Title: SPATIOTEMPORAL STATIONARITY OF THE MEAN RAINFALL
1SPATIO-TEMPORAL STATIONARITY OF THE MEAN RAINFALL
- Armand NZEUKOU
- University of Dschang Cameroon
-
- Henri SAUVAGEOT
- University of Toulouse III France
2Localization
- Tropical latitude with a seaward circulation
- The Climate of the Dakar area is of sahelian type
- The rainy season is reduced to about 3 months,
from early July to late September - Most rainfalls become weaker and then disappear
when crossing the coast and moving over the
nearby ocean - A few systems grow stronger, advance over the
sea, and seem eventually able to play a role in
the genesis of the hurricanes of the west
tropical Atlantic (Gray and Lansea, 1992)
3Gauge-based mean annual cumulative rainfall for
Senegal
- The mean annual cumulative rainfall displays a
strong meridional gradient, from 300, at the
latitude of Saint Louis, to 1500 mm at Cap
Skirring, which is 400 km away - As in most similar rain field representations,
isohyets end at the coast. Thus the rain field
characteristics over the sea are poorly
documented - In this work, we are to describe and to discuss
the characteristics of the rainfall distribution
in coastal area, which offer an opportunity to
observe the land sea contrast
It is possible to observe sea-land differences in
the distribution of the rain field
characteristics ?
Computed over 39 years (1951-1989) by LHote and
Mahé (1996). The dots are the synoptic
observational station managed by national
meteorological staff
4Studied area and radar dataset
- Land and sea areas are colored
- North and south areas are half-annular areas
located north and south of the latitude of the
radar between 60 and 180 km - Dataset
- The data acquisition was performed by the staff
of the Laboratoire de Physique de lAtmosphère
Siméon Fongang of the universitiy of Dakar, using
a SANAGA acquiring system develop by
Sauvageot. - Period of observation
- 7 years (1993 à 1999)
- sampling interval
- between 10 and 20 min
- number of scans
- 7407
Location and shape of the areas used to computed
the averaged parameters
5Parameters to characterize precipitation
- Cumulative rainfall
- Rainfall duration
- Average rain rate
- Standad deviation of rain rate
- Variation coefficient
6Distribution of the annual mean cumulative
rainfall ( H )
- The surface echoes area is very asymmetrical and
is mainly over the cape of Dakar. - Screening effects are observed for azimuths 225
and 240
Area average of the cumulative rainfall (H)
- Very strong sea-land and north-south gradient.
- The differences are 112 and 69 respectively
The scale is in millimeter
7Distribution of the annual mean rain duration
( T )
Area average of the rain duration (T)
- Very strong sea-land and north-south gradient.
- The differences are 97 and 72 respectively
- The T variation is almost the same as the H
- That suggests that the rain rate can be
considered constant in the area average
The scale is in days
8Probability density function of the rain rate
observed P(R)
For all the R values higher than the mode of
P(R), the frequency is lower over sea than over
land. That suggest a convection slightly less
vigorous over sea than over land
The north and south P(R) curves coincide almost
exactly despite the strong gradient of the
cumulative rainfall
The shape of P(R) is compatible with a lognormal
distribution which is defined by two parameters,
namely, the average (?R) and the variance (?R) of
the rain rate
9Distribution of the time-average rain rate ?R
Area average of the rain rate (?R)
- The ?R distribution is mostly homogeneous except
for the northwestern quater plan beyond 100 km.
- The differences between sea-land and north-south
are 11 and 3, respectively
- The ?R is almost constant for the whole observed
area, with sligthly lower values over the sea
The scale is in millimeter per hour
10Distribution of the standard deviation ?R of
rain rate
Area average of the standard deviation (?R)
- The ?R distribution is very homogeneous except
for the northwestern quater plan beyond 100 km.
- The ?R is very constant for the whole observed
area
The scale is in millimeter per hour
11Distribution of the variation coefficient
CV?R/?R
Area average of the variation coefficient (CV)
- The CV distribution is very homogeneous
- The mean value is 2.27 and very close to the
value proposed by Sauvageot (2.24) over large
space and time samples.
- Only the knowledge of the mean rain rate enables
the definition of ?R and P(R). It shows that the
observed rain rate fields are approximately
spatio-temporal stationary (or ergodic)
i.e., ?R and ?R do not differ when computed over
different data samples (e.g., Bendat and Piersol)
12CONCLUSION
- The rain volume or cumulative rainfall is higher
over land than over sea by 112
- The rain duration is longer over land than over
sea by 97
- The probability density distribution of the rain
rate is well represented by a lognormal function,
which is determined by two parameters, the mean
?R and the standard deviation ?R
- The stability of ?R and ?R through rain fields
implies the same stability for the probability
density function of R or P(R).
- The rain field studied is approximately
spatio-temporal stationary or ergodic and
justifies the validity of P(R) as a significant
rain field characteristic