# Polarimetric signatures and hydrometeor classification of West African squall lines

Raquel Evaristo, Georges Scialom, Nicolas Viltard, Yvon Lemaître

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Raquel Evaristo, Georges Scialom, Nicolas Viltard, Yvon Lemaître. Polarimetric signatures and hydrometeor classification of West African squall lines. Quarterly Journal of the Royal Meteorological Society, 2010, 136 (S1), pp.272-288. 10.1002/qj.561 . hal-00450805

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# Polarimetric signatures and hydrometeor classification of West African squall lines

Raquel Evaristo,* Georges Scialom, Nicolas Viltard and Yvon Lemaître

Laboratoire Atmosphères, Milieux, Observations Spatiales, IPSL, Vélizy, 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 ($Z_{DR}$), and specific differential phase ($K_{DP}$). 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 (<5 mm h⁻¹) below and snow or ice crystals above. These regions present downward flow below the melting layer, which prevents drop growth by condensation, consistent with the light rain observed. The stratiform part of these systems can be very non-uniform, due to the presence of old cells dissipating, associated with moderate rain and thus stronger reflectivities. Copyright © 2010 Royal Meteorological Society

**KEY WORDS:** tropical squall lines; polarimetric radar; microphysics; kinematics; dynamics-microphysics interaction

Received 15 December 2008; Revised 13 November 2009; Accepted 16 November 2009

## 1. Introduction

Squall lines are of great importance to the West African region due to the water supply they carry, especially in the soudano-sahelian region (Schrage et al., 2006). Because of their importance, they have been the subject of various experimental campaigns like GATE (Global Atmospheric Research Program's Atlantic Tropical Experiment) in 1974, COPT81 (CONvection Profonde Tropicale) in 1981, HAPEX-Sahel (the Hydrology-Atmosphere Pilot Experiment in the Sahel) in 1992, IMPETUS (Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa) in 2002, and recently AMMA (African Monsoon Multidisciplinary Analysis) in 2006. The structure, dynamics and thermodynamics of squall-line systems, as well as their time evolution, have been inferred from the observations gathered during those field campaigns. Essentially, a squall line consists of a convective line formed of discrete convective cells aligned perpendicularly to the system propagation direction, followed by an extensive trailing mesoscale anvil in the rear. Between these two regions there is often a minimum in reflectivity, known as the transition zone, usually associated with enhanced subsidence

(Biggerstaff and Houze, 1993). In the convective region, a strong updraught is observed, from the surface to the tropopause (Houze, 1977; Zipser, 1977; Roux, 1988). The air lifted by the convective updraught enters the anvil region and exits from the system in the upper layers, above the melting level. This flow is known as the front-to-rear flow. Just behind the convective updraught a convective downdraught below the melting layer, originating from evaporative cooling, melting and water loading, creates a cold return flow near the surface, known as the density current. This enhances low-level convergence and hence favours the formation of new convective cells in front of the convective region (Houze, 1977; Leary, 1979; Leary and Houze, 1979b; Chong et al., 1987). As the older cells start dissipating, they move to the rear of the system and become part of the anvil cloud (Houze, 1977; Zipser, 1977; Leary and Houze, 1979b). The stratiform part of the system is characterized by a mesoscale updraught above the 0°C isotherm, important in maintaining the supercooled water supply in the anvil cloud, and a mesoscale downdraught below, as a result of particle melting and evaporation. Of course, this is a description of an idealized squall line, which is not exactly what is observed in real cases.

Although much is known about the structure and dynamics of West African squall lines, the microphysics of these systems, particularly in the convective region, remains unclear, mostly because of the lack of large-scale microphysical observations covering a whole system as

<footnote>*Correspondence to: Raquel Evaristo, Valparaiso University, Dep. of Geography and Meteorology, Kallay-Cristopher Hall, 201-A 1809 Chapel Drive, Valparaiso IN, 46383. E-mail: raquelmpmreques@gmail.com</footnote>
well as its evolution. The most complete microphysical study based on observations was made by Leary and Houze (1979a), who made a quite detailed study of the microphysics of the stratiform part in a tropical oceanic squall line from observations from single polarization radar and satellites, and disdrometer and *in situ* measurements. Yet, until recently, most microphysical studies were based on model results (a few examples are Hauser *et al.*, (1988) for West Africa, and elsewhere: Randell *et al.* (1994), Szeto and Cho (1994), Yang and Houze (1995), and Tingkai and Zhemin (2001)). In this issue two other studies focus on squall-line microphysics (Bouniol *et al.*, 2009; Penide *et al.*, 2010), the first one by means of *in situ* and 95 GHz Doppler radar observations, and the second one using a numerical simulation of a mesoscale convective system with the BRAMS model (Brazilian Regional Atmospheric Modelling System). Both studies are mainly interested in the processes taking place in the anvil cloud. The general microphysical characteristics of squall-line systems, as described by the cited studies, include the following:

(1) Strong rain mixing ratios and consequent strong rain rates (10–100 mm h⁻¹) in the convective region contrasting with much lower values in the stratiform region (1–10 mm h⁻¹).

(2) The presence of snow in the upper levels in both the convective and stratiform parts.

(3) Graupel or hail (depending on the model) is first found in the convective region and later graupel is found in the stratiform part too, at heights between the rain and the snow.

(4) In the convective region drops grow mainly by accretion of cloud particles.

(5) A considerable amount (from 25 to 48%, depending on the study) of the condensed water in the leading part of the system is transferred to the rear.

(6) In the stratiform region, in the mesoscale updraught whenever the environment is saturated, ice growth is mostly due to aggregation of ice particles just above the melting layer, and to vapour deposition at higher levels,

(7) In the melting layer aggregates of ice crystals start melting forming large ice particles coated by liquid water which results in a characteristic reflectivity maximum known as *bright band*.

(8) Below that level, raindrops having formed by melting of ice undergo an evaporation process as they fall, responsible for the mesoscale downdraught observed below the mesoscale anvil cloud.

Presently, an important additional tool for microphysical studies of precipitating systems is available: the dual polarization radar.

During the AMMA campaign, polarimetric radar observations were made within a large variety of precipitating systems, including some typical West African squall lines, by the Ronsard (Recherche sur les orages et nuages par un système associé de radars Doppler) radar. The polarimetric variables are affected by the number density, dielectric

constant, shape, size and orientation of the targets and therefore are very useful in discriminating between the different hydrometeor types (Straka *et al.*, 2000; Zrnić *et al.*, 2001; Keenan, 2003; Ryzhkov *et al.*, 2007). In the case of the Ronsard radar the polarimetric variables retrieved are the differential reflectivity ($Z_{DR}$), the differential phase shift ($\Phi_{DP}$), and the cross-correlation coefficient at zero lag ($\rho_{HV}$, hereafter correlation coefficient). The differential reflectivity is defined as the ratio between the horizontal and vertical reflectivities. This variable is a good indicator of the mean axis ratio of the hydrometeors: a positive value of $Z_{DR}$ indicates the presence of oblate hydrometeors; conversely, prolate hydrometeors produce a negative $Z_{DR}$. Differential reflectivity also depends on the dielectric constant, and thus is much larger for rain than for ice particles of the same shape and size. The differential phase shift is the phase difference between the copolar horizontal and vertical waves. If the volume sampled is mostly filled with horizontally oriented particles, the phase of the horizontal wave will be delayed compared to the vertical wave phase, thus producing a phase shift. Its derivative with respect to the range direction $K_{DP} = \{\Phi_{DP}(r_2) - \Phi_{DP}(r_1)\}/2(r_2 - r_1)$ is known as the specific differential phase. This variable is insensitive to isotropic hydrometeors since these are statistically indistinguishable from spherical particles, thus producing similar phase shift for horizontal and vertical waves. Hence, $K_{DP}$ increases when flattened or elongated hydrometeors are present (anisotropic particles), but is not affected by tumbling particles or randomly oriented hail (isotropic particles). Like $Z_{DR}$, $K_{DP}$ also increases with increased dielectric constant. Both $Z_{DR}$ and $K_{DP}$ depend on the particle axis ratio but $K_{DP}$ depends on the total particle concentration and not $Z_{DR}$. The cross-correlation coefficient is defined as the correlation coefficient at 0 lag between the received horizontal and vertical signals. Generally $\rho_{HV}$ is close to 1 but decorrelation occurs when large and irregularly shaped particles or when mixtures of hydrometeors are sampled. Thus, $\rho_{HV}$ is a good variable to detect the presence of a melting layer, in addition to the reflectivity.

The microphysical information obtained from the polarimetric variables is by far more complete than any previous microphysical observations made within West African squall lines, in particular in terms of spatial coverage and temporal evolution. The objective of this work is to analyse the polarimetric signatures persistent in these systems and to relate these signatures to the microphysics. An identification of hydrometeors is also made based on the polarimetric variables. The interaction between dynamics and microphysics is investigated.

In the next section we will describe the data and methods used in this study. The large-scale environment and evolution of the three cases will be addressed in section 3. In section 4 the polarimetric signatures that characterize West African squall lines and the results of hydrometeor classification will be shown. In section 5 the kinematics of squall lines will be discussed and compared to the microphysics. In section 6 we will summarize the results and conclude.
## 2. Instrument and methods

### 2.1. Ouémé mesoscale observation site

Because of its multidisciplinary character, the AMMA campaign involved several different spatial and temporal observation scales. Since our interest is focused on the mesoscale events we centre our attention on the mesoscale observation site of the Ouémé valley, where the Ronsard and X-Port radars were deployed during the AMMA Special Observing Period (SOP) during summer 2006. Close to each one of the radars there was a disdrometer. At Parakou and Niamey a radiosonde was launched every six hours. These soundings were used in the hydrometeor classification process to define the freezing level. A wind profiler (UHF, ultra-high frequency) was placed near Djougou. For further details about the mesoscale site see Redelsperger et al. (2006).

### 2.2. Experimental data

This paper concerns only the Ronsard data. This radar was initially a single-polarization radar but was recently modified to include dual polarization capability (Scialom et al., 2009).

The radar performed Plan Position Indicator (PPI) volume scans at maximum ranges of 102.5 km or 205 km and gate widths of 100 m or 200 m respectively, depending on the operating mode (OM1 or OM2). For each gate we produce the following variables: $Z_{HH}$, $Z_{VV}$, $Z_{DR}$, $V_{HH}$, $V_{VV}$, $\rho_{HV}$ and $\Phi_{DP}$ (where $Z_{HH}$ and $Z_{VV}$ are the copolar reflectivities at horizontal and vertical polarizations and $V_{HH}$, $V_{VV}$ are the copolar radial velocities at horizontal and vertical polarizations). Low signal-to-noise ratio (SNR) signals are characterized by low reflectivities (−12 dBZ at 1 km distance) and incoherent velocities. Second trip echoes are also characterized by incoherent velocities but reflectivities are usually high. Thus noisy (low SNR) measurements and second trip echoes were eliminated using incoherence in the Doppler velocity. Since the Ronsard keeps both the horizontally and vertically polarized radial velocities, we eliminate the gates where the difference between the horizontal and vertical velocities is higher than 0.5 m s⁻¹. Ground clutter was eliminated by comparison with a digital terrain model. Reflectivity was calibrated by comparing with reflectivity from disdrometers and Tropical Rainfall Measuring Mission Precipitation Radar (TRMM PR) data. $Z_{DR}$ was calibrated by visual evaluation of CFAD (Contoured Frequency by Altitude Diagrams) type plots at high altitudes (see Scialom et al. (2009) for details). Attenuation and differential attenuation was corrected using the algorithm ZPHI (Testud et al., 2000). The attenuation correction is based on the linearity between the attenuation and the value of $\Phi_{DP}$. The algorithm possesses an internal calibration check that corrects eventual residual calibration errors (Le Bouar et al., 2001). Also, the sensitivity of the scheme to the variability of the raindrop size distribution is reduced since the ray is partitioned into several segments processed independently, instead of considering

the whole ray. The quality of the retrieved attenuation profile for $Z_H$ is checked through the comparison of the retrieved $\Phi_{DP}$ with the measured one. Filtering of backscattering differential phase and calculation of $K_{DP}$ were achieved as described in Hubbert and Bringi (1995). All the variables were interpolated onto a Cartesian grid with 1.25 km horizontal resolution for OM1 and 2.5 km for OM2 and a vertical resolution of 0.2 km using Cressman analysis.

### 2.3. Hydrometeor classification

Hydrometeor classification is made using the algorithm described in Zrnić et al. (2001). This is a fuzzy-logic method based on the construction of membership functions for each polarimetric variable and hydrometeor type. There are eleven different classes, as in Zrnić et al. (2001): light rain (LR, rain rates (RR) <5 mm h⁻¹), moderate rain (MR, RR from 5 to 30 mm h⁻¹), heavy rain (HR, RR >30 mm h⁻¹), rain dominated by large drops (LD), hail (H), rain/hail mixture (RH), graupel and/or small hail (G), dry snow (DS), wet snow (WS), horizontal ice crystals (HC) and vertical ice crystals (VC).

The classification algorithm chooses the most probable hydrometeor type. We have modified the algorithm in order to be able to retrieve, in addition to the most probable hydrometeor type, the second most probable one, especially in the cases where there is some ambiguity. This may happen when the probability of the most probable type is very close to the probability of the second one. We address the second hydrometeor whenever there is some uncertainty in the classification results.

The algorithm was first developed to be used in an S-band radar; of course the sensitivity of the polarimetric variables to the particle type depends on the wavelength of the radar signal. In particular, at C-band (wavelength ~5.5 cm), measurements are affected by resonance effects due to Mie scattering, especially in the case of intense rainfall, large raindrops or mixed-phase precipitation (whenever the particles equivolume diameter exceeds 4.5 mm) (Baldini et al., 2005; Ryzhkov and Zrnić, 2005; Ryzhkov et al., 2007). In those cases $Z_{DR}$ may reach much higher values at C-band than at S-band (maxima in $Z_{DR}$ at C-band are around 7 dB against 4 dB at S-band (Ryzhkov and Zrnić, 2005)). In rain $\rho_{HV}$ can be significantly lower, reaching a minimum of 0.93–0.94 at C-band compared to 0.99 at S-band (Baldini et al., 2005; Ryzhkov and Zrnić, 2005). $\Phi_{DP}$ and $K_{DP}$ are directly proportional to the radar frequency, which means that, under the same conditions, $\Phi_{DP}$ and $K_{DP}$ are twice as large at C-band as at S-band.

These differences require the membership functions from Zrnić et al. (2001) to be adapted to the Ronsard frequency. Therefore, the original membership functions were modified in order to take into account all the effects described above. The modifications were based on published works on the subject and consisted in: (1) increasing the upper threshold on $Z_{DR}$ by 0.5 dB on moderate rain, rain/hail mixture, and wet snow, and by 1 dB on heavy rain and large drops; and (2) decreasing
<table><caption>Table I. Thresholds of the polarimetric variables used in the membership functions for each hydrometeor type.</caption><thead><tr><th></th><th>LR</th><th>MR</th><th>HR</th><th>LD</th><th>H</th><th>R/H</th><th>G/SH</th><th>DS</th><th>WS</th><th>CR</th></tr></thead><tbody><tr><td>Z (dBZ)</td><td>10-35</td><td>35-45</td><td>45-55</td><td>20-45</td><td>&gt;50</td><td>&gt;50</td><td>30-50</td><td>10-35</td><td>30-45</td><td>5-30</td></tr><tr><td>Z<sub>DR</sub> (dB)</td><td>-0.5-2</td><td>0-3</td><td>1-4</td><td>1-6.5</td><td>-3-1</td><td>1-6</td><td>-0.2-2</td><td>0-1</td><td>0-3.5</td><td>0.5-3</td></tr><tr><td>ρ<sub>HV</sub></td><td>0.76-1</td><td>0.7-1</td><td>0.67<br>-0.97</td><td>0.67<br>-0.98</td><td>0.5<br>-0.95</td><td>0.5<br>-0.9</td><td>0.75<br>-0.97</td><td>0.86<br>-0.95</td><td>0.5<br>-0.85</td><td>0.84<br>-0.99</td></tr><tr><td>K<sub>DP</sub> (°/km)</td><td>0-2</td><td>0-5.5</td><td>1-16</td><td>0-5.5</td><td>-1.5-0</td><td>0-10</td><td>0-5</td><td>0-0.4</td><td>0-1</td><td>-1-1</td></tr><tr><td>T (°C)</td><td>&gt;0</td><td>&gt;0</td><td>&gt;0</td><td>&gt;0</td><td>-15-15</td><td>0-20</td><td>-35-0</td><td>&lt;0</td><td>0-10</td><td>&lt;0</td></tr></tbody></table>

LR: light rain, MR: moderate rain, HR: heavy rain, LD: rain dominated by large drops, H: hail, R/H: rain/hail mixture, G: graupel and/or SH: small hail, DS: dry snow, WS: wet snow, CR: crystals.

the lower threshold in ρ<sub>HV</sub> by 0.03 on heavy rain, large drops and wet snow, and 0.02 on rain/hail mixture. The weights given to the different variables were all the same, including the temperature, except for the reflectivity, which was twice the weight of the others. The thresholds of the polarimetric variables used in the membership functions for each hydrometeor type are shown in Table I.

### 3. Environments of the different cases

During the field campaign, the Ronsard observed 43 precipitating systems of different types (squall lines, isolated convection, stratiform precipitation). Among these, ten squall lines were sampled. Three of these squall lines are described in the present paper: the 28 July (OM1), the 12 September (OM2), and the 30 June (OM1). These specific systems were chosen because of the good view the Ronsard had of them. Although they are not very intense, especially the 12 September and 30 June systems, as we will see in this and the next section, the cases presented in this paper were the best cases observed by the Ronsard in terms of both intensity and organization. We believe that the intensity of the systems might be linked to the location of the Ronsard, close to the Gulf of Guinea, because further north, in Niamey, during AMMA, stronger systems were observed with the Massachusetts Institute of Technology (MIT) radar by Chong (2009) as well as during COPT, in Côte d'Ivoire (Chong et al., 1987; Roux, 1988). In the southern part of West Africa the environment is typically warm and moist which makes it easier for systems to initiate because of lower convective inhibition (CIN), although values for convective available potential energy (CAPE) are usually low. In contrast, the north is characterized by drier and colder mid-levels which contribute to a higher CAPE and thus stronger systems. This is just a hypothesis and further investigation is needed in order to validate it, but is a possible explanation to why the systems we observed were so weak compared to other observations of West African squall lines.

More intense convective cells were observed by the Ronsard, but we decided to focus on squall-line-type systems since these are known for supplying a large

percentage of the rainfall during the monsoon season, so important for the population of West Africa.

This section describes the large-scale evolution of these squall lines from their formation until they leave the Ronsard field of view. The description of the three systems is based on satellite images from Meteosat Second Generation (MSG2) and on the Ronsard reflectivity. The characteristics of the environment prior to the passage of the systems are addressed using radiosonde data.

#### 3.1. 28 July 2006

This system formed on 27 July around 1200 UTC over the Nigerian Highlands, as a small convective cell, likely triggered by orography. The atmosphere was characterized by a moist layer at lower levels, very close to saturation around 700 mb, and a drier layer (relative humidity between 43 and 68%) at mid-levels, as measured by the Parakou radiosonde on 27 July at 2240 UTC. The corresponding CAPE was 1675 J/kg, a moderate value in tropical conditions. The convective Richardson number was 20, resulting from a moderate CAPE and quite strong wind shear. The system moved westwards as it developed and merged with other cells, and on the 28 July, around 0400 UTC, it entered the Ronsard field of view (Figure 1 (a)). At this time the system was composed of several convective cells aligned from northwest to southeast in its main part, to the north; to the south it was oriented along a second line, from northeast to southwest (Figure 1(b)). A trailing mesoscale anvil started developing around 0500 UTC (Figure 1(c) and (d)). After 0624 UTC the system began to decay (Figure 1(e) and (f)). The line kept moving westward and by about 1000 UTC it had completely left the Ronsard domain.

#### 3.2. 12 September 2006

This system is created between two pre-existing systems around 1100 UTC over Western Nigeria. Convective cells developing from the northern part of the southern system merged with cells from the southern part of the northern system. These cells simultaneously intensified and moved
<figure><img src="image_3.png" /><figcaption>Figure 1. Reflectivity horizontal section (CAPPI) at 1 km for 28 July 2006: (a) 0416 UTC, (b) 0448 UTC, (c) 0520 UTC, (d) 0554 UTC, (e) 0624 UTC, (f) 0705 UTC. The horizontal axis represents the west–east direction and the vertical axis the south–north direction. The radar is located at x = 0, y = 0. The line in Figure 1(c) shows the location of the cross-section shown in Figure 4, and the square in Figure 1(d) shows the area in Figures 8 and 10.</figcaption></figure>
westwards. The Parakou radiosonde at 1643 UTC shows a very deep moist layer, very close to saturation throughout the whole atmosphere. Saturation was evident between 517 and 340 mb. The value of the CAPE was 744 J/kg, which is not a very high value. However, CIN was very low (6 J/kg) which is very easy to overcome to initiate convection. The convective Richardson number was 9 as a consequence of a small CAPE and strong wind shear. These values should be treated with caution since the radiosonde was located rather far away and at a time when the system was active. A better radiosonde would have been that of 1200 UTC in order to be in front of the system. By 1500 UTC the system entered the Ronsard domain, but, from the reflectivity intensity, it was a weak system. Later, around 1600 UTC, the system became much stronger (Figure 2(a) and (b)). It was aligned from north to south and kept moving west. It weakened around 1700 UTC (Figure 2(c) and (d)), but re-strengthened later, after 1800 UTC (Figure 2(e) and (f)). At this time the system was formed of a distinct convective part in the front, a reflectivity trough (transition region), and a laterally extensive stratiform part to the rear. This structure persisted until 1900 UTC, and after this time the line started dissipating (not shown).

3.3. 30 June 2006

The system originated at 0500 UTC as a convective cell over northeastern Benin. It expanded and intensified. Simultaneously, it merged with other cells that formed meanwhile as well as with a dissipating system to its south. Unfortunately, both the radiosondes on 29 June at 2232 UTC and on 30 June at 0442 UTC had problems, and the data cannot be used. At 0600 UTC the system entered the Ronsard field of view (Figure 3(a)). The convective line was oriented from northwest to southeast and moving to the southwest (Figure 3(b)). About 0730 UTC the convective part was weaker in terms of reflectivity intensity (Figure 3(c)), and at approximately 0800 UTC it had lost its linear structure (Figure 3(d)). Later, around 0900 UTC, some aligned but quite weak cells were observed, followed by an extensive trailing mesoscale anvil (Figure 3(e) and (f)). This structure remained until 1000 UTC, when the system began its exit from the domain observed by the radar.

# 4. Polarimetric signatures and hydrometeor classification results

## 4.1. Polarimetric signatures

This section will be dedicated to the description and interpretation of the polarimetric variables. Some polarimetric signatures previously observed in other kinds of convective systems (Illingworth et al., 1987; Brandes et al., 1995; Hubbert et al., 1998; Loney et al., 1999, 2002; Cifelli et al., 2002; Kumjian and Ryzhkov, 2008) are also observed in West African squall lines: ZDR columns,

KDP columns and mid-level ZDR rings, although they present some differences related to the storm structure and dynamics. These signatures are directly related to the microphysics of the systems. We will here make an attempt to infer what type of particles are responsible for those polarimetric signatures. Afterwards the hypothesis made here will be compared to the results of the automatic hydrometeor classification, in the next subsection.

Figures 4, 5 and 6 show vertical cross-sections for reflectivity (4(a), 5(a) and 6(a)), differential reflectivity (4(b), 5(b) and 6(b)), specific differential phase shift (4(c), 5(c) and 6(c)), and hydrometeor classification (4(d), 5(d) and 6(d)) corresponding to 28 July at 0520 UTC, 12 September at 1628 UTC and 30 June at 0622 UTC, respectively. The cuts are made along the lines shown in Figures 1(c), 2(c) and 3(a). The cross-sections were chosen so they would cross a convective cell (reflectivity maximum) and during a stage when the systems were well organized. They are representative of each one of the systems for long periods of time in terms of vertical development and polarimetric signatures, although the intensity of the different polarimetric variables may vary with time. The images also show the location of the 0°C isotherm as measured by the closest radiosonde at Parakou (approximately 100 km to the east of the radar). This should only serve as an approximate location for the 0°C isotherm since the temperature may vary in the domain as well as in time.

West African squall lines are characterized by strong reflectivity columns that indicate the convective region. Their intensity and height depend on the updraught strength and precipitation intensity.

On 28 July at 0520 UTC (Figure 4(a)) the reflectivity column (20 dBZ reflectivity isoline) reaches an altitude of 14.5 km, with values stronger than 44 dBZ from the surface to 4 km. An older cell is observed to the rear (to the right in the cross-section). On 12 September, at 1648 UTC (Figure 5(a)), the maximum height of the 20 dBZ reflectivity isoline is 8 km and reflectivity is stronger than 40 dBZ below 6.5 km or 48 dBZ below 3 km. On 30 June, at 0622 UTC (Figure 6(a)), the height of the reflectivity column is 8 km, and maxima greater than 40 dBZ are seen below 5 km, or 52 dBZ, below 3.5 km. Both the two later systems present neither weaker cells nor a stratiform part to the rear, suggesting that, by this time, these systems are in their early stages of development (Leary and Houze, 1979b). This fact is corroborated by satellite images (not shown), and by the horizontal reflectivity images (CAPPI, Constant Altitude Plan Position Indicator) shown in Figures 2 and 3, where the development of an extensive stratiform cloud is observed by 1700 UTC and 0800 UTC, respectively.

Vertical columns of differential reflectivity have been discussed by several authors (Illingworth et al., 1987; Brandes et al., 1995; Cifelli et al., 2002; Loney et al., 2002; Kumjian and Ryzhkov, 2008). They provide valuable information about the microphysics of the systems. This signature is present in all our cases for long periods of time.
<figure><img src="image_4.png" /><figcaption>Figure 2. Same as Figure 1 but for 12 September 2006: (a) 1607 UTC, (b) 1621 UTC, (c) 1648 UTC, (d) 1728 UTC, (e) 1823 UTC, (f) 1843 UTC. The lines in Figure 2(c) and (f) show the location of the cross-section shown in Figure 5 and 7 respectively.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3. Same as Figure 1 but for 30 June 2006: (a) 0622 UTC, (b) 0648 UTC, (c) 0727 UTC, (d) 0813 UTC, (e) 0854 UTC, (f) 0913 UTC. The line in Figure 3(a) shows the location of the cross-section shown in Figure 6.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4. Vertical cross-section of (a) reflectivity, (b) differential reflectivity, (c) specific differential phase shift, and (d) hydrometeor classification, for 28 July at 0520 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 vertical cut corresponds to the line in Figure 1(c), and the segment shown is the thicker segment. The dashed line indicates the height of the 0°C level.</figcaption></figure>

On 28 July at 0520 UTC (Figure 4(b)), a relatively strong positive Z<sub>DR</sub> column is seen between the surface and a height of 4 km. The maximum values, over 2 dB, are centred at *x* = 20 km, a little displaced to the left of the reflectivity maximum. This indicates that larger drops are concentrated in the front (left). A secondary maximum in Z<sub>DR</sub>, not reaching the surface, is observed at *x* = 30 km, coincident with the reflectivity maximum. Although in this figure the maxima of Z and Z<sub>DR</sub> do not coincide exactly, most of the time these two columns are co-located. On 12 September at 1648 UTC (Figure 5(b)), the Z<sub>DR</sub> column reaches 5 km height and maximum values exceeding 3 dB. The Z<sub>DR</sub> column is located in the same place as the reflectivity core. On 30 June, at 0622 UTC (Figure 6(b)), the maximum height of the Z<sub>DR</sub> column is 6.5 km and the maximum intensity, exceeding 4.5 dB, is found near the ground. For this case too, reflectivity and differential reflectivity cores are superposed.

The superposition of the Z and Z<sub>DR</sub> columns suggests that high concentrations of relatively large drops are being observed. This seems consistent with the fact that Z<sub>DR</sub> values are not extremely strong, especially for a C-band radar, but are still well above zero. This differs from previous studies of Z<sub>DR</sub> columns in supercell storms (Loney *et al.*, 2002; Kumjian and Ryzhkov, 2008) that show low concentrations of very large drops associated with the observed offset between the reflectivity and differential reflectivity columns.

Another polarimetric signature widely studied is the K<sub>DP</sub> column (Hubbert *et al.*, 1998; Loney *et al.*, 1999, 2002; Kumjian and Ryzhkov, 2008). These signatures consist of regions of enhanced K<sub>DP</sub> frequently extending several kilometres above the freezing level. K<sub>DP</sub> columns also seem to be characteristic of West African squall lines since they are observed in all three cases presented here and they persist for practically the whole time the systems were in the domain observed by the Ronsard.
<figure><img src="image_7.png" /><figcaption>Figure 5. Same as Figure 4, but for 12 September at 1648 UTC. The vertical cut corresponds to the line in Figure 2(c), and the segment shown is the thicker segment.</figcaption></figure>

On 28 July at 0520 UTC (Figure 4(c)) this column, centred at $x = 30$ km, reaches a maximum altitude of 4 km with highest values over $3^\circ \text{ km}^{-1}$. As in the $Z_{DR}$ column, $K_{DP}$ maxima also coincide with the reflectivity maximum. The strong values at the edges of the cloud are probably an artefact, due to the smoothing of $\Phi_{DP}$. It does not seem to affect the hydrometeor classification, as will be shown later. On 12 September at 1648 UTC (Figure 5(c)), the $K_{DP}$ column, near $x = 27$ km, is 6 km high and stronger values are between 3 and $4^\circ \text{ km}^{-1}$. In this case the $K_{DP}$ column is also co-located with the reflectivity core. A weaker $K_{DP}$ column is seen to the rear (to the right, in the figure) of the strong one, coincident with a secondary reflectivity maximum. This feature is also seen on 30 June (Figure 6(c)). The $K_{DP}$ column reaches a maximum height of 4 km. Values stronger than $4^\circ \text{ km}^{-1}$ are observed below 3 km. In this case too, the reflectivity core and the $K_{DP}$ column are superposed.

In previous studies $K_{DP}$ columns have been associated with raindrops shed from hailstones (Hubbert *et al.*, 1998), large ($D > 2$ mm) mixed-phase hydrometeors

(melting ice particles) above the melting layer (Loney *et al.*, 2002) or with high concentrations of raindrops and melting graupel (Kumjian and Ryzhkov, 2008). Our results show that the $Z_{DR}$ and $K_{DP}$ columns are co-located, which is not the case in the previous studies just mentioned. Kumjian and Ryzhkov (2008), who observed an offset between these two columns in supercells, hypothesized that the offset might be related to the environmental shear or updraught strength, although further investigation is needed.

The co-location of $Z$, $Z_{DR}$ and $K_{DP}$ columns, suggesting the presence of strong rainfall, seems to be a signature of the convective zone of tropical squall lines. This is consistent with the widely known fact that strong rain rates ($10–100 \text{ mm h}^{-1}$) are associated with the convective region in squall lines.

## 4.2. Hydrometeor classification results

Figure 4(d) shows a vertical cross-section of the hydrometeor classification results for the 28 July case at 0520
<figure><img src="image_8.png" /><figcaption>Figure 6. Same as Figure 4, but for 30 June at 0622 UTC. The vertical cut corresponds to the line in Figure 3(a), and the segment shown is the thicker segment.</figcaption></figure>

UTC. The leading cell is composed essentially of heavy rain below the melting level and graupel above, up to 9 km. A layer of wet snow is seen, just below the 0°C isotherm. To the right, the rear cell is formed by heavy rain in lower levels, with some rain/hail mixture between 3 and 4 km. Above, a small amount of graupel is identified, but mainly dry snow is observed. Dry snow and some ice crystals (both horizontally and vertically oriented) are found elsewhere in upper levels. Another interesting feature observed in this figure is the presence of raindrops above the environmental melting level in the core of the convective region. This indicates the presence of updraughts that are carrying liquid particles above the melting layer.

Although only one image is shown for this event, it is representative of the hydrometeor classification results during the strongest phase of the system, between 0501 UTC and 0630 UTC. Occasionally a narrow column of large drops is observed in front of the main cell. Around 0600 UTC there are a few indications of rain/hail mixture in the centre of the heavy rain region. Intense

lightning was reported by observers on the ground during the passage of this system, which is consistent with the identification of large ice particles, like graupel and hail (Williams, 2001). From 0630 UTC on, no more heavy rain is observed but moderate rain remains until 0726 UTC. After this time only the stratiform part is being observed. The predominant hydrometeor in lower levels is light rain. Graupel is observed as late as 0904 UTC, but is confined to the levels around the melting layer.

The 12 September squall line is much weaker than the one on 28 July, as noted before. The hydrometeor classification reflects the intensity difference between the two systems. A cross-section of the hydrometeor classification for 12 September at 1648 UTC is shown in Figure 5(d). The convective region is characterized by heavy and moderate rain below the melting level. Above, graupel is identified at a maximum height of 8.5 km. Dry snow and both kinds of ice crystals are also seen. All around the strongest rain region there is light rain, with some wet snow just below the melting layer. This system passed through different stages while observed
by the Ronsard, as was described in subsection 3.2. The temporal evolution of the microphysics (not shown) shows that the system passed through three intense stages (around 1628, 1720 and a longer period from 1756 to 1913 UTC). The first two intense periods are well represented by Figure 5(d). The last period however is dominated by an extensive trailing anvil, with very little heavy rain but still a large amount of graupel between 2 and 4.5 km height, and moderate rain below. More details on the stratiform part of this system are addressed below in this section.

Figure 6(d) shows a vertical cross-section of the hydrometeor classification results at 0622 UTC for 30 June. The hydrometeor classification reveals a relatively shallow system, the 20 dBZ reflectivity isoline reaching only 8 km, as already noted in the reflectivity cross-section (Figure 6(a)). The evolution of the system shows only small changes in the vertical extent, thus the weak vertical extent seems to be a characteristic of this particular case. In front of the main cell, to the left in the figure, a zone composed of large drops is observed. In the convective region, below the melting layer, the main hydrometeor types are moderate and heavy rain. A narrow column of rain/hail mixture is observed from the surface to 2 km height. Graupel and large drops are observed above the melting level. Some wet snow is identified in the melting layer, between 3.5 and 4.5 km. The occurrence of hail at the surface in this area is not very common, but was reported for other days. We verified that the second most probable hydrometeor for the rain/hail points is pure hail (not shown), which reinforces the hypothesis that hail was probably occurring at the time. Moreover, large drops were identified well above the melting level, up to 7 km altitude. The presence of supercooled water drops suggests that we are in the presence of strong updraughts, generally required for hail formation. Besides, these drops could be the origin of the hail formation by riming resulting from the accretion of these raindrops on ice particles. Calculations from Rasmussen and Heymsfield (1987a, 1987b) and Williams (2001) indicate that, in order for hail to arrive at the surface with a 0.5 cm diameter, its diameter at the melting level should be between 1.25 and 1.8 cm, depending on the density. Updraught velocities between 15 and 17 m s⁻¹ are required to form particles of this size. However, in the calculations of Rasmussen and Heymsfield (1987a, 1987b), the melting level was found to be at 5.2 km, whereas, in our case, it is found at 4.5 km. So the existence of hail in this case requires slightly smaller vertical velocities. Vertical Azimuth Display (VAD) analysis between 0727 UTC and 0819 UTC shows that updraughts of 0.5 m s⁻¹ are observed throughout the whole atmosphere. These are not strong, but these calculations are an average for a 100 km radius domain, which includes the stratiform areas. Locally, this indicates the possibility of strong updraughts; however, no more details on the intensity of the updraughts can be made with this kind of analysis. Although VAD analysis should only be used in stratiform and widespread precipitation conditions, the use of this

technique in convective conditions gives an idea of the mean flow, so the fact that we obtain a positive vertical velocity means that we have a mean upward flow and that convection is occurring in some portion of the domain.

Although hail has not been described in previous studies of West African squall lines, at least at the surface, its presence seems to be consistent for this particular case. In fact, hail has been observed at the surface during AMMA in Benin, in particular at Djougou on 9 July 2006 at 1240 UTC (Françoise Guichard, Fred Cazenave and Luc Seguis, personal communication) even though details are not available. Anyway, the occurrence of hail does not seem to be a general characteristic of squall lines, but rather an exception, since it is seldom observed.

In general, these results agree well with the expected types of particle discussed in subsection 4.1. Also, rain rates were calculated from disdrometer data, located near the radar. The comparison between these and the results of the hydrometeor classification at low levels are shown in Figure 7 for the 28 July case. The plot shows the rain rate obtained by the disdrometer and the different shades indicate the local rain intensity as identified by the classification algorithm (dark grey corresponds to heavy rain and light grey to moderate rain). The hydrometeor classification results seem to agree quite well with the disdrometer calculated rain rates. A similar result was obtained for the 12 September case, but unfortunately there are no disdrometer data for the 30 June case. This verification shows that the hydrometeor classification algorithm is working well in lower levels.

Figure 8 shows a cross-section in the trailing stratiform part of the 12 September system, at 1843 UTC, along the line in Figure 2(f). Figure 8(a) shows the reflectivity and 8(b) the hydrometeor classification. The reflectivity cross-sections reveal a very non-uniform stratiform region, with a relatively strong zone between -10 and 20 km in range, where reflectivities exceed 36 dBZ, surrounded by weaker zones, with lower reflectivities. The stronger zone is characterized by moderate rain below the melting level, a mixed-phase layer where there is wet snow, large drops and some graupel, between 4 and 6 km, and dry snow and horizontal ice crystals above. This region is probably an old dissipating cell. It extends up to a height of 7 km, which is at least 1 km higher than the surroundings, suggesting that some convection may still be occurring. Elsewhere, light rain is the predominant hydrometeor. A thick melting layer (1.5 to 2 km thickness), formed of wet snow, is present over most of the domain shown.

Some differences exist between these and previous descriptions of squall lines. In particular, the vertical extent of the systems observed by the Ronsard seems to be much weaker than observed in previous studies. This could be due to the different environmental conditions closer to the Gulf of Guinea, where the water vapour supply is greater than in the north and thus systems are generated more easily but have lower intensities due to lower values of CAPE, although this hypothesis
<figure><img src="image_9.png" /><figcaption>Figure 7. The line shows the rain rate calculated from the disdrometer data and the different shades indicate the local rain intensity as identified by the classification algorithm (dark grey corresponds to heavy rain and light grey to moderate rain). This figure is available in colour online at www.interscience.wiley.com/journal/qj</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 8. Vertical cross-section for 12 September at 1843 UTC showing (a) reflectivity and (b) hydrometeor classification. The vertical cut corresponds to the line in Figure 2(f), and the segment shown is the thicker segment. The dashed line indicates the height of the 0°C level.</figcaption></figure>

needs further investigation. Another difference in our results is that we observed hail reaching the ground. Nevertheless, in general, the results shown here seem to be consistent with previous microphysical studies. The strong rain rates associated with the convective region and the contrast with the low rainfall to the rear had been described by several studies, such as Chong *et al.* (1987) and Hauser *et al.* (1988). The presence of graupel first in the convective zone and later in the stratiform part, which was observed in each one of the

presented cases, had been observed in simulations by Randell *et al.* (1994) for Darwin, Australia and Szeto and Cho (1994) for an idealized case. At higher levels and outside the convective region snow and ice crystals are observed here, consistent with the observations from Leary and Houze (1979a) in a tropical oceanic squall line and with several studies such as Hauser *et al.* (1988) in West Africa and others like Randell *et al.* (1994), Szeto and Cho (1994), Yang and Houze (1995), and Tingkai and Zhemin (2001).
## 5. Interaction between dynamics and microphysics

The three-dimensional (3D) wind field was retrieved using Doppler measurements from a single radar (mono-Doppler). Some assumptions have to be made in order to properly retrieve the three wind components in mono-Doppler conditions. We assume that the squall line translates without internal deformation. Under this hypothesis, two observations of the same system at different times may be considered as two simultaneous observations from two radars. The same kind of approach was used by Chong (2009, in this issue), in the analysis of the 11 August 2006 squall line further north, in Niamey with the MIT radar. Accurate advection must be inferred, which can be complicated, especially in cases like the 28 July, where the northern and southern parts of the system move at different speeds. After all these steps, the 3D wind retrieval itself is achieved using the Multiple Analytical DOPpler (MANDOP) technique (Scialom and Lemaître, 1990; Caillault and Lemaître, 1999). We retrieved the 3D wind field for the 28 July event using the combined volume scans at 0520 UTC and 0527 UTC as the first pseudo-radar and at 0617 UTC and 0624 UTC as the second pseudo-radar. The propagation speed of the system is evaluated at $11 \text{ m s}^{-1}$ in the east-west direction and $4 \text{ m s}^{-1}$ in the north-south direction. The dynamics retrieved is brought at a certain time between the two pseudo-radar observations, chosen as 0601 UTC so it can be compared to one of the sequences of hydrometeor classification.

The horizontal cross-section showing the relative wind field at 0.5 km is presented in Figure 9. We can see that it does not possess a two-dimensional structure, but rather a 3D one. In the northern portion of the system, the wind comes from the north, almost parallel to the orientation of the line. To the rear, near $y = -10 \text{ km}$, there is a flow from the southwest that is part of a typical divergence zone just behind the leading line. To the south, a convergence zone centred at $x = 10$, $y = -40$ is a result of both easterly winds colliding with westerly winds and northerly winds with southerly winds. A vertical cross-section showing the wind field (Figure 10(a)) fits better previous descriptions of squall-line dynamics. This section traverses the region where stronger convergence is observed at the surface. Strong updraughts are observed from the surface to an altitude of 12 km. Just behind, a region of downdraughts feeds the rear-to-front flow, at the surface. In turn, the rear-to-front flow converges with the ambient flow maintaining the convective updraught. A corresponding cross-section with the hydrometeor classification results at 0601 UTC is shown in Figure 10(b). Note that this is only a comparative example, since the dynamics is a composite figure built from four volume scans, as explained before. Therefore, we cannot match the exact location of the different flows in Figure 10(a) with the hydrometeor types in Figure 10(b), but we can still compare both figures and make physical connections. The leading cell in Figure 10(b) (hydrometeor vertical section) is characterized by moderate rain and graupel at lower

levels. Graupel is observed as low as 1.5 km and extends up to 7.5 km. Above, dry snow is the main observed particle, with some horizontally oriented ice crystals in smaller quantity. This column corresponds probably to the leading updraught in Figure 10(a), which could be responsible for lifting rain drops above the melting level and producing graupel by riming of supercooled drops on ice particles. Afterwards, the relatively large fall speeds of graupel particles ($2-4 \text{ m s}^{-1}$) cause them to fall to quite low altitudes before melting. VAD analysis shows that, around this time, particles have terminal fall velocities of $8 \pm 3 \text{ m s}^{-1}$. According to Rasmussen and Heymsfield (1987a, 1987b) this could correspond to graupel particles having initial diameters between 0.5 and 2 cm, and according to the same study, particles with an initial diameter of 1.5 cm would melt above 1 km height, which agrees well with our observations. Behind the leading cell there are two weaker cores mostly formed by moderate rain below and graupel above. These attain much lower altitudes (around 8 km). Graupel probably melts in the melting layer, as it is not seen below. This corresponds to particles with an initial diameter of 0.5 cm or less (Rasmussen and Heymsfield, 1987a, 1987b). Updraught is probably still present in both cells, and this explains the presence of graupel. Moderate rain is being created directly from the melting of graupel and also by the growth of smaller drops. The coexistence of updraughts and moderate rain is consistent with the growth by coalescence, since the upward velocity maintains drops in the atmosphere for longer periods, giving them time to collide with other particles and consequently grow.

To the rear of these cells, light rain is observed, below a thin layer of dry snow. This region appears to correspond to the downdraughts observed at lower levels, just behind the updraughts in Figure 10(a). As mentioned in section 1, this zone of enhanced subsidence just behind the convective region corresponds to the transition region. Previous studies show that these downdraughts are maintained by cooling associated with melting and evaporation (Biggerstaff and Houze, 1993). This downward flow keeps water drops from growing, due to the short residence times, as

<figure><img src="image_11.png" /><figcaption>Figure 9. Horizontal wind field at 0.5 km for 28 July at 0601 UTC. The line shows the location of the vertical cut shown in Figure 10. Shading is reflectivity (dBZ).</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 10. Vertical cross-section for 28 July at 0601 UTC for: (a) wind field and reflectivity (dBZ), and (b) hydrometeor classification. 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 cross-section corresponds to the line in Figure 9.</figcaption></figure>

opposed to what happens in the updraught zone, explain-
ing the presence of light rain.

As noted before, this event presents a 3D structure, and thus a vertical cross-section is not sufficient information to explain every single process as is usually done in squall-line analysis. As an example, the vertical section showing the hydrometeor classification (Figure 10(b)) shows some wet snow particles between 3.5 and 4.5 km, within the secondary cells, although not much snow is observed above. The presence of wet snow is however easily explained by advection. In fact, a horizontal cross-section of hydrometeor classification at 3.8 km (Figure 11) shows that this particle type is also present to the west of this region, from where the wind is blowing. Similar images at higher altitudes (not shown) show dry snow particles to the west. These particles are probably advected during their descent and are observed in the melting layer as wet snow in regions far from where they originated.

**6. Summary and conclusions**

Three squall lines observed during the AMMA campaign
by the Ronsard radar are studied here. These systems
present a clearly defined convective region formed as an
alignment of convective cells.

In this paper the polarimetric signatures of West
African squall lines were investigated, together with their
microphysics. Although the three examples shown here
present significant differences from case to case, namely
in terms of intensity and maximum height, they still
possess similar characteristics that allow some general-
ization. These systems present cells on the leading edge
with strong reflectivity cores that indicate the convective
region. This part of the storms is also characterized by
relatively strong Z<sub>DR</sub> and K<sub>DP</sub> columns. The co-location
of Z, Z<sub>DR</sub> and K<sub>DP</sub> columns suggest that this is a zone of
high concentration of raindrops with a considerable size
(equivolume diameter >3 mm).

<figure><img src="image_13.png" /><figcaption>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.</figcaption></figure>

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
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 non-uniform, 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 Ouémé 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 C-band 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.

* Bouniol D, Delanoë J, Dudoure C, Protat A, Giraud V, Penide G. 2010. Microphysical characterisation of West African MCS anvils. *Q. J. R. Meteorol. Soc.* **136**(s1): 324–345.

* Brandes EA, Vivekanandan J, Tuttle JD, Kessinger CJ. 1995. A study of thunderstorm microphysics with multiparameter radar and aircraft observations. *Mon. Weather Rev.* **123**: 3129–3143.

* Caillault K, Lemaître Y. 1999. Retrieval of three-dimensional wind fields corrected for the time-induced advection problem. *J. Atmos. Oceanic Technol.* **16**: 708–722.

* Chong M. 2009. The 11 August 2006 squall-line system as observed from MIT Doppler radar during the AMMA SOP. *Q. J. R. Meteorol. Soc.* **136**(s1): 210–227.

* Chong M, Amayenc P, Scialom G, Testud J. 1987. A tropical squall line observed during the COPT 81 experiment in West Africa. Part 1: Kinematic structure inferred from dual-Doppler radar data. *Mon. Weather Rev.* **115**: 670–694.

* Cifelli R, Petersen WA, Carey LD, Rutledge SA, da Silva Dias MAF. 2002. Radar observations of the kinematic, microphysical, and precipitation characteristics of two MCSs in TRMM LBA. *J. Geophys. Res.* **107**: 8077, DOI:10.1029/2000JD000264.

* Hauser D, Roux F, Amayenc P. 1988. Comparison of two methods for the retrieval of thermodynamic and microphysical variables from Doppler radar measurements: Application to the case of a tropical squall line. *J. Atmos. Sci.* **45**: 1285–1303.

* Houze Jr RA. 1977. Structure and dynamics of a tropical squall-line system. *Mon. Weather Rev.* **105**: 1540–1567.

* Hubbert J, Bringi VN. 1995. An iterative filtering technique for the analysis of copolar differential phase and dual-frequency radar measurements. *J. Atmos. Oceanic Technol.* **12**: 643–648.

* Hubbert J, Bringi VN, Carey LD, Bolen S. 1998. CSU-CHILL polarimetric radar measurements from a severe hail storm in eastern Colorado. *J. Appl. Meteorolol.* **37**: 749–775.

* Illingworth AJ, Goddard JWF, Cherry SM. 1987. Polarization radar studies of precipitation development in convective storms. *Q. J. R. Meteorolol. Soc.* **113**: 469–489.

* Keenan TD. 2003. Hydrometeor classification with a C-band polarimetric radar. *Aust. Meteorolol. Mag.* **52**: 23–31.

* Kumjian MR, Ryzhkov AV. 2008. Polarimetric signatures in supercell thunderstorms. *J. Appl. Meteorolol. Clim.* **47**: 1940–1961.

* Leary CA. 1979. Behavior of the wind field in the vicinity of a cloud cluster in the intertropical convergence zone. *J. Atmos. Sci.* **36**: 631–639.

* Leary CA, Houze Jr RA. 1979a. Melting and evaporation of hydrometeors in precipitation from the anvil clouds of deep tropical convection. *J. Atmos. Sci.* **36**: 669–679.

* Leary CA, Houze Jr RA. 1979b. The structure and evolution of convection in a tropical cloud cluster. *J. Atmos. Sci.* **36**: 437–457.

* Le Bouar E, Testud J, Keenan TD. 2001. Validation of the rain profiling algorithm 'ZPHI' from the C-band polarimetric weather radar in Darwin. *J. Atmos. Oceanic Technol.* **18**: 1819–1837.

* Loney ML, Zrnić DS, Ryzhkov AV, Straka JM. 1999. 'In-situ and multiparameter radar observations of an isolated Oklahoma supercell at far range.' *29th Conference on Radar meteorology, Montreal, Quebec, Canada*.

* Loney ML, Zrnić DS, Straka JM, Ryzhkov AV. 2002. Enhanced polarimetric radar signatures above the melting level in a supercell storm. *J. Appl. Meteorolol.* **41**: 1179–1194.

* Penide G, Giraud V, Bouniol D, Dubuisson P, Duroure C, Protat A, Cautenet S. 2010. Numerical simulation of the 7 to 9 September 2006 AMMA mesoscale convective system: Evaluation of the dynamics and cloud microphysics using synthetic observations. *Q. J. R. Meteorolol. Soc.* **136**(s1): 305–323.

* Randell SC, Rutledge SA, Farley RD, Helsdon Jr JH. 1994. A modeling study on the early electrical development of tropical convection: Continental and oceanic (monsoon) storms. *Mon. Weather Rev.* **122**: 1852–1877.

* Rasmussen RM, Heymsfield AJ. 1987a. Melting and shedding of graupel and hail. Part I: Model physics. *J. Atmos. Sci.* **44**: 2754–2763.

* Rasmussen RM, Heymsfield AJ. 1987b. Melting and shedding of graupel and hail. Part II: Sensitivity study. *J. Atmos. Sci.* **44**: 2764–2782.

* Redelsperger J-L, Diedhiou A, Flamant C, Janicot S, Lafore J-P, Lebel T, Polcher J, Bourlès B, Caniaux G, De Rosnay P, Desbois M, Eymard L, Fontaine B, Geneau I, Ginoux K, Hoeppfner M, Kane SCE, Law K, Mari C, Marticorena B, Mougin E, Pelon J, Peugeot C, Protat A, Roux F, Sultan B, Van den Akker E. 2006.
AMMA, une étude multidisciplinaire de la mousson ouest-africaine.
*La Météorologie* **54**: 22-32.

Roux F. 1988. The West African squall line observed on 23 June 1981 during COPT 81: Kinematics and thermodynamics of the convective region. *J. Atmos. Sci.* **45**: 406–426.

Ryzhkov A, Zrnić D. 2005. 'Radar polarimetry at S, C, and X bands: Comparative analysis and operational implications.' *Proceedings 32nd Conference on radar meteorology, Albuquerque, New Mexico, USA, 22–29 October 2005*. American Meteorological Society.

Ryzhkov AV, Zrnić DS, Zhang P, Krause J, Park HS, Hudak D, Young J, Alford JL, Knight M, Conway JW. 2007. 'Comparison of polarimetric algorithms for hydrometeor classification at S and C bands.' *33rd Conference on radar meteorology, Cairns, Australia, 6–10 August 2007*. American Meteorological Society.

Schrage JM, Fink AH, Ermert V, Ahlonsou ED. 2006. Three MCS cases occurring in different synoptic environments in the sub-Sahelian wet zone during the 2002 West African monsoon. *J. Atmos. Sci.* **63**: 2369–2382.

Scialom G, Lemaître Y. 1990. A new analysis for the retrieval of three-dimensional mesoscale wind fields from multiple Doppler radar. *J. Atmos. Oceanic Technol.* **7**: 640–665.

Scialom G, Faroux J, Giraud M, Ney R, Evaristo R, Lemaître Y, Viltard N. 2009. RONSARD radar: Implementation of dual polarization on a C-band Doppler weather radar. *IEEE Geosci. Remote Sensing Lett.* **6**: 132–136.

Straka JM, Zrnić DS, Ryzhkov AV. 2000. Bulk hydrometeor classification and quantification using polarimetric radar data: Synthesis of relations. *J. Appl. Meteorol.* **39**: 1341–1372.

Szeto KK, Cho H-R. 1994. A numerical investigation of squall lines. Part I: The control experiment. *J. Atmos. Sci.* **51**: 414–424.

Testud J, Le Bouar E, Obligis E, Ali-Mehenni M. 2000. The rain profiling algorithm applied to polarimetric weather radar. *J. Atmos. Oceanic Technol.* **17**: 332–356.

Tingkai S, Zhemin T. 2001. Numerical simulation study for the structure and evolution of tropical squall line. *Adv. Atmos. Sci.* **18**: 117–138.

Williams ER. 2001. The electrification of severe storms. Pp 527–561 in *Severe convective storms, Doswell III CA (ed). Meteorological Monograph Series 28*: No. 50. American Meteorological Society: Boston.

Yang M-H, Houze Jr RA. 1995. Sensitivity of squall-line rear inflow to ice microphysics and environmental humidity. *Mon. Weather Rev.* **123**: 3175–3193.

Zipser EJ. 1977. Mesoscale and convective-scale downdrafts as distinct components of squall-line structure. *Mon. Weather Rev.* **105**: 1568–1589.

Zrnić DS, Ryzhkov A, Straka J, Liu Y, Vivekanandan J. 2001. Testing a procedure for automatic classification of hydrometeor types. *J. Atmos. Oceanic Technol.* **18**: 892–913.