| dc.description.abstract | This study was undertaken to establish the geographical
and seasonal distribution of the diurnal variation of precipitation
in East Africa. An analysis of the diurnal variation of
precipitation at about fifty different stations allover East
Africa for the months of January, April, July and October
revealed that there are seven main rainfall regimes occurring
in areas with similar orographic features that ~each their
maximum intensity at different times of the day. Each of the
seven groups displays its own form of diurnal oscillation, the
highest amplitude occurs over the highlands which get as much as
40% of the day's rainfall in the late afternoon, between 1600
and 1900 hours local time and the lowest is over the semi-arid
areas of Kenya and Tanzania which have one weak maximum in the
afternoon and another one during the night. Three and six
hourly rainfall amounts have been represented on maps along with
orography. This gives us a clear picture of the influence of
orography on precipitation intensity and its time of occurrence over East Africa. Analysis has also been done on low level
cloudiness at 0900 and 1500hours and the afternoon induced
cloudiness.
The seasonal variation of the diurnal variation of
precipitation and afternoon induced cloudiness were studied by
looking at the month's departure from the four month mean.
Three areas, lake basin and its east and west facing slopes;
the north east highlands of Tanzania and the coastal areas showed
large seasonal variations in the diurnal variation of precipitation.
The same was true of the afternoon included cloudiness
except for the lake basin and its east and west facing slopes.
To explain the above observed variations of the
diurnal and seasonal variations of the diurnal variation of
precipitation and low level cloudiness, analyses were carried
out on pilot balloon data over East Africa for the years 1952-
1972 using streamline-isotach method at five levels, 825;780;
720;620 and 530 robs for the four months mentioned above. Using
these charts, divergence was calculated for the morning period,
averaged at 0900 hours, and for the afternoon averaged at 1500
hours EA~T. With the incorporation of the orographically
induced vertical motion at the lower boundary, the equation of
continuity was integrated to calculate the vertical motion at
the intermediate le7els up to 500 robs. To get a clear picture
of the interaction between the mesoscale circulations and the
synoptic scale flow graphs of the East-West and North-South
surface wind 24-hour acceleration!:! were drawn for Entebbe,
Kisumu, Nairobi and Mombasa. Charts are also presented for the
afternoon induced wind vectors for each of the four months.
The calculated divergence field and resulting
vertical motion explain the observed diurnal variations of
precipitation and low level cloudiness and their seasonal
variations. Further explanations are given by the E-W and N-S
accelerations at the four stations, the afternoon induced wind
vectors and also the afternoon induced vertical motion which
shows strong upward motion centred over the highlands becoming
weaker where the afternoon weather is moderate or light | en |
