3.1. Study area and environmental characteristics
The 2006 July–September (JAS) season was found to have small precipitation anomalies (less than 5%) from climatological values at each location using TRMM 3B42 gridded precipitation data. A time–longitude diagram using data from 2006 averaged over 12–17°N is shown in Figure 2. Data below 12°N (southern humid region in continental Africa) may skew regional analyses via the introduction of an area with less variability (Mohr et al., 2009), and was therefore excluded. The depiction of precipitation fraction data allows the cycle of convective decay and regeneration to be observed as a function of system propagation. Streaks of variable precipitation were observed, corresponding to westward propagating PFs—similar to outgoing longwave radiation diagrams (Laing et al., 2008; Cifelli et al., 2010). More than 10 advecting (or propagating) modes are evident in Figure 2, with varying life cycles in terms of precipitation intensities, duration, and phase speed. Objectively identified AEW trough tracks are overlaid (solid black lines). Ten AEW troughs were associated with the continental site, eight with the coastal site and 12 with the maritime site. Propagating modes averaged a speed of 14.9 m s−1, while mean AEW speed was 8.5 m s−1. In some cases, westward propagating precipitation events were evident along AEW trough tracks, while other trough regimes were void of precipitation. It is possible that variations in thermodynamic conditions and topography could have driven precipitation irregularity observed in the propagating modes (Laing et al., 2008).
Figure 2. Time–longitude plot of TRMM 3B42 gridded rainfall product averaged between 12 and 17°N. Contours represent percentage of rainfall above threshold value (0.8 mm h−1, mean value during 2006 season), with greater values representing increased areas of rain rates in observed systems—a proxy for size of precipitating system. Exclusion of data below 12°N was used to reduce ‘noise’ present from the southern track of precipitation associated with the summer monsoon. Objectively identified AEW trough tracks are overlaid (black lines), with all tracks that persist for less than 1.5 days and 8° in longitudinal length filtered out. Vertical, dashed lines show the location of each radar system in the study. The bottom plot denotes mean elevation between 12 and 17°N.
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Focusing on the radar locations, time series of radar reflectivity-estimated precipitation, CAPE, and CIN are shown for the continental (Figure 3), coastal (Figure 4), and maritime (Figure 5) locations, with AEW trough passages superimposed (hatched shading). Precipitation events of long duration and large spatial coverage were generally represented by unconditional rain rates (mean over entire scan domain) greater than 0.5 mm h−1. Continental convective systems were linear in organization and faster moving than those found over the east Atlantic (Laing and Fritsch, 1993; Hodges and Thorncroft, 1997) resulting in narrower peaks inland.
Figure 3. Time series of (a) radar-estimated unconditional rainfall rate, (b) CAPE, and (c) CIN for the continental location, 19 August–16 September 2006. Hatched, vertical bars indicate the presence of AEW troughs within 500 km of the site. Arrow heads along the CIN plot abscissa indicate the first radar echo occurrence of mesoscale convective systems of large spatial (>1000 km2) and temporal (>3 h) extent.
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Time series of maritime CAPE in Figure 5(b) showed more variability than one might expect, likely because soundings were launched from an island large enough for nocturnal surface cooling to help establish a low-level inversion prior to daytime heating. Small values of CIN were prevalent at the coast, with greater variability at the continental and maritime sites. The largest values of CIN were observed with more frequency at the continental site. No significant correlations or anti-correlations (including lag correlation) were found between time series at each radar location. Spectral analysis of the time series showed no common precipitation or environmental periodicities between variables plotted in Figures 3–5, suggesting little dependence upon wave-driven dynamics. However, AEW and no-wave regime environmental variable populations were shown to be significantly different (discussed later).
More frequent occurrence of MCSs ahead of AEW troughs at the continental and maritime sites (60%) and an even distribution ahead and behind the trough at the coastal location were observed, in agreement with earlier studies (Carlson, 1969b; Payne and McGarry, 1977; Duvel, 1990; Machado et al., 1993; Diedhiou et al., 1999; Kiladis et al., 2006). During MCS events at each radar site, the AEJ was predominantly located north of the radar, which agrees with observations from Mohr and Thorncroft (2006), who found that the most intense convective systems occurred south of the jet axis in September.
Interpretation of PF results was dependent upon understanding the environment within which convection occurs. Mohr and Thorncroft (2006) showed that environments of high shear and high CAPE can result in a high probability of the most intense convective systems (SLs) in West Africa, in agreement with simulations (Weisman and Klemp, 1982). Vertical wind shear is an essential component to linearly organized convective systems (Bluestein and Jain, 1985; Weisman et al., 1988; Coniglio et al., 2006). Nicholls and Mohr (2010) found that the top 10th percentile West African convective systems exhibited significantly stronger low-level shear. Though MCSs exist in environments with a wide range of shear, organization and system strength tend to increase with increasing shear (also true in this study despite low correlation values). Figure 6 depicts the relative frequency of CAPE (top row), CIN (middle row), and low-level shear magnitudes (bottom row) at each site. The distribution of CAPE at the maritime location (Figure 6(a); 1090 J kg−1 median value) was skewed toward lower values, while the coastal location displayed a tendency toward larger CAPE values (Figure 6(d); 1842 J kg−1 median value). The continental site (Figure 6(f); 1044 J kg−1 median value) was centered about more moderate CAPE values; though extremely large quantities up to 6000 J kg−1 were observed (not shown), but confined to less than 1% of cases. Extreme CAPE values at the continental site were mostly unrealized, occurring in unfavourable conditions for convection (e.g. lack of synoptic-scale convergence, very little vertical shear, and large CIN). The continental domain exhibited a 50% larger CAPE value during AEW regimes (found to be significant to the 95% confidence level), while the coastal and maritime locations remained nearly unchanged between wave and no-wave regimes. Median values and distributions of CAPE are generally consistent with Fink et al. (2006) and Nicholls and Mohr (2010).
Figure 6. Frequency distribution of calculated environmental variables for the maritime (a–c), coastal (d–f), and continental (g–i) locations. CAPE (top row), CIN (middle row), and low-level shear (bottom row) are shown. Black bars represent values calculated during an AEW regime, while gray bars denote no-wave regime calculations.
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Distributions skewed toward small CIN values were observed at each location. Maritime and continental values (Figure 6(b) and (h)) showed occasional large CIN, with tails extending beyond 400 J kg−1, while the relative occurrence of small CIN was most frequent at the coastal site (Figure 6(e)). Occurrence fraction of sub-MCS features (fraction of time when sub-MCS convection was present) at the coastal site was 23%, while it was only 16% at the continental site, indicating that smaller CIN at the coastal site may have allowed for a higher relative occurrence of sub-MCS systems. Convective storms able to overcome the larger convective cap (shown by larger CIN values) inland resulted in more ‘intense’ convection in terms of vertical growth and reflectivity statistics (shown later) and is consistent with Nicholls and Mohr (2010), who found both larger CAPE and CIN values were present during intense events when compared to less-intense occurrences.
Similar median low-level wind shear values were observed for the maritime (easterly 3.7 × 10−3 s−1), coastal (easterly 4.1 × 10−3 s−1), and continental (easterly 4.1 × 10−3 s−1) locations; however, distributions differ for each location. Mean vertical wind profiles in Figure 7 show the presence of the AEJ near 650 hPa, and a westerly low-level jet (LLJ) near the surface for both the continental and coastal sites. This configuration is consistent with the higher frequency of larger easterly shear values at these locations. Southwesterly flow at the surface gives way to easterly flow aloft inland. At the coast, mean southwesterlies were overlaid by northeasterlies up to the AEJ level. The largest difference between the AEW and no-wave regime wind profiles occurs at the coast, where a difference of approximately 3 m s−1 existed throughout the profile. In addition, the westerly LLJ was more pronounced during the no-wave regime. Calculations of shear from the surface to the westerly LLJ (not shown) revealed that the coastal site exhibited larger mixing potential at low levels during wave passage. Despite prominent changes in environmental wind profiles between AEW and no-wave regimes, the coastal location exhibited the smallest inter-regime changes in precipitation and convective characteristics in the study (shown later). Along with the relative homogeneity of CAPE mentioned earlier, this suggests that when favourable large-scale dynamics are absent, MCSs at the coastal location draw upon buoyancy to maintain their intensity, despite less environmental shear.
Figure 7. Mean zonal (solid lines) and meridional (dashed lines) vertical wind profiles for the (a) maritime, (b) coastal, and (c) continental locations. Mean rawinsonde profiles are shown for the African easterly wave (black) and no-wave (gray) regimes.
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Mean VAD divergence profiles (Figure 8) may be used to assess the effect of MCSs on the large-scale environment. Convective cells are characterized by convergence at the surface and divergence aloft, while stratiform regions display divergence at the surface, midlevel convergence and divergence aloft (Gamache and Houze, 1982; Mapes and Houze, 1993a). Standard deviation associated with the profiles was too large to yield significant differences between the AEW and no-wave regimes. The maritime profile (Figure 8(a)) exhibited the same structure as the intermediary case (a system during the conversion process from being convective to stratiform in nature) discussed in Mapes and Houze (1993b), also for an oceanic profile. The coastal profile (Figure 8(b)) showed divergence near the surface, mid-level convergence, and divergence aloft. The continental site exhibited the same general pattern (Figure 8(c)), with decreased divergence at the surface and peak convergence occurring lower in the atmosphere. This suggests distinct heating profiles for each location. It should be noted that these profiles could be driven by time-of-arrival of propagating MCSs that were often in a similar stage of development (see section 3.3).
Figure 8. Mean velocity azimuth display divergence profiles for the (a) maritime, (b) coastal, and (c) continental locations. Profiles of 40 km annuli centered about ranges of 24, 44, 60, and 76 km from the radar are averaged for AEW (black) and no-wave (gray) regimes. Profiles are made up of both convective and stratiform components.
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3.2. Precipitation characteristics
Table III lists statistics derived from PF analysis for the study time period, along with the statistics for both AEW and no-wave regimes. Less than 4% of total scans over the continent and even less over the coastal and maritime locations contained MCS events. Even though MCSs where infrequent, MCS rain volume fractions (of total observed precipitation) were large, in line with previous studies using IR (80–90%; Mathon and Laurent, 2001) and TRMM microwave satellite data (60–80%; Mohr et al., 1999; Nesbitt et al., 2006), with a mean Sahelian value near 80% of annual precipitation.
Table III. Convective system characteristics derived from precipitation feature analysis for all study times and within AEW and no-wave regimes.
|Location||Regime||MCS occurrence fraction (%)||MCS rain volume fraction (%)||MCS area fraction (%)||Convective (stratiform) rain volume fraction (%)||Convective (stratiform) area fraction (%)||Number of precipitation features identified |
|Maritime||All||0.6||83||53||64 (36)||9 (91)||14661|
|(TOGA)||AEW||0.3||81||57||61 (39)||9 (91)||6622|
| ||No wave||0.4||73||47||66 (34)||9 (91)||8039|
|Coastal||All||1.4||85||72||63 (37)||17 (83)||9507|
|(NPOL)||AEW||0.6||89||81||56 (44)||16 (84)||3031|
| ||No wave||0.9||81||65||68 (32)||18 (82)||6476|
|Continental||All||3.6||92||83||51 (49)||12 (88)||6468|
|(MIT)||AEW||2.0||95||88||53 (47)||12 (88)||2620|
| ||No wave||1.9||88||79||49 (51)||12 (88)||3848 |
A marked decrease at successive westward locations is observed in MCS area fractions (echo area coverage contributed by MCS-scale features; Table III). The percentage of area covered by continental and coastal MCSs was larger than sub-MCSs, while maritime MCSs and sub-MCSs covered an equivalent percentage of area, which agrees with Liu et al. (2008), who showed that the population of large satellite-observed systems decreased from West Africa into the East Atlantic at this latitude.
Contrary to results from previous studies, the stratiform precipitation fraction increased from west to east. Stratiform precipitation fractions for the maritime (36%) and coastal (37%) regions generally agree with Schumacher and Houze (2006), while the continental site (49%) was larger by nearly 10%. This difference may be explained by the fact that this study used only 1 month of data compared to 5 years in (Schumacher and Houze, 2006) and that spaceborne precipitation estimates do not account for the evaporation of precipitation in the boundary layer. Stratiform area accounts for 90% of MCS area, which may lead to underestimation from ground-based observations, which view a much smaller domain than space-borne observations and may not sample the entire MCS area. Strong, easterly low-level shear in this region produced leading convective line, trailing stratiform MCSs that greatly affected boundary layer properties. Boundary layer relative humidity (not shown) increases an average of 5% (>20% in some cases) with the passage of the convective line of these MCSs (denoted by arrow heads along the bottom abscissa in Figure 3), thereby retarding evaporation of the following stratiform precipitation and increasing observed stratiform precipitation fraction. Generally, upper-level humidity increases via transport by strong convective updrafts were observed during periods of high precipitation.
To further investigate precipitation in terms of vertical structure, precipitation contributions as a function of two characteristic reflectivity levels were calculated at each vertical level. First, 20 dBZ (Figure 9(a)–(c)) echo top heights were chosen to closely match the minimum threshold of the TRMM PR and minimize contamination from spurious echo missed in the radar QC process. Second, 30 dBZ (Figure 9(d)–(f)) echo top heights were chosen to identify intense convective cells with significant mixed-phase processes (DeMott and Rutledge, 1998). Data for all occurrences are shown; exclusion of sub-MCSs did not affect the distributions.
Figure 9. Contribution by precipitation features, at the maritime (solid line), coastal (dotted line) and continental (dashed line), to total volumetric rainfall as a function of mean (a–c) 20 dBZ and (d–f) 30 dBZ echo-top heights. Convective (b, e) rain volume-weighted reflectivity occurrence distributions normalized by maximum occurrence were used to illuminate the convective mode of precipitation. The same methodology was used for stratiform (c, f) contributions.
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A strong bimodal distribution at the maritime location (9 and 15–17 km peaks), a weak bimodal distribution at the coast (9 and 15 km peaks), and a broad, unimodal distribution (15 km peak) at the continental site were seen in the 20 dBZ distributions. The maritime and coastal distributions suggest distinct modes, while the continental site appears to be influenced by a deeper spectrum of vertical development. Convective precipitation controls the contribution from the deep mode at all sites, while the stratiform precipitation occurs at a lower height. The stratiform contribution generally exhibits a more narrow vertical distribution, with the exception of the broad stratiform distribution at the continental site.
The 30 dBZ distributions indicate that the continental and coastal locations had deeper, more intense convective modes than the maritime site. The continental and coastal distributions fall off less rapidly from the 7–9 km peak, with a secondary peak in the coastal distribution near 13 km. More vertically developed storms display a greater propensity for mixed-phase processes (DeMott and Rutledge, 1998; Nesbitt et al., 2006), enhancing the stratiform region and leading to larger precipitation contribution over the course of the study from deep convection observed over the continent (Figure 9(e)–(f)). DeLonge et al. (2010) showed that MCSs transitioning from land to ocean exhibit signs of disorganization, resulting in less intense convective characteristics over the ocean. Figure 4 indicates that storms at the coast experienced a higher likelihood to enter a region with higher CAPE, which would theoretically produce stronger updrafts and significant lofting of precipitation-sized particles. Greater low-level shear over the land could act to enhance linear organization, resulting in two distinct modes (ocean and land) of vertical development present during the study period.
It is well established that the diurnal cycle of precipitation in West Africa is largely controlled by propagating MCSs (Reed, 1978; Shinoda et al., 1999; Mohr, 2004; Fink et al., 2006; Laing et al., 2008; McGarry and Rickenbach et al., 2009) and is a function of distance from genesis and redevelopment regions (e.g. high terrain; Hodges and Thorncroft, 1997; Mohr, 2004). This pattern was confirmed in this study in conjunction with Meteosat imagery (not shown), showing peak precipitation occurring near 0800 LT at the continental site, 0200 LT at the coastal and maritime sites.
3.3. Convective characteristics and easterly waves
Analysis of longer time period radar-estimated precipitation at the continental site showed a peak precipitation interval every 3–4 days (Nieto Ferreira et al., 2009), suggesting a correlation to AEW trough passage at this longitude. While precipitation events did occur near trough passages during the time frame of this study, many events also occurred when no wave was identified (see Table II). As a result, no significant correlation between AEWs and precipitation was noted at the continental site. Given the current debate concerning the impact of AEWs on precipitation, it was of interest to compare convective system characteristics during periods of AEW passage and periods of no AEW forcing. The intent was to take advantage of radar data from the three sites to further elucidate effects of AEW forcing on convective characteristics (i.e. vertical and horizontal structure) and to understand possible feedbacks of these mesoscale features onto the larger scale (i.e. MCS latent heat release).
Table IV lists contributions of AEW regime PFs during the study period, with 37–57% (32–45%) of total rain volume (feature area) associated with the AEW regime at all sites; less than previous results. Only the continental site showed that greater than half of the total estimated precipitation occurred during the AEW regime. Laing et al. (2008) found that about 80% of the deep convective area (as identified by satellite cold cloud streaks) was associated with AEWs for a region from 10°W to 10°E. The discrepancy with the current study may be attributed to differences between the PF definition used here and the classification of convection based upon minimum IR brightness temperatures. Ground-based radar observations yield a more direct picture of the spectrum of precipitating features, while IR precipitation estimates are based upon persistent, high cloud shields associated with MCSs. Therefore the estimation and temporal evolution of precipitation may differ between these methodologies and result in partitioning differences.
Table IV. Contribution of AEW regime precipitation features as a function of study period totals.
|Location||Total feature fraction (%)||Total rain volume fraction (%)||Convective rain volume fraction (%)||Stratiform rain volume fraction (%) |
To further consider differences during AEW and no-wave regimes, cumulative frequency distributions (CFDs) of the feature area (Figure 10(a)) and rain volume (Figure 10(b) and (c)) were created and the distribution differences (CFDAEW − CFDno−wave) analyzed. Regime populations were found to be significantly different from the 99% confidence level. The AEW regime was associated with broad increases in PF size at the continental and coastal locations, with the coastal peak increase offset to larger systems. Feature size decreased during the AEW regime at the maritime location, with a maximum decrease at the sub-MCS scale. Examination of convective and stratiform precipitation volume distributions revealed that continental convective precipitation (Figure 10(b)) was enhanced during the AEW regime, while stratiform precipitation (Figure 10(c)) decreased. Little change was observed at the coast for convective precipitation, with an increase in stratiform precipitation during the AEW regime. Convective precipitation decreased at the maritime site during the AEW, while stratiform precipitation showed little deviation between regimes. Inspection of PF distribution along with environmental variables may help clarify the differences shown in convective and stratiform precipitation.
Figure 10. Difference in cumulative frequency distributions between AEW and no-wave regimes for (a) precipitation feature area, (b) convective and (c) stratiform volumetric rainfall. Maritime (solid line), coastal (dotted line), and continental (dashed line) locations are shown. Positive values correspond to an increase during the AEW regime. Less (more) volumetric rainfall may be interpreted as a decrease (increase) in precipitation rate and/or increase (decrease) in precipitation spatial coverage.
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The increase in system size and convective rain volume at the continental site (Figure 10(a)) is consistent with increased CAPE, decreased CIN, and weaker shear, resulting in a shift of sub-MCSs to MCS-scale that occurred during the AEW regime compared to the no-wave regime. Increased large system population at the coast may have been driven by increased CIN and vertical shear, which resulted in a greater thermodynamic triggering barrier and provided increased organization for larger systems at the expense of smaller systems. A reduction in CIN and weaker vertical shear in the lowest 3 km were observed during the wave regime at the maritime site, explaining the formation of weaker, smaller convection.
Differences in convective and stratiform contributions can be further elucidated in terms of mean vertical reflectivity profiles (Figure 11). Convective (stratiform) profiles for each site are similar, with near surface mean values of 36–40 (22–28) dBZ. The decrease in reflectivity with height is similar for all three locations. Continental and maritime AEW regime convective profiles were more intense and exhibited higher reflectivity values aloft compared to the no-wave regime, suggesting hydrometeor loading aloft due to strong updrafts. Note that the number of points was an order of magnitude less for the continental no-wave regime, also suggesting less vertical growth overall. The coastal site exhibited very little difference in convective reflectivity profiles for AEW and no-wave regimes, which agrees with convective precipitation differences noted earlier (Figure 10(b)). Land-to-ocean transitioning MCSs (coastal site) have been shown to diminish in strength (e.g. DeLonge et al., 2010) due to less favourable thermodynamic (e.g. lower specific humidity) and dynamic (e.g. reduced vertical wind shear) conditions. The changes associated with this transition may have mitigated enhanced synoptic-scale moisture flux convergence and potential vorticity during the AEW regime to mediate vertical reflectivity profiles.
Figure 11. Vertical radar reflectivity profiles for (a) convective and (b) stratiform regimes at the maritime (green), coastal (blue), and continental (red) locations. Mean profiles for AEW passages (solid line) and no-wave (dashed line) are shown along with the difference between the AEW and no-wave regime profiles (dotted line). Secondary plots to the right of each main plot show the number of points averaged at each vertical height. Note that the order of magnitude is the same throughout the bottom 11 km.
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A brightband signature, owing to the melting of aggregates common in organized MCSs (Houze et al., 1989), was observed near 3–5 km in the stratiform profile at the coastal and continental sites. Decreasing reflectivity below this level toward the surface is a signature of droplet evaporation below cloud base, consistent with mesoscale descent. The continental AEW regime stratiform profile decreased more rapidly with height when compared to the no-wave regime, consistent with the reduction in stratiform rain area (Figure 10(c)) noted earlier. The order of magnitude difference in the continental number of points profile might suggest the importance of large-scale dynamics during the AEW regime on MCSs for the maintenance of the stratiform shield at this site. The consequences of these profiles are that MCS heating profiles at the coastal and maritime locations are comparable for the wave and no-wave regimes, whereas at the continental site differences arise due to the modification of stratiform structure. Weaker upper-level stratiform signal during the AEW regime results in a reduction of heating aloft and a less top-heavy heating profile (Mapes and Houze, 1995).