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# Rainfall diurnal cycle along a latitudinal transect from Cotonou to Kandi: some hints on the climatology of the West African Monsoon in Benin

Christian Depraetere, M. Alassani, J. Anese, Marielle Gosset, Andreas H Fink, Francis Didé

► To cite this version:

Christian Depraetere, M. Alassani, J. Anese, Marielle Gosset, Andreas H Fink, et al., Rainfall diurnal cycle along a latitudinal transect from Cotonou to Kandi: some hints on the climatology of the West African Monsoon in Benin. 2007. hal-05006534

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https://hal.science/hal-05006534v1

Preprint submitted on 26 Mar 2025

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# Rainfall diurnal cycle along a latitudinal transect from Cotonou to Kandi: some hints on the climatology of the West African Monsoon in Benin

Depraetere C (1), Alassani M.(2), Anese J. (3), Gosset M. (4), Fink A. (5), Didé F. (6)

(1) IRD/LTHE,Grenoble, France (2)Université d'Abomey-Calavi, République du Bénin (3) Université Joseph Fourier, Grenoble France

(4) IRD, Cotonou, Bénin (5) University of Köln (6) Direction de la Météorologie Nationale, DMN, Cotonou, Bénin

A program of digitalization of rain diagrams from 1965 to 2004, archived by the Direction de la Météorologie Nationale (DMN) of Bénin has been conducted in collaboration with the LTHE, the university Joseph Fourier and the university of Köln. The work was achieve on the 6 synoptic meteorological stations which constitute a latitudinal transect from 6° to 11°N. The objectives are twofolds:

i) to help to perpetuate a valuable data set (“data rescue”), and save it online in a data bank.

ii) to carry out a climatological studies thanks to a unique long serie of rain data along a transect.

Evolution of hourly occurence of rainfall events according to the distance to the Gulf of Guinea are specifically studied in this paper. The rationale is to explore the characteristics of rainfalls diurnal cycles and their relations with the prevailing West African Monsoon processes (WAM) or continental surface conditions.

The datasets includes 19,500 digitized daily diagrams over 39 years and 6 stations, thus about 83 diagrams/year/station. The cross validation of this data with daily rainfalls suggests that there is 20% of missing rainfall diagrams but those gaps only represents 13% of the total amount of rain. The data analysis shows that this database void does not constitute a major limitation for interannual climatological analysis but is a major drawback for studies on a yearly basis. Further detailed and comparative studies proves that the data is relevant to depict the diurnal cycle. They also rise that they should be considered with caution for rainfall intensity studies due to the graphic innaccuracy of rainfall curve at small time steps.

The climatological “Benin transect” of 6 stations from 6.3° to 11.2°N is mostly located along the “dry diagonal” on the eastern side of the Atacora highlands. Therefore, its representativity compare to the entire West Africa region is questionable. Despite that peculiar location, the Hovmoeller diagramme over the Benin transect compare to its West African counterpart suggests that the local condition of the WAM dynamic are quite similar with a clearly defined “monsoon jump” at the end of June. Apart from the pluviometric deficit and the lack of northward decrease of rainfall, the Benin transect is then assumed to reflect dominant processes at larger scale.

The rainfall diurnal cycle along the Benin transect over the period 1965-2004 is indicating significant differences according to latitude, local geographical context and the period of time. Compare to other stations, Cotonou on the coastline appears as an exception with maximum of rainfall occuring in the morning (6h-14h) and no evolution of this tendency during the rain season. The station of Natitingou located between the two main ranges of the Atacora mountains of northern Benin shows a typical maximum of rainfall at the late afternoon or the evening (14h-22h) and this slight prevailing occurence is observed all along the year. For these two stations, local geographical conditions may be the factor with the most controls on the time occurence of rainfall events. The four other stations located inland are far apart from the main ridges of Atacora. Their rainfall diurnal cycle is changing all along the rain season with a sharp shift from overwellming night rainfall (22h-6h) before the “monsoon jump” to dominant late afternoon rainfall (14h-22h) that becomes dominant at the end of july. At the end of September and october, the hourly distribution of rains tends to become again similar to what it was before the monsoon jump. This general evolution is more or less well defined and shows slight difference according to the latitude between Bohicon (7,17°N) and Kandi (11,13°N).

These first results on interannual average of hourly rainfall could give some hints on the evolution of dominant meteorological processes according to local and latitudinal. At this stage and without further validations, only general hypothesis based on changes of surface conditions at the continent surface or meteorological conditions in the upper atmosphere may explain this sudden change at the end of june. This may contribute to understand the threshold effect underlying the monsoon jump along the Benin transect, but also on the entire sub-saharian West Africa.

**Keywords:**

Daily cycle, rainfall diagrams, Benin, West African Monsoon, Hovmoeller diagram.

Contact: Christian Depraetere, Institut de Recherche pour le Développement, LTHE, BP 5,3 38041 Grenoble cedex 9, France. Tel.: 33-476 82 50 36 (from abroad) Email: christian.depraetere@hmg.inpg.fr
Location and climatological context of the Benin transect:

Benin is a latitudinally elongated country within what has been called the « Dry diagonal » (diagonale de sècheresse, Bokonon-Ganta, 1987) that reaches the Gulf of Guinea between Bénin and Ghana (Map 1). This stunning deficit of rainfall along the coast is also called the “Anomaly of Accra”. The six synoptic stations of this country form a transect of climatological records from the coast (Cotonou, 6.35°N) to the soudano-sahelian zone at the station of Kandi at 11,13°N (Map 2). They are all located far apart on the eastward side of the Atacora highlands but Natitingou which stands between the two main ranges.

<figure><img src="image_2.png" /><figcaption>Fig. 1: The «Dry diagonale» in West Africa</figcaption></figure>

The fact that this transect is located along the dry diagonal requires that their climatology ought to be compare to others at the same latitude in West Africa. For that purpose, we will compare the hovmoeller diagrammes as they have been computed over West Africa (fig. 3) and over a quadrangle including all the synoptic station (fig. 4, quadrangle 1.4°E to 3°E and 6.25°N to 11.2°N). The results suggest that the rain season is quite similar between the two scales despite the specific context of Benin. This is particularly true for the “monsoon jump” which occures at the the end of jump with the maximum of rainfall suddenly occuring at about 10°N instead of 6°N previously. This is consistent with what was identified by previous authors on other regions (Le Barbé et al., Redelsperger J.L. et al. 2006).

The lack of northward decrease of rainfall is the main peculiarity of the Benin transect compare to other locations. In fact, the maximum of rainfall of 1200 mm is located between 9 and 10°N between Parakou and Natitingou while annual rainfall is only about 1000 mm around Cotonou.

<figure><img src="image_3.png" /><figcaption>Fig. 2: Location of the Benin transect
compare to the Atacora Highlands</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3: Hovmoeller diagram over West Africa and the Gulf of Guinea for the Period 1979-2000 (from Redelsperger J.L. et al. 2006)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4: Hovmoeller diagram over the Benin transect for the period 1979-2000</figcaption></figure>
Analysis of rainfall diagrams dataset:

The rainfall diagrams come from the archives of the Direction de la Métérologie Nationale of Bénin (DMN). These archives of the national synoptic stations network have never been used before that work. The digitizing of rainfall diagrams was executed at the DMN in collaboration with IRD and the University of Köln first of all as a data rescue operation. The resulting dataset includes 19,500 digitized daily diagrams over 39 years and 6 stations (Table 1), thus about 83 diagrams/year/station (fig. 5).

<figure><img src="image_6.png" /><figcaption>Fig. 5: Annual rainfall on the Benin transect and number of rainfall diagrams per station per year</figcaption></figure>

The cross validation of this data with daily rainfalls suggests that there is 20% of missing rainfall diagrams but those gaps only represents 13% of the total amount of rain (Table 2). The data analysis shows that the void does not constitute a major limitation for interannual climatological analysis because the interannual regime of rainfall is still correctly sampled as illustrated on the case of Kandi with 13% of gaps (fig.6).
<table><caption>Table 1: yearly rainfalls and rainfall diagrams dataset on the Benin transect from 1965 to 2004</caption><thead><tr><th rowspan="2">Year</th><th rowspan="2">Rain<br>mm</th><th rowspan="2">Type<br>of<br>year</th><th colspan="6">Benin transect (including 5 stations inland + Cotonou)</th><th colspan="3">Data on transect</th></tr><tr><th>Kandi<br>D005</th><th>Natitingou<br>D020</th><th>Parakou<br>D034</th><th>Savé<br>D049</th><th>Bohicon<br>D059</th><th>Cotonou<br>D075</th><th>6(all)<br>Stations</th><th>5(inland)<br>stations</th><th>NbDigrams/<br>Station</th></tr></thead><tbody><tr><td>1963</td><td>1587</td><td>Humid</td><td>71</td><td>110</td><td></td><td>22</td><td>105</td><td>124</td><td>5</td><td>4</td><td></td></tr><tr><td>1964</td><td>1084</td><td></td><td>53</td><td>67</td><td></td><td>52</td><td>26</td><td>11</td><td>5</td><td>4</td><td></td></tr><tr><td>1965</td><td>1118</td><td></td><td>64</td><td>104</td><td>136</td><td>117</td><td>76</td><td>110</td><td>6</td><td>5</td><td>99</td></tr><tr><td>1966</td><td>1149</td><td></td><td>94</td><td>80</td><td></td><td>116</td><td>87</td><td>102</td><td>5</td><td>4</td><td></td></tr><tr><td>1967</td><td>1189</td><td></td><td>89</td><td>116</td><td></td><td>109</td><td>71</td><td>88</td><td>5</td><td>4</td><td></td></tr><tr><td>1968</td><td>1454</td><td>Humid</td><td>84</td><td>6</td><td></td><td>123</td><td>108</td><td>147</td><td>5</td><td>4</td><td></td></tr><tr><td>1969</td><td>1175</td><td></td><td>93</td><td>3</td><td>46</td><td>92</td><td>66</td><td>120</td><td>6</td><td>5</td><td>60</td></tr><tr><td>1970</td><td>1076</td><td></td><td>85</td><td></td><td>89</td><td>96</td><td>66</td><td>111</td><td>5</td><td>4</td><td></td></tr><tr><td>1971</td><td>1022</td><td></td><td>88</td><td>117</td><td>68</td><td>106</td><td>74</td><td>102</td><td>6</td><td>5</td><td>91</td></tr><tr><td>1972</td><td>1031</td><td></td><td>94</td><td>104</td><td>92</td><td>98</td><td>80</td><td>98</td><td>6</td><td>5</td><td>94</td></tr><tr><td>1973</td><td>1082</td><td></td><td>76</td><td>96</td><td>114</td><td>101</td><td>74</td><td>79</td><td>6</td><td>5</td><td>92</td></tr><tr><td>1974</td><td>1122</td><td></td><td>84</td><td>103</td><td>115</td><td>94</td><td>88</td><td>60</td><td>6</td><td>5</td><td>97</td></tr><tr><td>1975</td><td>1137</td><td></td><td>105</td><td>122</td><td>109</td><td>105</td><td>86</td><td>103</td><td>6</td><td>5</td><td>105</td></tr><tr><td>1976</td><td>1000</td><td>Dry</td><td>70</td><td>127</td><td>102</td><td>94</td><td>56</td><td>86</td><td>6</td><td>5</td><td>90</td></tr><tr><td>1977</td><td>859</td><td>Dry</td><td>70</td><td>98</td><td>97</td><td>77</td><td>47</td><td>83</td><td>6</td><td>5</td><td>78</td></tr><tr><td>1978</td><td>1170</td><td></td><td>93</td><td>124</td><td>127</td><td>99</td><td>70</td><td>113</td><td>6</td><td>5</td><td>103</td></tr><tr><td>1979</td><td>1296</td><td>Humid</td><td>82</td><td>111</td><td>115</td><td>91</td><td>77</td><td>114</td><td>6</td><td>5</td><td>95</td></tr><tr><td>1980</td><td>1065</td><td></td><td>68</td><td>107</td><td>97</td><td>89</td><td>65</td><td>100</td><td>6</td><td>5</td><td>85</td></tr><tr><td>1981</td><td>970</td><td>Dry</td><td>80</td><td>116</td><td>96</td><td>106</td><td>79</td><td>72</td><td>6</td><td>5</td><td>95</td></tr><tr><td>1982</td><td>934</td><td>Dry</td><td>82</td><td>106</td><td>113</td><td>80</td><td>61</td><td>76</td><td>6</td><td>5</td><td>88</td></tr><tr><td>1983</td><td>751</td><td>Dry</td><td>58</td><td>78</td><td>58</td><td>47</td><td>54</td><td>67</td><td>6</td><td>5</td><td>59</td></tr><tr><td>1984</td><td>1005</td><td>Dry</td><td>73</td><td>64</td><td>106</td><td>80</td><td>77</td><td>63</td><td>6</td><td>5</td><td>80</td></tr><tr><td>1985</td><td>1090</td><td></td><td>68</td><td>76</td><td>97</td><td>84</td><td>8</td><td>67</td><td>6</td><td>5</td><td>67</td></tr><tr><td>1986</td><td>958</td><td>Dry</td><td>25</td><td>110</td><td>100</td><td>49</td><td>75</td><td>60</td><td>6</td><td>5</td><td>72</td></tr><tr><td>1987</td><td>1001</td><td>Dry</td><td>67</td><td>87</td><td>42</td><td>55</td><td>80</td><td>116</td><td>6</td><td>5</td><td>66</td></tr><tr><td>1988</td><td>1284</td><td>Humid</td><td>65</td><td>100</td><td>105</td><td>75</td><td>55</td><td>88</td><td>6</td><td>5</td><td>80</td></tr><tr><td>1989</td><td>1212</td><td></td><td>82</td><td>81</td><td>104</td><td>76</td><td>78</td><td>95</td><td>6</td><td>5</td><td>84</td></tr><tr><td>1990</td><td>1005</td><td>Dry</td><td>52</td><td>136</td><td>78</td><td>51</td><td>64</td><td>31</td><td>6</td><td>5</td><td>76</td></tr><tr><td>1991</td><td>1329</td><td>Humid</td><td>61</td><td>129</td><td>116</td><td>74</td><td>53</td><td>143</td><td>6</td><td>5</td><td>87</td></tr><tr><td>1992</td><td>1002</td><td>Dry</td><td>37</td><td>122</td><td>75</td><td>73</td><td>47</td><td>102</td><td>6</td><td>5</td><td>71</td></tr><tr><td>1993</td><td>1057</td><td></td><td>81</td><td>132</td><td>35</td><td>54</td><td>61</td><td>108</td><td>6</td><td>5</td><td>73</td></tr><tr><td>1994</td><td>1182</td><td></td><td>69</td><td>130</td><td>101</td><td>73</td><td>46</td><td>107</td><td>6</td><td>5</td><td>84</td></tr><tr><td>1995</td><td>1241</td><td></td><td>80</td><td></td><td>102</td><td>89</td><td></td><td>107</td><td>4</td><td>3</td><td></td></tr><tr><td>1996</td><td>1149</td><td></td><td>31</td><td>107</td><td>69</td><td>68</td><td>30</td><td>89</td><td>6</td><td>5</td><td>61</td></tr><tr><td>1997</td><td>1123</td><td></td><td>67</td><td>102</td><td>99</td><td>22</td><td>78</td><td>109</td><td>6</td><td>5</td><td>74</td></tr><tr><td>1998</td><td>1213</td><td></td><td>84</td><td></td><td>101</td><td>83</td><td>73</td><td>45</td><td>5</td><td>4</td><td></td></tr><tr><td>1999</td><td>1293</td><td>Humid</td><td>92</td><td></td><td>77</td><td>65</td><td>120</td><td>129</td><td>5</td><td>4</td><td></td></tr><tr><td>2000</td><td>980</td><td>Dry</td><td>75</td><td>115</td><td>104</td><td>10</td><td>103</td><td></td><td>5</td><td>5</td><td>81</td></tr><tr><td>2001</td><td>958</td><td>Dry</td><td>89</td><td>101</td><td>107</td><td>57</td><td>92</td><td></td><td>5</td><td>5</td><td>89</td></tr><tr><td>2002</td><td>1111</td><td></td><td>80</td><td>91</td><td>90</td><td>100</td><td>105</td><td></td><td>5</td><td>5</td><td>93</td></tr><tr><td>2003</td><td>1257</td><td>Humid</td><td>85</td><td>111</td><td>132</td><td>112</td><td>97</td><td></td><td>5</td><td>5</td><td>107</td></tr><tr><td>2004</td><td></td><td>?</td><td>84</td><td>61</td><td>97</td><td>115</td><td>103</td><td>126</td><td>6</td><td>5</td><td>92</td></tr></tbody></table>

<table><caption>Table 2: interannual rainfall differences over the period 1965-2004</caption><thead><tr><th rowspan="2">Station</th><th colspan="2">Rainfall depletion 1965-2004</th></tr><tr><th>(mm)</th><th>(%)</th></tr></thead><tbody><tr><td>KANDI</td><td>-129</td><td>13,6</td></tr><tr><td>NATITINGOU</td><td>-271</td><td>22,7</td></tr><tr><td>PARAKOU</td><td>-181</td><td>16,3</td></tr><tr><td>SAVE</td><td>-94</td><td>9,2</td></tr><tr><td>BOHICON</td><td>-216</td><td>20,9</td></tr><tr><td>COTONOU</td><td>-62</td><td>4,9</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Fig. 6 : difference of rainfalls between daily rainfall and rain diagrams at the station of Kandi for the period 1965-2004</figcaption></figure>

On the other hand, it is a major drawback for studies on a yearly basis and complete data set on all the stations of the transect are only available for few years (table 3).

<table><caption>Table 3: Complete annual set of data for the Benin transect over the period 1965-2004</caption><thead><tr><th>Annual Data on Benin transect (1965-2004)</th><th>Dry year R&lt;1020mm</th><th>Normal year</th><th>Humid year R&gt;1250mm</th><th>Nb Years</th></tr></thead><tbody><tr><td>Complete set</td><td>8</td><td>12</td><td>4</td><td>24</td></tr><tr><td>(Example of years)</td><td>(1976 1983)</td><td>(1965 1975 1978)</td><td>(1979 1991)</td><td></td></tr><tr><td>VOID*(station or data)</td><td>4</td><td>10</td><td>2</td><td>16</td></tr><tr><td>Nb Years</td><td>12</td><td>22</td><td>6</td><td>40</td></tr></tbody></table>

The paper diagrams are representing daily cumulative rainfall from a rain gauge measuring rainfall with a tipping bucket of 0.5mm capacity. The mechanic part between the tipping bucket and the pen is smoothing the original information made of 0.5 mm increment of 0.5 mm. The assessment of this data downgrading will be evaluate on a subset of data on the station of Parakou for few periods in 2002, 2003 and 2004 for which recording rain gauge data are available.

The data should be considered with caution for rainfall intensity studies due to the graphic innaccuracy of rainfall curve at small time steps. This point is illustrated on one rainfall event observed at the station of Parakou (fig. 7). It is worth noting that at least the total amount of rain and the duration of the rainfall event are quite similar. Conversely, it does not reflect clearly the sharp difference of rainfall intensities between the convective part and the stratiform part of this typical Mesoscale Convective System (MCS).

Considering now the entire reference subset on Parakou over the period 2002-2004, the comparison between the two sets of rainfall intensities proves that rain diagrams data depicts very poorly the statistical distribution of rainfall intensity (fig.8). From rain gauge, the intensities greater than 10 mm/hour totalize more than 60% of the rain while they only represent less than 10% according to rain diagram data. Those results illustrates the limitations of use of rain diagram for rainfall intensities studies but suggests that they could be as source of information on global characteristics of rainfall events such as cumulate rainfall and duration.

We will develop in this paper another major application of rain diagrams. This dataset is providing a unique source of information on time occurrence of rain and could therefore help to depict the average diurnal cycle of rainfall.

<footnote>*at least one station is missing or many missing diagrams</footnote>
<figure><img src="image_8.png" /><figcaption>Fig. 7: raingauge and rain diagram intensities at Parakou 16/8/2002 rainfall event</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8 : distribution of rainfall intensities according to rain gauge and rain diagram at Parakou from 2002 to 2004</figcaption></figure>
The diurnal cycle of rainfall according to latitude and the time period:

The diagram of hourly versus daily rainfall computed over a long period of time is an efficient method to study the evolution of diurnal cycle during the rain season. This fact is exemplified on the four stations of Kandi, Parakou, Save and Bohicon which constitute the **inland Eastern Benin transect** (fig. 9). These Hourly/Daily diagrams shows the occurence of rain per ¼ hour for all the considered period. Later on, we will see that the amount of rain instead of rain occurence would have given similar results on the whole despite highest rainfall intensity of late afternoon showers.

Considered globally, the four stations
points up a steady trend northward for the
decrease of late afternoon rainfalls.
Conversely, rainfall occuring at night
progressively becomes the most frequent
type of events. There is therfore a sharp
difference between Bohicon where late
afternoon rainfall are largely
overwellming all the other and Bohicon
which show a dominant pic of frequency
at night with a secondary maximum
during late afternoon hours. In between,
the station of Save and Parakou appears to
be intermediary stages between these two
extrem cases.

The results also put in the forefront
an evolution during the rain season.

<figure><img src="image_10.png" /><figcaption>Fig. 9 : Diagram of hourly versus daily rainfall on the eastern inland transect (1965-2004)</figcaption></figure>
**_The two stations of Cotonou and Natitingou_** are apart from the four stations of the inland eastern transect Bohicon-Kandi (Figure 10).

The latitude of Natitingou is located between Kandi and Parakou but on the western piedmont of the Atakora Highlands (see map of figure 2). This location may cause orographic convection and induces a more complex diurnal cycle.

Cotonou is on the coastline and influence the diurnal wind cycle: from the see at the end of the night and early morning, from inland in the afternoon and the evening. This is why the diurnal cycle of rainfall in Cotonou are clearly different from all the other stations.

<figure><img src="image_11.png" /><figcaption>Fig. 10 : Diagram of hourly versus daily rainfall on Natitingou and Cotonou for the period 1965-2004</figcaption></figure>
Eventhough it is obervable on the
hourly/daily diagram and for
convenience of the demonstration, we
will divide the days in tree laps of time
of 8 hours: morning from 6h to 14h,
afternoon from 14 to 22h and night from
22h to 6h (fig. 11). If we consider now
the evolution of rain during these tree
periods all along the rain season, some
significant changes can be identified.
The most acute one occures at the
station of Bohicon at the end of June,
precisely during the monsoon jump (fig.
11a). In june, night rainfall are dominant
in the proportion of 2/3, while they are
representing slightly less than 1/3 from
mid july to the end of august. This
period which corresponds to the apex of
the rain season is characterized by an
ewuibrobability of rain. The proportion
of nocturn showers then starts to rise
again in September and represent 1/2 of
rainfall up to the end of the rain season.
The same chronology is observed at
Parakou but in a less demonstrative way
(fig. 11b).

Bohicon and Save are presenting a
different chronology compare to the
previous northern stations (fig. 11c and
11d). Rainfalls occuring during the
afternoon and the evening are dominant all
along the year in the proportion of 1/2. The
only exception is observed from the
beginning of July and mid August with an
occurence of afternoon rain equal or
greater than 2/3. This period correspond to
the little dry season further south at
Cotonou. It is not well defined on the two
inland stations of Save and Bohicon with
amount of rainfall which are alsmost
equivalent to what is observed at the
beginning of the rain season.

<figure><img src="image_12.png" /></figure>

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

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 11: Evolution of diurnal cycle during the rain season along the Eastern Benin transect (1965-2004)</figcaption></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

The global consistency of the result along the transect suggests that the latitudinal evolution reflects changes in the dominant processes responsible for rainfall.

Rainfall intensities will help to identify specific processes as shown on figure 12. Intensities are computed at 15 minutes time steps by dividing the amount of rain by the occurrence. The main fact stands on the highest intensities observed on all stations at late afternoon hours. The maximum of 2.5mm/15minutes (10mm/h) happens between 16 and 18 hours apart from the northern station of Kandi were it occurs sooner between 13 and 16 hours. The rains occurring at night present significantly lower intensities ranging bet 1.6 and 1.9 mm/15m (6 to 8 mm/h).

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 12: Diurnal cycle along the Eastern Benin transect (1965-2004)</figcaption></figure>
The station of Natitingou is located between the latitude of Parakou and Kandi but does not fit with the evolution of diurnal cycle as observed on the Eastern Benin Transect. The geographic peculiarity of this station is that it stands just between the main ridges of the Atacora. The major difference stands on high occurrence of afternoon rainfall with a maximum between 14h and 18h. The second difference is that the timing of rain does not change as much all along the rain season despite some analogy with what is observed at Parakou. The decrease of night rain frequency after the monsoon jump is still noticeable but is gradual and not sharply defined compare to the two other stations. The last difference come from the low intensities of night rainfall compared to all other stations.

<figure><img src="image_20.png" /><figcaption>Fig. 13: Evolution of diurnal cycle and rainfall according to hours at Natitingou (1965-2004)</figcaption></figure>

Cotonou is located on the coast of the Gulf of Benin and is directly influenced by the daily cycle of sea-breezes all along the year. Consequently, the rainfall mainly occurs at late night or morning hours while the afternoon hours are significantly much drier. The diurnal cycle is stable all along the rain season apart from the months of March and April where nocturne rainfall are more frequent than others. The rain intensities are also globally similar all day long with values which are comparable to what was observed at afternoon hours on other stations.

<figure><img src="image_21.png" /><figcaption>Fig. 14: Evolution of diurnal cycle and rainfall according to hours at Cotonou (1965-2004)</figcaption></figure>
**References:**

Bokonon-Ganta E. B. (1987) : Les climats de la région du Golfe du Bénin. (Afrique Occidentale). Thèse de doctorat du 3ème cycle. Paris IV, Sorbonne. 248p + Annexes.

Redelsperger J.L. et al. (2006) : AMMA, une étude multidisciplinaire de la Mousson Africaine. La Météorologie, n°54, août 2006, pp. 22-32.

L'Hôte et Mahé, 1996 : Afrique centrale et de l'Ouest : précipitations moyennes annuelles (période 1951-1989). IRD (ORSTOM) Éditions, Bondy, 1 carte.
(D'après 2005, Mestre J. & Chiffaud J. Dernière mise à jour : Décembre 2004)

ARCHIVES :

Hu, Qi (2003) :A Multidecadal Variation in Summer Season Diurnal Rainfall in the Central United States. Journal of climate (J. clim.), 2003, vol. 16, no1, pp. 174-178 [5 page(s) (article)] (12 ref.), American Meteorological Society, Boston, MA, USA.

Abstract:

Recent studies have identified two sources alternating their dominant roles in the interannual summer rainfall variations in the central United States. One is the ENSO cycle in the tropical Pacific, and the other is the interannual variability in the intensity of the southerly flow from the Gulf of Mexico. The ENSO cycle affected the rainfall variation through an atmospheric teleconnection, which was particularly strong in 1871-1916 and 1948-78. When the teleconnection weakened in 1917-47 and 1979-2002, the southerly flow from the Gulf of Mexico invigorated its effect on the interannual rainfall variation. Because the effect of the two sources could result in different rainfall processes in the central United States, their alternation should have built similar variation into the region's summer season diurnal rainfall pattern. An hourly rainfall dataset was used to examine this hypothesis. **Results showed a multidecadal variation in the diurnal rainfall pattern.** In the decades when the southerly flow dominated the rainfall variation, the diurnal pattern had large rainfall in late night/morning hours with a sharp rainfall peak in the midnight hour. In the decades when the southerly flow effect weakened, a different diurnal pattern emerged, with small late night/ morning hour rainfall and a broad plateau of rainfall in the late night/early morning hours. This diurnal pattern change was happening simultaneously with variations of the southerly moisture flux and moisture convergence in the central United States. These coherent variations show that the summer season diurnal rainfall pattern varied at a multidecadal scale consistent with the alternation frequency of the two sources. In addition to showing the variation in the diurnal rainfall pattern, results of this study provide the knowledge for understanding the climate of extreme events, particularly heavy rainfall and floods, in the central United States.

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