Introduction
Precipitation
is one of the most important meteorological elements which determines
vegetation zonation of the tropical Andes (Lauer, 1981; Richter,
2003). Spatio-temporal information on rainfall amount and dynamics in
these areas is rather poor for several reasons (e.g. Barry, 1992).
Rain gauge data are only sparsely distributed and frequently located
in drier valleys and are hence less representative for the whole
mountainous area. Furthermore, measurements with rain gauges reveal
specific errors especially in windy environments such as the tropical
high mountains (Sevruk 1986). Numerical models are currently not yet
reliable enough for representing rainfall dynamics in a high
spatio-temporal resolution as is required for climate ecological
studies in tropical high mountains due to several restrictions
(Golding, 2000; Mecklenburg et al., 2000). Major progress in
the retrieval of precipitation from satellite data (optical and
microwave sensors) has been achieved over the last few years (e.g.
Bendix, 2000; Reudenbach et al., 2001; Bendix et al., 2003;
Levizzani, 2003). However, the spatial/temporal resolution of
rainfall data as provided from the geostationary/polar orbits does
not match the requirements of limited-area ecological research.
In some countries, operational ground
based weather radar networks mostly operating in the C-band offer
great opportunities to observe precipitation dynamics at a high
spatio-temporal resolution. A spatial resolution which is more
appropriate for climate-ecological studies can be provided by X-band
radar technology. Unfortunately, neither operational radar networks
nor local radar sites are available in most countries such as
Ecuador. Experimental cost-effective weather radar systems which are
based on ship radar technology have recently become available and
seem to be an appropriate tool for climate-ecological studies (Jensen
2002, Rollenbeck & Bendix 2005b).
Another important factor contributing
to the hydrological balance is scavenging of fog and cloudwater. Many
efforts have been made to detect this form of water input
(Walmsley&Schemenauer 1996, Bruijnzeel 2000), but no general
methodology was developed and results of different locations and
studies are hardly comparable. It seems probable that only combined
methods of landscape modeling and in-situ measurement are capable to
quantify this atmospheric contribution to the water balance.
Study area and data
The
study area is located in southern Ecuador covering an altitudinal
range between 800 and 3,600 m asl (Fig. 1). The region covered by the
radar comprises the humid eastern Andean slopes, which are
characterised by tropical rain forests up to ~1,800 m asl, and
tropical evergreen cloud forests at altitudes >1,800 m asl, as
well as the relatively dry basins and valleys west of the main
Cordillera Oriental de los Andes, which are partly covered by
xerotropical vegetation (Richter 2003). Spatio-temporal
rainfall dynamics are analysed by using weather radar images of an
X-band local area weather radar (LAWR) setup as part of the project.
The radar site is located within the
Reserva Biósfera de San Francisco, the central investigation
area of the joint research group. At the research station Estacíon
Científica San Francisco (ECSF, 3°58' S / 79°04'W)
data are compiled and archived (Fig. 1, right, refer also to Beck &
Müller-Hohenstein, 2001).
Fig. 1: Study area and location of the Reserva Biológica San Francisco
The LAWR is a low-cost
alternative to professional weather radar systems. It provides the
typical range of X-band systems (60 km radius) but lacks some of the
more sophisticated capabilities of operational systems, as e.g.
vertical scans or Doppler technology (Rollenbeck et al. 2005b). As
far as we know, it is the first weather radar installed in the Andes
and the first to be used in eco-climatological studies. The system is
installed on the highest peak of the study area, the Cerro de
Consuelo at 3200 m asl. This site guarantees the least obstructed
view in the highly mountainous region. The range extends far over the
central study area, which provides insight into larger scale rain
formation processes and thus helps to understand the climatological
and orographic influence of mountains on rainfall distribution in
general. Images are generated every 5 minutes and have a resolution
of 500x500 m. Hence, the central study area is covered by about 50
radar pixels, which is a great advantage as compared to the three
point observations from the existing climatic stations (Fig. 2).
However, calibration of the LAWR data strongly relies on the existing
station network that was supplemented by 6 additional totalling
raingauges along the altitudinal gradient.
Fog water input is
determined by six quadratic polypropylene mesh collectors which are
installed along the altitudinal gradient as well. The mesh collectors
were chosen due to their simple setup and are exposed to the
prevailing easterly wind direction (frequency of winds from the
easterly sector ~90 %) (Rollenbeck et al. 2005a). To evaluate the
characteristics of these manual collectors, a central location (1960
m asl) in the study area, close to the main research station, was
chosen to install a set of more sophisticated automatic instruments,
providing data on clouds and precipitation with a temporal resolution
of five minutes: (a) A BIRAL VPF-730 scatterometer measures
atmospheric extinction and horizontal visibility, rain rate, particle
number, droplet spectra and rain type. (b) Fog water is sampled by an
automatic impact collector (Eigenbrodt NES210 coupled to fog detector
ONED 250). An automatic rain water sampler collects samples of wet
deposition, determines rain rate and measures the values of pH and
electric conductivity online. Sample bottles are automatically
changed every day. Operation time is recorded on a standard PC in
order to calculate average liquid water content for the sample
intervals. (c) An automatic climate station provides hourly data of
temperature, relative humidity, windspeed and –direction and
global radiation (Fig. 2).
Fig. 2: Experimental setup along the altitudinal gradient
The measurements inside
the central research area are extended by additional stations in a
wider environment: 10 km to the west, the site "El Tiro"
is equipped with another fog collector, a rain gauge and an automatic
climate station. Five automatic weather stations and five official
principal stations of the Ecuadorian weather service (INAMHI) operate
in the range of the weather radar LAWR. Data are used for calibration
of the measured reflectivities (conversion to rain rate).
Weather
satellite data for the entire area of Ecuador are obtained by the
project-owned HRPT (High resolution picture transmission format)
receiving station which is operated in cooperation with INAMHI at the
headquarters in Quito (Bendix et al. 2003). On average two
daily passes of the NOAA satellite series (AVHRR imagery) are
captured and transferred via internet to Marburg (Germany) for
further analysis.
In a
field campaign from December 2001 to April 2003, additional
measurements of vertical rainfall profiles were performed by using a
vertical K-Band Doppler rain radar (METEK MRR-2, Klugmann et al.
1996). These measurements were generally made at the ECSF station
building in the Rio San Francisco valley at 1850 m asl., with
supplementing mobile measurements being made in the range of the LAWR
radar (up to 45 km distance).
To ensure continous
observation of the synoptical situation in the San Francisco valley,
a simple digital weather cam was installed at the ECSF station
building. An image is taken automatically every five minutes. These
data allow monitoring and detection of the cloud base altitude (local
condensation level) in the neighborhood of the ECSF research area.
Results
Precipitation distribution in time and
space
The region of the study
area, the so called "Nudo de Loja" (knot of Loja, s. Fig.
1) is characterised by an extreme spatial variability of climate.
Whereas the town of Zamora 20 km to the east of the study area
receives >2000 mm of rain per year in 12 humid months, the valley
of Catamayo 70 km further west is fully arid (12 arid months) and
receives only 300 mm rainfall on average (Richter 2003). The huge
climatic contrast is caused by the configuration of the main
cordillera which is not characterised by two well separated chains as
in the rest of Ecuador and major parts of Peru and Colombia, but is
grouped into several mountain chains that form a cascade of barriers
and valley intersections. This causes an intensive mixture of
ecoclimatic conditions and the lowest traverse for air masses
crossing the Andes.
The extreme
climatological variability is well reflected in the radar
measurements and the satellite imagery. The total of all calibrated
radar images (Fig. 3) shows values of up to 6000 mm
Fig. 3: Annual average total of precipitation as measured by the LAWR radar (2002-2004)
mm per year for some
exposed parts of the eastern slopes of the Andes, meanwhile the whole
western half of the image hardly exceedes 600 mm, even at the
mountain tops.
Distribution of cloud
frequency (not shown here; s. Bendix et al. 2003) shows similar
results. The eastern slopes have higher cloud frequencies than the
lowlands of the amazon basin. West of the Loja region a low cloud
frequency reveals arid conditions. The central study area is
characterised by a "tongue" of high cloud frequency,
entering from the east and continuing to north-west.
The annual variability
of rain distribution cannot only be attributed to the prevailing
easterly flow. The season between September and April shows a more
varying distribution of rainfall. These are times when the less
frequent westerly flow causes heavy convective rainfall at the
western slopes of the Cordillera which are normally under
"Föhn"-influence induced by the tradewinds. In
contrast to that, the season from May to September shows an almost
unique pattern of rainfall formation. Very frequent but less heavy
precipitation is responsible for the high totals at the eastern
slopes.
The data from the
vertical pointing MRR-radar show an altitudinal increase in rain
intensity measured at the station building and hence representative
of the San Francisco valley. Rain intensity in the free atmosphere is
increasing by 35 % to the altitude of 3200 m asl or 1400 m above
the measurement point. This value is also supported by the
registering rain gauges and fog collectors (s. below). The average
annual total for the research area was calculated to be 2050 mm
(period 1998-2004) at an altitude of 1960 m, increasing to ~4400 mm
at the Cerro de Consuelo (3200 m). This corresponds to a vertical
gradient in atmospheric water input of ~150 mm/100 m. Actually this
gradient is not linear: up to 2600 m rain fall increases by 250
mm/100m, then the gradient decreases to 100 mm/100m. For fog input
this gradient is 40 mm/100m up to 2600 m, then increases sharply
to 180 mm/100m (s. Fig. 4).
Fig. 4: Height/time section of monthly totals of rain and fog/horizontal rain in the central research area.
The nonlinearity of the
gradient coincides well with results of personal observations and
synoptical data from the weather cam images. Generally, condensation
level and highest cloud frequency is observed between 2500 m and
3400 m. Fog frequency for these altitudes is about 80 % (3200m).
The relative numbers
show, that with increasing altitude fog water (and horizontal rain)
plays a more important role for the total of atmospheric water input.
An areal sum of the total precipitation for the study area is in the
range of 3600 mm per year, which is 80 % higher than the conventional
measurements of the primary climate station at 1960 m indicate.
Dynamic processes
The data of the LAWR
radar, the vertical K-Band radar and satellite imagery were combined
to interprete the temporal and spatial distribution of rain
generating processes. Besides the well known phenomena of exposition,
which controls the formation of orographic clouds and rain, diurnal
oscillations were detected that indicate an interaction between the
andean mountain chain and the amazon basin east of the cordillera.
Untypical for tropical environments, the daily precipitation maximum
at the ECSF station occurs in the early morning hours, between 4:00
and 6:00 local time (LT).
By analysing several
case examples, the formation of mesoscale cloud systems (MCS) could
be observed in the satellite images (NOAA-AVHRR), that showed low
cloud top temperatures which may indicate deep convection cells.
These MCS typically develop around midnight south-east of the study
region (s. Fig. 5) and start to travel slowly with the trade winds in
westerly directions. In the morning hours these cloud complexes are
exposed to orographic uplifting at the eastern escarpment of the
andean chain and produce stratiform rain fall, which can be seen in
the radar images as extended rain fields covering the whole center of
the observed range (Fig. 6).
Fig. 5: Fully developed mesoscale cloud system at 8:00 LT reaching from about 150 km distance on to the study region (22.07.2002)
Fig. 6: Radar image of rain distribution 22.07.2002, 6:30 LT showing the northwestern margin of the developing MCS shown in Fig.5
A possible
interpretation of this process is based upon the fact, that
local/regional katabatic winds occur every night and transport masses
of cold air into the amazon basin. These airflows are channelized by
the larger valleys of Rio Marañon and Rio Zamora to the
foreland, were they meet with the warm and moist air of the easterly
tradewinds. Convergence and the temperature difference are able to
cause the formation of local cold fronts with the associated deep
convection, which consequently produces rain fall. These systems
decay during the course of the night, but the remaining clouds often
reach the higher chains of the Andes and the study region, where they
cause siginificant amounts of rain in the morning hours.
Further local and
regional dynamic processes are involved with the development of
westwind situations, which sometimes produce convective showers, but
sometimes also cause total rain free situations. Further work has to
be done to analyse the underlying processes, but certainly analysis
of wind fields, derived from tracking radar-observed rain storms will
help to understand the dynamics of the rain formation in the region
of the Nudo de Loja and in the andean mountain chain in general.
Conclusions
The expected altitudinal
gradient of precipitation input for tropical montane regions can be
confirmed by results of this study. The additional fog input shows
that significant amounts of the water balance have been neglected in
the past. Further investigations will focus on structural parameters
of the vegetation cover, thereby enabling the setup of a fog
scavenging model to convert atmospheric fog water content to actual
fog water input. Intensive cooperation with other groups will be
necessary.
Climatological variables
like condensation levels, fog and cloud frequency and microphysical
precipitation characteristics are used to further analyse rain
generating processes. Advective and convective cloud processes
directly affect precipitation intensity and frequency.
To unify the research
effort in the Reserva Biologica San Francisco, a nested multi-scale
model will be developed that can enable the simulation of the most
important interactions between atmosphere, landcover and soil in a
quantitative and impacts of potential land use changes and climate
variability.
Acknowledgements
The Project PREDICT is kindly funded
(FOR 402/1, BE 1780/5-1 /5-2) by the German Research Society (DFG).
We also acknowledge the invaluable support of NCI Ecuador, E.
Palacios (INAMHI) for the operation of the NOAA-AVHRR receiving
station and M. Richter for supplying rain-gauge data.
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