Spatio temporal stationarity of the mean rainfall
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SPATIO-TEMPORAL STATIONARITY OF THE MEAN RAINFALL. Armand NZEUKOU University of Dschang Cameroon & Henri SAUVAGEOT University of Toulouse III France. Localization. Tropical latitude with a seaward circulation The Climate of the Dakar area is of sahelian type

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SPATIO-TEMPORAL STATIONARITY OF THE MEAN RAINFALL

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Spatio temporal stationarity of the mean rainfall

SPATIO-TEMPORAL STATIONARITY OF THE MEAN RAINFALL

Armand NZEUKOU

University of Dschang Cameroon

&

Henri SAUVAGEOT

University of Toulouse III France


Localization

Localization

  • 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)


Gauge based mean annual cumulative rainfall for senegal

Gauge-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

Computed over 39 years (1951-1989) by L’Hote and Mahé (1996). The dots are the synoptic observational station managed by national meteorological staff

It is possible to observe sea-land differences in the distribution of the rain field characteristics ?


Studied area and radar dataset

Studied 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 l’Atmosphère Siméon Fongang of the universitiy of Dakar, using a « SANAGA » acquiring systemdevelop 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


Parameters to characterize precipitation

Parameters to characterize precipitation

  • Cumulative rainfall

  • Rainfall duration

  • Average rain rate

  • Standad deviation of rain rate

  • Variation coefficient


Distribution of the annual mean cumulative rainfall h

Distribution 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


Distribution of the annual mean rain duration t

Distribution 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


Probability density function of the rain rate observed p r

Probability density function of the rain rate observed P(R)

The north and south P(R) curves coincide almost exactly despite the strong gradient of the cumulative rainfall

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 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


Distribution of the time average rain rate r

Distribution 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


Distribution of the standard deviation r of rain rate

Distribution 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


Distribution of the variation coefficient cv r r

Distribution 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)


Conclusion

CONCLUSION

  • 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


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