

Measurement of atmospheric emission in two frequency bands:

A: 22.235 –31.4 GHz, 7 frrequencies on the upper wing of the water vapour absorption line and in the atmospheric window (IWV,LWP)

B: 51.26 – 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 DEC100

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cloudy (< 7500 m)clear skyno measurements

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DECIWV kgm-2

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Rainmm/day

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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/m2 in dry season)

• April: transition to wet season (IWV from 10 to 52 kg/m2)

• July: wet season, low variability (standard deviation only 3 kg/m2)

• December: dry season, no moist air masses

JULY DECEMBER

JANUARY APRIL

IWV statistics

JANUARY APRIL

AUGUST DECEMBER

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/m2

• 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: 700-0 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 AprilIWV 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 mixedwind from STemperature in

Nangatchori:solid line: 1.2 m agldashed: 2.5 m agldash-dotted: 4 m agl

Wind direction:o Djougoux 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

12 18 0 6 1212 18 0 6 12Time (UTC)

T

Θe

12 18 0 6 12

Θ

12 18 0 6 12

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 observedwith 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

LWP IWV

Total Sky Imager

IR, LWP and IWV inpolar 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

13:00-14:30convection in NW direction

after 15 UTC: humidity increase

For clear sky cases:Strong correlation betweenIR- 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