Jan 26, 2010 - The microphysical information obtained from the polarimetric variables .... were observed with the Massachusetts Institute of Tech- nology (MIT) ...
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 136(s1): 272–288 (2010) Published online 26 January 2010 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.561
Polarimetric signatures and hydrometeor classification of West African squall lines Raquel Evaristo,* Georges Scialom, Nicolas Viltard and Yvon Lemaˆıtre Laboratoire Atmosph`eres, Milieux, Observations Spatiales, IPSL, V´elizy, France
ABSTRACT: During the AMMA campaign, the Doppler and polarimetric radar Ronsard was deployed in Kopargo, Benin. In the present paper, three squall lines sampled during that period are described. It was the first time this type of system had been observed with a polarimetric radar in the West African region. The strongest case occurred on 28 July 2006, and the others on 12 September and 30 June. The polarimetric characteristics of these systems are described and a hydrometeor classification algorithm is applied in order to identify the microphysics of the systems. Finally, the 3D wind field is retrieved for the 28 July event. The convective part of the squall lines is generally characterized by maxima of reflectivity, differential reflectivity (ZDR ), and specific differential phase (KDP ). The results of the hydrometeor classification show that the convective region is composed of strong (>30 mm h−1 ) to moderate rain (5–30 mm h−1 ) below the melting level, and graupel above. This zone seems to be characterized by relatively strong updraughts (5–10 m s−1 ), responsible both for the formation of graupel by riming and for the growth of drops due to coalescence. To the rear the systems are characterized by light rain (50
>50
30–50
10–35
30–45
5–30
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0–3
1–4
1–6.5
−3–1
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0.5–3
0.76–1
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0.67 –0.97 1–16
0.67 –0.98 0–5.5
0.5 –0.95 −1.5–0
0.5 –0.9 0–10
0.75 –0.97 0–5
0.86 –0.95 0–0.4
0.5 –0.85 0–1
0.84 –0.99 −1–1
>0
>0
>0
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−15–15
0–20
−35–0
3 mm). c 2010 Royal Meteorological Society Copyright
Figure 11. Horizontal section showing hydrometeors at 3.8 km, just below the base of the melting layer for 28 July at 0601 UTC. LR: light rain, MR: moderate rain, HR: heavy rain, LD: rain dominated by large drops, H: hail, RH: rain/hail mixture, G: graupel and/or small hail, DS: dry snow, WS: wet snow, HC: horizontal ice crystals, and VC: vertical ice crystals. The line indicates the vertical cross-section showed in Figure 10.
The microphysics of these systems is examined from the results of the hydrometeor classification algorithm. In general, it seems that the convective part of West African squall lines is characterized by heavy and moderate rain below the melting level. Sporadically, hail can be observed in the centre of the convective region reaching the ground, although the presence of hail was not frequently reported in this area. Above the melting level in the convective region, the main precipitation type is graupel. Occasionally, supercooled drops are seen above the melting layer. This zone seems to be characterized by relatively strong updraughts (5–10 m s−1 ), responsible both for the formation of graupel by riming and for Q. J. R. Meteorol. Soc. 136(s1): 272–288 (2010)
POLARIMETRIC SIGNATURES AND HYDROMETEORS OF SQUALL LINES
the growth of drops due to coalescence. Immediately to the rear of the convective region the systems are characterized by light rain below and snow or ice crystals above. Below the melting level this region is a zone of downdraughts. The downward flow prevents drop growth, which is consistent with the light rain observed. These results are in accordance with what had previously been suggested from other studies of the dynamics and microphysics of tropical squall lines. The analysis in the stratiform part of one of the systems has shown that these regions can be very nonuniform, due to the presence of old cells dissipating, associated with moderate rain and thus stronger reflectivities. Another interesting result is the presence of graupel in the stratiform region, close to the melting level, and this was observed in all three cases. However, the most common hydrometeors around this altitude are wet and dry snow, in agreement with the results from Bouniol et al. (2010, this issue), that found rimed aggregates as the predominant particle above the 0◦ C isotherm in the anvil cloud. Several other squall-line cases were observed during the AMMA SOP either by the Ronsard and the X-Port in the Ou´em´e valley, or by the MIT radar, further north, in Niamey. The systematic study of the microphysics and dynamics of those systems could be made and compared to the present results to evaluate the degree of generality of the present results. Moreover, such a study could help define the differences between squall lines that occur in different stages of the West African monsoon and in evaluating the role of different continental surfaces on the vigour of convection.
Acknowledgements Based on a French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, especially from France, United Kingdom, USA and Africa. It has been the beneficiary of a major financial contribution from the European Community’s Sixth Framework Research Programme. Detailed information on scientific coordination and funding is available on the AMMA International web site http://www.ammainternational.org. Raquel Evaristo is supported by the Science and Technology Foundation (Portugal) as part of the funds assigned by the Social European Fund. We would like to thank Teresa Bals-Elsholz for her help with the radiosonde data, and Earle Williams for his helpful comments in the review process. References Baldini L, Gorgucci E, Chandrasekar V, Peterson W. 2005. ‘Implementations of CSU hydrometeor classification scheme for Cband polarimetric radars.’ Proceedings 32nd Conference on radar meteorology, Albuquerque, New Mexico, USA, 22–29 October 2005. American Meteorological Society. Biggerstaff MI, Houze Jr RA. 1993. Kinematics and microphysics of the transition zone of the 10–11 June 1985 squall line. J. Atmos. Sci. 50: 3091–3110. c 2010 Royal Meteorological Society Copyright
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