versão On-line ISSN 0717-6538
Gayana (Concepc.) v.68 n.2 supl.TIProc Concepción 2004
Gayana 68(2) supl. t.I. Proc. : 83-90, 2004 ISSN 0717-652X
DYNAMICS OF GENERATION OF EXTREME PRECIPITATION IN THE NORTHERN OF PERU
Mauricio Carrillo & Juan Bazo
This article studies the influential factors in the occurrence of extreme rainfall in the northern Peru. The favorable conditions are analyzed under the following hypothesis: warming up of the sea surface temperature (SST), the southward movement of the Inter Tropical Convergence Zone (ITCZ), and the advection of humidity transferred from the east part of the Andes.
Extreme rainfall has been periodically observed, it occurs during El Niño and non El Niño events. The main goal is to quantify the critical threshold for this occurrence. These limits will be associated to the teleconnections of the SST, ITCZ and winds.
The generation of a warm "nucleus" of SST facilitates the creation of convergence "nucleus" in low levels due to positive temperature advection. It may weaken the thermal inversion and raise the atmospheric instability.
To explain the dynamics of generation of extreme precipitation, the relationship of these three important factors: SST, ITCZ and winds, is analyzed using MODIS, GOES, and simulation of winds with the Regional Atmospheric Modeling System (RAMS). The results of this study will help to alert extreme rainfall events.
Extreme rainfall in the north of Peru occur during El Niño years, which causes huge economic loss and social impacts; making this area of Peru highly vulnerable to this events. However, cases of extreme rainfall also occur during not El Niño years, so this will be the main subject of this paper.
The dynamics of rainfall in the equatorial west part of South America has been studied by several researches; from all of them, the explanation by Horel and Cornejo-Garrido (1986) presents as main mechanism of precipitation the system of sea-land breezes. Also Bendix (2000) makes a genesis of the sea-land breezes based on AHVRR images, adding humidity flow transported from the Amazon through the Andes, which enhance the amount of humidity in the west part of the Andes.
During the period February-March 2002, two events of extreme rainfall were identified in the north of Perú.
The analysis was carried out with precipitation data by the National Service of Meteorology and Hydrology (SENAMHI) of Peru.
As is shown in figure 1 and 2 extreme rainfall occurs in 4-5 February and 18-19 March. To analyze the temporal variation of the events at bigger range of time, rainfall at different stations are averaged and for this grouping a lower, middle and higher time series is shown in figure 3.
To analyze the behavior of the SST near the northern coast of Peru, images of the MODIS-TERRA Level 3 mapped 4 KM daily and weekly mean products were acquired from the Goddard Space Flight Center (GSFC) and Distributed Active Archive Center (DAAC).
The data of the Sea Surface Temperature (SST), daily and nightly time, has been worked on a daily and weekly composition base to have an image weekly as well as a composition from 5 days.
The first case of study was carried out from a composition of daily data from the MODIS SST, for the first five days of February 2002. The area represented by these images is: from the northern coast of Perú to 25 miles off shore. As it is shown in figure 4, where the SST presents average values between 18° and 20°C. This range is lower than 26°C, threshold for the generation of rainfall according to Woodman (1999).
Figure 1: Measure event of event one. The numbers in the maps represent rainfall for February 4 and 5. The shaded color is the topography of the region.
Figure 2: Measure event of event two. The numbers in the maps represent rainfall for February 4 and 5. The shaded color is the topography of the region.
Figure 3. Average rainfall for February and March of 2002. The average is for a group of stations, which are selected from the lower, medium and high basin. That are related to the high. As it is shown higher rainfall occur in the high basin.
Figure 4. SST for case one.
Furthermore, the weekly image MODIS SST, for the period between 2-9 February, reports SST of 24 C, which are below the threshold for the generation of deep convective activity. Waliser and Graham (1993) propose a value of 29.5°C as threshold of SST for outbreak of deep convection.
The second case of study, a composition of daily and weekly data has been made from MODIS SST for the period 15-19 March. For these days the values of SST are between 25°C to 27°C, as it is shown in figure 5. These values could facilitate the generation of convective activity in the area of study.
Figure 5. SST for case two.
Finally, the composition of images from MOSDIS SST of weekly data, for the days between 14-21 March, confirms that in areas near the shoreline, the average of SST is 26°C, while for a 100 miles far off shoreline the SST ended up reaching 28.5°C.
In this investigation we analyzed only two cases of extreme rainfall, and winds are used to support the explanation of the dynamics of the rainfall in the northern of Peru, It will be shown further ahead that this is a very important component in the system.
To have a detailed representation of the wind fields, a numerical simulation is the way to have a full representation of this field on different levels of the atmosphere. A regional model was used, and its ability to simulate well rainfall is discussed ahead.
The model used is the Regional Atmospheric Modeling System (RAMS), developed initially by Colorado State University. In this experiment RAMS is initialized with data from the Reanalysis Project (National Center for Atmospheric Research NCAR). Moreover, the coupling between the ocean and atmosphere has two methods: First, the oceanic component is fed with data estimated from satellite (AVHRR) and the atmospheric components (winds, temperature, geopotential height and humidity) are feed with data from Reanalysis. Second, the same components of the atmospheric field and the oceanic components are feed from climatological data from AVHRR sensor during period 1992-1993.
The simulation has two domains; the first with a resolution of 80 km with an horizontal grid of 47x47 for Peru and the other grid with a resolution of 20 km and a horizontal grid 50x35, figure 6. In both domains the number of vertical levels is 30, with a vertical grid ratio of 1.15, so in the land-atmosphere interface the density of levels is bigger than in the top of the vertical level.
Figure 6. Coarse and nested domain in the used simulation. The shaded color represent the domain for Piura which has a 20 Km resolution, the biggest domain has a 80 Km resolution.
The atmospheric conditions to RAMS are from the Reanalysis Project data set. Thus, the boundary conditions are provided every six hours. The land surface conditions were standard of RAMS. An analysis about variations of the atmospheric conditions due to the initial state of the land surface conditions has not been analyzed, even when the potential temperature in surface is one precursor (Horel and Cornejo-Garrido, 1986) for the outbreak of rainfall. It is expected that the initialization lags previous to the atmospheric instability cause these conditions to balance and make them less critical in the analysis. Thus, global conditions should quickly upgrade these conditions to the regional model.
Two cases of extreme rainfall, occurred between February and March of 2002 (not El Niño year) are analyzed. These cases are used to explain the dynamics of extreme rains in the northern of Peru. The maximum rainfall occurs in the high lands of the Andes on February 5th and March 19th of 2002.
The sea-land breezes originated by the thermal gradient between ocean and continent is an important factor in the generation of rainfall, because this process generates the migration of humidity from the ocean towards continent; humidity that could enhance the occurrence of convective processes, for that reason the current condition of SST are important.
In analyzing this case we found for the event in February, that the sea surface temperature near shoreline of Piura, does not reach the critical thresholds to generate convective activity.
The sea surface temperature near shoreline reached maximum values of 27°C in March. However, far from the coast it has registered temperatures of 28,5°C, which would allow to generate convective activity in the area of study. It is important to indicate that the value of temperature is not bigger than the suggested by Woodman (1999), but we propose that rather important than high temperature in the ocean is the sea-land thermal gradient.
The RAMS simulation coherently represents the daily cycles of sea-land and land-sea breeze. In the cases for the days 4 and 5 of February (fig. 7), and of 18 and 19 of March 2002, an intensified effect is observed for the breeze sea-land for this period and a contrary effect to the complementary period.
The statement of Horel and Cornejo-Garrido (1986), is true in these two cases, but lower convection is not a necessary and sufficient condition to the occurrence of extreme rainfall. We propose that other additional condition may happen.
The hypothesis is to suppose that the necessary, complementary mechanism to the breezes, for the generation of rainfall is vorticity in high levels. If it is considered the research originally development by Ekman on boundary layer and developed later by Cushman-Roisin, it is claimed that a geostrofic flow limited in the bottom for an inhomogeneous terrain, the vertical speed in the Ekman's layer is defined for:
Figure 7. Sea-land and land-sea breezes according for RAMS model.
Where: b = b(x, y) is the lands irregularity.
are the speed far from the layer of Ekman. The first term is the influence of the terrain and the second term is the circulation in the high levels, the addition of both indicates the total field of vertical speed ascent of the fluid, we want to note that this equation is valid for a geostrophic flow, where only effect of rotation and viscosity are included. This is a simple model but useful to explain our hypothesis.
Figure 8 shows the vorticity simulated by RAMS in the vertical column of the atmosphere for days February 1 to 7. These graphics show an intense nucleus of positive vorticity during 4th and 5th February, which coincides with the days of extreme rainfall. As it is shown vorticity is transformed in ascent of air masses in low level, which is a necessary factor to the occurrence of rainfall. The others days, negative values of vorticity were observed, which coincides with the period of rainfall did not occur. This value of negative vorticity will generate subsidence which not allows the formation of deep convection.
Figure 8. Vorticity in a vertical column of the atmosphere, from 1 to 7 of February. a), b) and c) are set in longitude -80, and latitude varies from -4 to -6. d), e) and f) are set in longitude -76, and latitude varies from -4 to -6.
The main factor in the generation of atmospheric instability besides the about mentioned by other authors (potential temperature, sea surface temperature, breezes due to the daily cycle) is vorticity in high levels. This variation of vorticity in high levels coincides with precipitation changes. The others parameters of the atmosphere system remain quite invariable during the simulation period, so vorticity makes the role of outbreak to rainfall.
The effect of sea-land breezes is not necessary and enough condition to cause the outbreak of the rainfall, because by itself it only implies a migration of air mass to lower levels to enhanced for humidity from ocean. We should notice that the upwelling effects in a fluid limited in the boundary this governed by the dynamics explained initially by Ekman, where it is explained that the vertical elevation of air mass in the Ekman's layer is a consequence of the vertical component of vorticity in high atmosphere. This migration of vertical fluid is small compared to the convergence in low levels, but it is enough to start the feedback mechanism for the generation of extreme rainfall, and the sea-land breeze contributes to this system as a mechanism to accumulate humidity, and transform it in a thermodynamic feedback process.
Images GOES-IR for those mentioned cases show the advection of moist air in middle levels from the Amazon to the west Andes. The images are not shown, but we want to indicate that this advection and the sea-land breezes became the studied area in a system of thermal instability. We did not test cases with advection and without presence of strong gradients of sea-land breezes that generate strong instability. We think that in this case the system is stable, so slow vertical displacement of fluid will not be important in considering this rainfall dynamics.
The RAMS simulations represent well the rainfall observed in the area of interest which can be verified in figures 9. Additionally, figure 10 present a serial time data of rainfall for the low, middle and high basin, with the respective lags. Thus, it can be observed how in the different beginnings RAMS can represent the rainfall for the cases of interest. This shows that the atmospheric circulation from RAMS in those cases validate the mentioned conclusions. But it is necessary understand that excess of rainfall simulated by RAMS, in the coastal area of Piura, should not be taken as a negative indicator of its performance to estimate the rainfall in the region. The important thing is that simulation with RAMS can simulate the outbreak of the atmospheric instability.
Figure 9. Rainfall simulated for RAMS model. Pictures from a) to f) are the precipitation from 1st until 6th February. Look at how the extreme precipitation appears in the day 4 and 5.
a. The mechanisms mentioned by Horel and Cornejo-Garrido (1986) , doesn't fully explain the intense rainfall in the equatorial west part of South America.
b The sea breezes are necessary mechanisms to formation of deep convective activity. But they don't imply the presence of extreme rainfall.
c. The values of more at 27°C SST (Woodman) or 29,5°C (Graham) are important to generate deep convection. We claim that it is more important the thermal gradient between the ocean and continent, that generates bigger migration of fluid from sea to land.
d. In the studied cases the decisive factor for the occurrence of extreme rainfall is the vorticity in high levels, it is a necessary condition.ACKNOWLEDGEMENTS
We thank to the National Service of Meteorology and Hydrology of Peru (SENAMHI), for the support in this research; and the facility to use their data set, as well as their computer infrastructure. Also an special thank to LABTEL-UNMSM of the National University San Marcos for their advice in topics about remote sensing. Finally, to MODIS-TERRA daily and weekly products from Goddard Space Flight Center (GSFC), and Distributed Active Archive Center (DACC).REFERENCES
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