HESSHydrology and Earth System SciencesHESSHydrol. Earth Syst. Sci.1607-7938Copernicus PublicationsGöttingen, Germany10.5194/hess-21-5165-2017Convective rainfall in a dry climate: relations with synoptic systems and flash-flood generation in the Dead Sea regionBelachsenIditidit.belachsen@mail.huji.ac.ilMarraFrancescohttps://orcid.org/0000-0003-0573-9202PelegNadavMorinEfratefrat.morin@mail.huji.ac.ilhttps://orcid.org/0000-0001-6671-7926Hydrology and Water Resources Program, Hebrew University of Jerusalem, 91904, IsraelInstitute of Earth Sciences, Hebrew University of Jerusalem, 91904, IsraelInstitute of Environmental Engineering, Hydrology and Water Resources Management, ETH Zurich, SwitzerlandIdit Belachsen (idit.belachsen@mail.huji.ac.il) and Efrat Morin (efrat.morin@mail.huji.ac.il)12October201721105165518019April20172May201710August201718August2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://hess.copernicus.org/articles/21/5165/2017/hess-21-5165-2017.htmlThe full text article is available as a PDF file from https://hess.copernicus.org/articles/21/5165/2017/hess-21-5165-2017.pdf
Spatiotemporal patterns of rainfall are important characteristics that influence runoff generation and
flash-flood magnitude and require high-resolution measurements to be
adequately represented. This need is further emphasized in arid climates,
where rainfall is scarce and highly variable. In this study, 24 years of
corrected and gauge-adjusted radar rainfall estimates are used to (i)
identify the spatial structure and dynamics of convective rain cells in a dry
climate region in the Eastern Mediterranean, (ii) to determine their
climatology, and (iii) to understand their relation with the governing
synoptic systems and with flash-flood generation. Rain cells are extracted
using a segmentation method and a tracking algorithm, and are clustered into
three synoptic patterns according to atmospheric variables from the
ERA-Interim reanalysis. On average, the cells are about 90 km2 in size, move 13 m s-1 from west to
east, and live for 18 min. The Cyprus low accounts for 30 % of the
events, the low to the east of the study region for 44 %, and the Active
Red Sea Trough for 26 %. The Active Red Sea Trough produces shorter rain
events composed of rain cells with higher rain intensities, longer lifetime,
smaller area, and lower velocities. The area of rain cells is positively
correlated with topographic height. The number of cells is negatively
correlated with the distance from the shoreline. Rain-cell intensity is
negatively correlated with mean annual precipitation. Flash-flood-related
events are dominated by rain cells of large size, low velocity, and long
lifetime that move downstream with the main axis of the catchments. These
results can be further used for stochastic simulations of convective rain
storms and serve as input for hydrological models and for flash-flood
nowcasting systems.
Introduction
A flash flood is a rapid runoff response of a catchment to intense
precipitation. Owing to their short response time and high intensity,
flash floods are difficult to predict and result in economic damages and
casualties . In fact, they are among the most dangerous
meteorological hazards affecting the Mediterranean countries
. Many factors contribute to flash-flood
generation, such as rainfall conditions (e.g., amount, intensity, and spatial
and temporal distribution), catchment morphological properties (e.g., slope
and surface cover), and hydrological preconditions (e.g., soil saturation).
The magnitude of a flash flood is determined by the interactions between
these factors .
Map of the Eastern Mediterranean area presenting radar location and
coordinates used to derive sea level pressure (SLP) differences for synoptic
classification (a). A map of the study area with the Darga and Teqoa
catchments (b). Isohyets over the study region represent long-term
(30 years, 1980–2010) mean annual rainfall (mm).
In particular, rainfall spatial and temporal variability is a key factor in
runoff response prediction . found
great sensitivity in flash-flood generation and magnitude to the intra-storm
rain intensity distribution. and
reported that neglecting rainfall spatial variability
resulted in a considerable degradation of the modeling results.
observed high sensitivity in the response of an arid
catchment to location, direction, and velocity of the convective storm.
The need to account for rainfall variability is accentuated in arid and
semi-arid regions, where rainfall is often of a convective nature and
characterized by extremely variable, high-intensity, short-duration events
(; ;
). Although antecedent soil-moisture conditions are known
to play a role in runoff generation, studies conducted in semi-arid and arid
areas ascribed them only a minor influence on flood response, due to low
infiltration capacities of the ground, high evaporation rates
, and long dry spells between rainfall
events . Hence, high-resolutions in space and time, and
over large areas, are required to adequately represent rainfall
spatio-temporal distributions. These can be best achieved by remote sensing
tools such as weather radars e.g.,.
The spatial distribution of rainfall in convective environments is often
examined by focusing on the properties of the convective rain cells (abbreviated
hereafter as rain cells), which can be directly derived by exploiting the full
three-dimensional structure of the cells or, more commonly, extracting the convective two-dimensional
segments from radar data . A widely used approach
requiring only two-dimensional information is to define them as areas in
which the rain intensity exceeds a certain threshold. This simplified
representation of the rain field allows focusing on the high flash-flood
generating potential portion of the storm, and is used in synthetic rainfall
generators and hydrological models e.g.,.
Morphological and hydrological characteristics of the Darga and Teqoa
catchments.
Catchment propertyDargaTeqoaArea (km2)73140Height range (m above sea level)-19 to +813-20 to +992Mean channel gradient (–)0.0270.029Mean hillslope gradient (–)0.1140.135Percentage of desert soils (%)4237Maximal observed peak discharge (m3 s-1)*61.2158.5Average number of flow events per year*1.962.13Threshold discharge value (according to Shamir et al., 2013) (m3 s-1)0.252
* Data record of hydrological years 1990/1991–2014/2015.
Rain cells can be represented by fitting an ellipse around the local rain maxima
in a radar image and geometrical
properties of the cells such as area, axes length, orientation angle, and
maximal intensity can then be derived. Some studies accounted for rain-cell dynamics
by monitoring their progress over time with tracking algorithms
e.g.,. This allowed the derivation of additional parameters such as
rain-cell lifetime, velocity, and direction of movement.
The atmospheric conditions generating a rainfall event are expected to
influence the properties of rain cells and, consequently, the
rainfall–catchment interactions and the runoff response. The objective of
this study is to quantify the properties of rain cells originated within
different synoptic systems in an arid climate region and to understand the
rain cell–catchment interactions with the generation of flash floods. Specific
questions motivating this study include the following. (i) What are the property distributions
of convective rain cells in dry environments? (ii) How do they vary between
different synoptic systems? (iii) How do rain-cell characteristics change within
the study region? (iv) What are the cell properties that dominate the formation
and magnitude of flash floods? These questions are examined through a
statistical analysis of rain cells derived from 24 years of weather radar
data over the western tributaries of the Dead Sea and flash flood data from
two catchments within this region (Fig. 1).
The paper is organized as follows. The study area and data are described in
Sect. 2. Section 3 analyzes the relations between the properties of the rain
cells and the governing synoptic system. Section 4 presents the impact of
rain-cell properties on flash-flood generation. The results of the study are
discussed in Sect. 5. Section 6 reports the concluding remarks.
Regional background and data
This study focuses on the western tributaries of the Dead Sea in the Eastern
Mediterranean (EM, Fig. 1) that drain from the Judean Mountains water divide
(600–1000 m a.s.l.) towards the Dead Sea (currently 430 m below sea
level). The study area is of 3315 km2 (∼ 50 km west to east and
∼ 80 km north to south).
Climate
The area is dominated by semi-arid and arid climates except for the
northwestern part that is governed by a Mediterranean climate
. Mean annual precipitation shows a steep gradient from
over 500 mm in the northwestern portion of the area to about 150 mm and
even less than 50 mm in the northeastern and southern parts, respectively
(Fig. 1b). The west-to-east gradient is due to the rain shadow effect caused
by the Judean Mountains and by the low topography of the Dead Sea valley. The
north-to-south gradient is related to the distance from the shoreline and
from the main tracks of Mediterranean storms. Rainfall occurs from October to
May, with no rain during summer . Intensities and
duration of extreme events differ dramatically within the study area, with
the relative frequency of high rainfall intensities increasing as the mean
annual precipitation decreases .
An example of a rain-cell derivation for 3 December 1996, 13:10
(UTC) over the Darga and Teqoa catchments. Radar rainfall for the examined
time (a), a segment of an identified rain cell and its fitted
ellipse (b). The spatial properties of the presented cell are the
following: area, 90 km2; orientation, 51∘; major axis length,
15 km; minor axis length, 9 km; ellipticity, 0.58; max rainfall intensity,
47 mm h-1; and mean rainfall intensity, 20 mm h-1.
Most intense rainfall episodes over the study area, and the EM in general,
are associated with the cold fronts of mid-latitude lows: the Cyprus low – a
Mediterranean low located around Cyprus, and the Syrian low – a
well-developed Mediterranean low located over Syria
. The region is
also affected by more localized convective showers associated with the Active
Red Sea trough (ARST), a surface low-pressure trough extending from eastern
Africa along the Red Sea towards the Middle East in its active phase
. The ARST is more frequent during the transition seasons
and its contribution to rainfall and flash floods in the EM decreases going
north (; ). According to
, the ARST accounts for most of the major floods over the
arid catchments in the south of the study area, followed by the Syrian low.
Some rare events of relatively widespread rainfall leading to flash floods in
the region are associated with the subtropical jet and the conveying of air
moisture of tropical origin over Africa to the Eastern Mediterranean; a
system often referred to as tropical plume
Hydrology
Two side-by-side gauged catchments, located in the northern part of the study
area (Fig. 1b), are chosen for the analysis due to the availability of long
and concurrent records of water discharge and radar data; these are the Darga (73 km2) and
Teqoa (140 km2). These catchments have ephemeral dry channels, and
flash-flood events occur on average twice a year (Table 1). The surface is
characterized by large areas of bare rock, shallow soils of low permeability,
and sparse vegetation.
Water discharge data were obtained from the Israel Hydrological Service for the 24
hydrological years (October to September) 1990/1991–2013/2014. In order to
avoid analysis of low flows, events with peak discharges lower than
0.25 m3 s-1 for Darga and 2 m3 s-1 for Teqoa were
excluded from the analysis. These thresholds were based on
who developed a method for determining minimal flash-flood thresholds for
arid regions using geomorphic indexes.
Weather radar
Rainfall data used for the research are based on the Shacham weather radar, a
C-band non-Doppler instrument, located within the Ben-Gurion international
airport (Israel), 50–125 km northwest of the study area (Fig. 1). The
observation geometry of the radar is characterized by a spatial polar
resolution of 1.4∘× 1 km and a temporal resolution of
about 5 min per volume scan. Its archived data record is of 24 hydrological
years 1990/1991–2013/2014. Such a long record represents a clear advantage
for climatological and hydrological studies in an arid region and data from
this radar were fruitfully used for research in many studies so far
. An
extensive description of the quantitative radar precipitation estimation and
of its assessment is provided in . The distance from the
radar increases going south and east in the study area, so that the
instrument samples higher atmospheric levels (sampling elevation between 1000
and 4500 m). The radar data are corrected taking into account the vertical
profile of reflectivity, but overshooting of precipitation in the study area
is possible. However, this problem is expected to be negligible for this
study, in which vertically developed systems, such as the convective cells,
are examined.
Average SLP (hPa, black contour lines) and geopotential height at
500 hPa (m, in color) for each cluster; (a) Cyprus low,
(b) low to the east, and (c) Active Red Sea Trough (ARST).
Rain cell characterizationRain cell identification and tracking
The identification of rain cells is done by partitioning each radar image
into segments, and is followed by a cell tracking algorithm
. Rain cells are defined as connected radar pixels with the following:
(i) more than 5 mm h-1 rain intensity, (ii) at least one peak
exceeding 10 mm h-1, and (iii) an area larger than 9 km2. These
thresholds were suggested and used in previous studies
e.g., and allow focusing on
the convective part of the rain without excluding moderate rainfall
intensities that could be relevant for the flood generation. It should be
noted that the selection of the thresholds can affect some of the derived
properties (e.g., cell area and mean areal rain intensity), but it should not
affect the comparison of these properties between different groups of cells.
The spatial properties extracted from each segment are the following: area
(km2), length of the major and minor axes of the ellipse fitted to the
segment (km), ellipse center location, ellipticity (minor-to-major axis
ratio), orientation (angle of the major axis in degrees relative to the
west–east orientation, positive values are counter-clockwise), maximum rain intensity
(mm h-1), and mean areal intensity (i.e., the mean intensity over the
area of the segment, in mm h-1). Figure 2 presents an example of one radar
image and a derived rain cell with its spatial properties.
The cell tracking algorithm links rain cells in consecutive images and allows
characterizing of the rain-cell lifetime and average advection (velocity and
direction). The algorithm, developed by and modified by
, is based on Pearson's correlation between shifted
successive images. The term “lifetime” relates to the length of the
individual cell's life, while “duration” to the length of the rain event.
Frequencies of different tracking categories during the cell's full life
cycle (i.e., frequency of splits and merges) were left out of the
analysis as no added value to the presented results was given. A total of
10 447 rain cells (composing 2632 tracks) were derived. The rain record was
divided into 424 rain events, defined as separated by dry spells longer than
6 h. This allowed associating each rain cell to the governing synoptic
system of the rain event. Rain events for which less than 80 % of the
radar scans were available have been removed.
Synoptic classification
Rain events were classified into synoptic types using cluster analysis. The
clustering was aimed at relying mainly on the sea level pressure (SLP) map,
the most commonly used map for synoptic classification in the EM
and in other places in
the world . In addition, surface temperature
was used to distinguish between an ARST and a cold Mediterranean low, since
the former is usually initiated by thermal instability caused by differential
heating between the surface (where a warm advection from the southeast takes
place) and the upper atmospheric levels . Other atmospheric
variables (e.g., specific humidity at 700 hPa and temperature at 850 hPa)
were tested and found to have a negligible influence on the clustering. Each
rain event was linked to the ERA-Interim global reanalysis atmospheric
variables – SLP and near-surface (1000 hPa) air
temperature, obtained for the time closest to the rain event's center of mass
(chosen out of Era-Interim 4 times daily available times – 00:00, 06:00,
12:00, 18:00 UTC).
A hierarchical agglomerative clustering technique using Ward's criterion was
applied based on the following: (i) location of the minimum SLP within the EM region
(25.5–42.75∘ E and 22.5–37.5∘ N); (ii) north–south SLP
difference between two points – (33.75∘ E, 33∘ N) and
(33.75∘ E, 24.75∘ N); (iii) west–east SLP difference
between two points – (30.75∘ E, 34.5∘ N) and
(39∘ E, 34.5∘ N); and (iv) near-surface temperature at the
grid point closest to the center of the study area (35.25∘ E,
31.5∘ N). All the mentioned points are presented in Fig. 1a.
The rain events were found to be best described by three clusters (Fig. 3).
The first cluster (128 events, Fig. 3a) describes a Mediterranean low located
west of the shoreline, and is associated with a Cyprus low (CL). The second
cluster (186 events, Fig. 3b) describes a low to the east (LE) and could be
associated with a Syrian low, or with any other Mediterranean low settled
east of the study area. The third cluster (110 events, Fig. 3c) describes a
surface trough extending from the south and is associated with an ARST. The
radar rain record did not include tropical plume events and therefore this
synoptic type is not considered in the present analysis.
On average, both Mediterranean lows (Fig. 3a and b) are accompanied by a
pronounced 500 hPa trough extending from eastern Europe towards the EM, with
an axis orientation of north–south to northeast–southwest. Besides the known
effect of an upper level trough on the intensification of the low on the
surface , under such atmospheric conditions the
northwesterly flow is enriched with moisture from the sea, increasing the
probability of rainfall in the southern EM . The
upper trough of the ARST (Fig. 3c) is shallower and has a similar axis
orientation.
Empirical probability density functions of mean areal (blue) and
maximum (black) rain intensities of rain cells over the Dead Sea (smoothed).
μ represents the mean and M represents the median.
Validation of the clustering results was done according to expert
examinations. Maps displaying contour lines of SLP and wind directions at
850 hPa for 30 randomly chosen events were given to three experts to
evaluate. The experts were asked to choose the synoptic system that best
describes a given map out of four possible options: CL, LE, ARST, and “none
of the above”. Mismatches between the three experts' classifications and the
automated clustering were then counted in relation to the extent of
disagreement between the experts. For 19 maps there was an agreement between
the experts and the automated procedure. Three out of 30 maps were agreed on
between all experts but were classified differently from the automated
procedure, resulting in a 10 % classification error. For 8 maps there was
no agreement among the experts, resulting in 7 maps with matches between only
some of the experts and the automated procedure and 1 map with no matches.
Mean and standard deviation (in parentheses) of spatial and temporal
properties of events and derived convective rain cells.
aP value of the ANOVA test
applied for the different rain-cell properties between the three synoptic
systems. All data sets were tested first for variance heterogeneity using
Levene's test (with squared deviations). The Box-Cox transformation technique
was applied to properties of unequal variances to obtain normality. In these
cases, Welch's test followed by a multiple comparison using the Bonferroni
method was used. b Pair of groups of significance
difference at 0.05 level: (1) ARST-CL, (2) ARST-LE, (3) CL-LE.
Spatial and temporal rain-cell characteristics
In this section, the differences between properties of rain cells originated
by different synoptic systems are analyzed. Event properties and average
spatial and temporal characteristics of rain cells are presented in Table 2.
The use of averaged cell characteristics allows for neutralizing the dependency
between individual rain cells of the same event and is crucial for the
statistical comparison between the properties of cells generated by different
synoptic systems. From this point on, unless stated otherwise, all values of
rain-cell properties mentioned are the average value during the event.
Advection direction is defined following the meteorological standard, i.e., it
represents the direction of the origin (e.g., direction of 270∘
represents a movement from west to east).
The average duration of all rain events is 5.4 h. On average, the rain cells
are 92 km2 in area, advecting from west to east (274∘) at a
velocity of 12.8 m s-1 and living 18.1 min. Ellipticity of cells is
0.57 (minor to major axis length ratio of 3 : 5). The major axis is close
to alignment (18∘) with their direction of movement. Similar values of
ellipticity and orientation were found in previous studies conducted close to
the study area , and
in other regions such as Catalonia and France . The empirical probability density functions of the cells' rain
intensities are shown in Fig. 4. The mean areal and maximal rain intensities
are 12.3 and 26.6 mm h-1, respectively, and both functions are
positively skewed (skew coefficients of 7.4 and 2.7, respectively) as a
result of extreme rainfall events.
Comparison of rain-cell properties (averaged for each rain event)
of the three synoptic systems: CL (dark blue), LE (light blue) and ARST
(orange); (a) average event duration (h), (b) cell area
(km2), (c) minor axis length (km), (d) major axis
length (km), (e) ellipticity (–), (f) mean areal rain
intensity (mm h-1), (g) average velocity (m s-1), and
(h) average lifetime (min). The black line in each boxplot marks the
median, black diamond the mean, boxes lower and upper borders mark the 25 and
75 % quartiles, respectively, and the whiskers mark minimum and maximum
values unless these values exceed 1.5 ⋅ IQR (inter quartile range –
the distance between lower and upper quartiles). Outliers are not shown. See
Table 2 for numerical values of the mean and standard deviation.
Effect of synoptic system
Spatial and temporal properties of rain cells originated by different
synoptic systems are compared (Table 2 and Figs. 5 and 6) using one-way analysis of
variance (ANOVA) followed by a multiple pairwise comparison of the three
groups' means using Tukey's honest significant difference criterion.
Statistically significant differences have been found and are highlighted in
Table 2.
The LE rain events are characterized by the highest average duration of all
synoptic systems (6.3 h compared to 5.5 and 3.8 h of the CL and ARST
events, respectively, Fig. 5a). The area of the ARST rain cells was found
to be smaller than the area of the Mediterranean lows (76 km2 compared to 110
and 87 km2 of the CL and LE, respectively, Fig. 5b), but cells were
found to live longer than cells in other synoptic systems (20.8 min compared
to 18.1 and 17.7 min of the CL and LE, respectively, Fig. 5h). Moreover,
ARST rain cells' mean areal rain intensity (14.5 mm h-1) and maximal
rain intensity (31.5 mm h-1, Table 2) were found to be higher than
both CL intensities (10.6 and 22.8 mm h-1) and LE intensities (12.2
and 26.4 mm h-1) and these events have the highest variability in mean
areal rain intensities (Fig. 5f).
Advection (velocity and direction) distributions of
CL (a), LE (b), and ARST (c) events.
The rain cells generally preserve the same orientation for all three synoptic
types, but their shape is different: CL cells are characterized by lower
ellipticity (0.54 compared to 0.58 and 0.57 of the LE and ARST,
respectively). General mean orientation is west to east with a
12–16∘ counterclockwise tilt from the west–east axis (Table 2). The
CL events are characterized by rain cells generally moving from west to east
(268∘, Fig. 6a), whereas the LE and ARST events are characterized by
a slightly stronger northwestern component (about 277∘, Fig. 6b and
c). In the ARST case, the direction distribution is bimodal, with cells
originating from west-southwest and west-northwest (Fig. 6c). ARST events are
characterized by lower average velocities (11.4 m s-1 compared to 14.5
and 12.3 m s-1 of the CL and LE, respectively, Fig. 5g).
Calculated root mean square vector error (RMSVE) between average advection components and wind zonal
and meridional components obtained from Era-Interim reanalysis for pressure
levels of 500–1000 hPa for each synoptic system.
Significant differences (P value < 0.05) are found between the following: (i) ARST
and CL cell areas, minor axis length, maximum rain intensities, and mean areal
rain intensities; (ii) ARST and LE cells average lifetime and event duration;
(iii) CL and both ARST and LE major axis length, average direction, and
average velocity of rain cells; and (iv) CL and LE cells' ellipticity. Rain
cells are broadly advecting with the mean wind through some deep tropospheric
layer, in which the cloud is embedded . To
identify the layers that rain cells are commonly moving in over the study area,
the advection vector components were compared with zonal and meridional wind
components at pressure levels 500–1000 hPa (increments of 50 hPa),
extracted from Era-Interim reanalysis for the times closest to each rain
event's mass center at a grid point closest to the center of the study area
(35.25∘ E, 31.5∘ N). A root mean square vector error
(RMSVE) was calculated for each pressure level and synoptic system (Fig. 7).
The pressure levels with minimum RMSVE are in general 700 to 850 hPa, and in
particular 700–850 hPa for CL, 800–850 hPa for LE, and around 700 hPa
for ARST.
Mean rain-cell properties along a north–south (right-to-left
axes) orientation of the different synoptic systems:
(a) number of cells1, (b) cell area
(km2), (c) average velocity (m s-1), (d) mean
areal rain intensity (mm h-1). Potentially related variables along the
same axis: (e) maximal topographic height (m a.s.l.),
(f) distance from shoreline (km), and (g) mean annual
rainfall (mm). Each point represents the mean or maximal value in a west to
east strip (in panels a–d, the strip is 2.5 km wide and a running
average of 7.5 km is applied).
Effect of location
The variations in rain-cell characteristics along the north–south
latitudinal gradient were examined (Fig. 8). The total number and the average
cell area of the two Mediterranean lows is decreasing from north to south
(Fig. 8a and b), whereas their velocity is increasing (Fig. 8c). The mean
areal rain intensity of CL cells is increasing from north to south, but LE
cells show an increasing trend only between latitudes 31.6 and 31.1∘ N
(Fig. 8d).
A positive correlation between topographic height and cells area (Fig. 8e
and b) and a negative correlation between distance from shoreline and number
of cells (Fig. 8f and a) is seen in both Mediterranean lows in the northern
part of the study area, and especially for the LE system.
The ARST rain cells follow different trends: (i) the total number of rain
cells shows a smaller variation with latitude (Fig. 8a), (ii) the cells
moderately increase in size along the north–south axis reaching a peak
around latitudes 31.3–31.5∘ N (Fig. 8b), (iii) the cells have a
moderate decrease in their velocity with latitude (Fig. 8c), and (iv) no
clear trend in mean areal rain intensity is seen (Fig. 8d).
The region with higher mean annual rainfall (Fig. 8g) overlaps the regions of
maximal number of cells (Fig. 8a), maximal cell area (Fig. 8b), and low
velocities (Fig. 8c) of both Mediterranean lows. The region of maximal mean
areal rain intensity (Fig. 8d), however, is not collocated with maximal
rainfall amounts. This fits previous findings that in dryer regions rainfall
is generally more intense over short durations .
The moderate increasing trend in velocity of Mediterranean lows' cells along
the north–south axis may have resulted from a bias in favor of stronger
storms in the southern part, i.e., regions that are most distant from the sea
and from the Mediterranean storm tracks. Mediterranean storms that produce
rainfall over those regions were most likely deep lows of stronger winds that
had managed to transport clouds from the Mediterranean Sea far inland
.
Relations between rain-cell properties and flash floods
The relationship between properties of rain cells and the occurrence and
magnitude of flash floods in the Darga and Teqoa catchments (Fig. 1b) was
explored. A flash flood was defined according to the criteria specified in
Sect. 2.2. No distinction between the two catchments was made due to their
similar morphology, and their small and narrow shape relative to an average
size rain cell (Fig. 2). Out of the 424 detected rain events, 173 events had
rain cells tracked above the catchments, 29 of which (532 rain cells) were
associated with flash-flood events. Since the same rain event can potentially
lead to a flash flood in both catchments, the 29 rain events corresponded to
41 measured flash floods (21 in Darga and 20 in Teqoa). The remaining 144
rain events (988 rain cells) were classified as “non-flash-flood” events.
Examining the event duration and total rain depth over the catchments
(Fig. 9) reveal that the recorded flash floods were produced from events of a
few minutes to 2 days long and areal rain depth from 1 to 100 mm. In
general, long duration and high rain depth are conditions favoring the
occurrence of flash floods. A number of rain events with similar duration and
depth did not lead to flash floods, confirming that short duration
intensities are also important. In fact, ARST events leading to flash floods
generate high peak discharges, in spite of their general shorter durations
and lower rain depths than both Mediterranean lows.
Number of cells is normalized to the relative strip area.
Scatter plot of rain events' mean areal rain depth over the
catchments vs. duration, for flash-flood-associated (filled circles) and
non-flash-flood events (empty circles), with respect to different synoptic
types and different unit peak discharges of the flash-flood-related events.
In case of flow in both catchments, maximum unit peak discharge is presented.
Over 90 % of the watershed area is covered by rainfall during
flash-flood-related events (79 % for the non-flash-flood-related events).
Event duration refers to the part of the event where rain cells were found
over the catchments.
Flash floods in the desert are often triggered by one or two rain cells
. In this study, the “dominant rain cell”
of each event, i.e., the rain cell that contributed the largest amount of
rainfall over the two catchments, was identified. The mean, median, and
interquartile ranges of the properties characterizing the dominant cells of
flash-flood and non-flash-flood events during their lifetime over the
catchments were compared, and are presented in Fig. 10. If not stated
otherwise, the results presented below are significant at 0.05 level.
Results show that dominant rain cells associated with flash floods are
(i) larger than dominant cells not associated with flash floods
(247 km2 compared to 159 km2; Fig. 10a) and, accordingly, cover a
larger portion of the catchments area (77 km2 compared to 55 km2;
Fig. 10b); (ii) have lower average velocities (11.6 m s-1 compared to
14 m s-1, significance level 0.1; Fig. 10c); and (iii) persist longer
(41 min compared to 20 min; Fig. 10d). The mean areal and maximum rain
intensities of flash-flood-related dominant cells are higher than
non-flash-flood dominant cells (Fig. 10g and h), though this difference was
found to be not statistically significant.
Comparison of dominant flash-flood-related cells (blue, N=29)
and non-flash-flood-related cell (grey, N=144) properties:
(a) cell area (km2), (b) areal coverage (km2),
(c) average velocity (m s-1), (d) average lifetime
(min), (e) ellipticity, (f) orientation (∘),
(g) mean areal rain intensity (mm h-1), and
(h) maximum rain intensity (mm h-1). Values are averaged over
the dominant cell's lifetime. Boxplot properties are as specified in Fig. 5.
Reported P values are of the ANOVA test applied between dominant
flash-flood-related cells and non-flash-flood-related cells. All data sets
were tested first for variance heterogeneity using Levene's test (with
squared deviations).The Box-Cox transformation technique was applied to
properties of unequal variances to obtain normality. In these cases, Welch's
test was used.
Figure 11a and b shows the distribution of the advection of dominant cells for
non-flash-flood and flash-flood events. Non-flash-flood dominant cells are
generally more westerly and characterized by higher velocities
(14 m s-1 and 275∘, on average) than flash-flood dominant
cells (11.6 m s-1 and 286∘). Flash-flood dominant cells are
characterized by a bimodal distribution with low velocity,
north-northwesterly cells (generally, < 12 m s-1,
300–360∘) and higher velocities, westerly cells (generally,
> 12 m s-1, 240–300∘). Considering only high magnitude
flash floods (with peak discharge larger than the median) it is found that
dominant rain cells are related to low velocities and north-northwesterly
directions (9.8 m s-1 and 301∘ on average; Fig. 11c) that
match the main drainage axis of the two studied catchments (Fig. 1b).
Discussion
The properties of convective rain cells in the arid area of the Dead Sea
western tributaries are discussed in relation to the governing synoptic
system, location, and to flash-flood generation.
Variation between different synoptic systems
Rain-cell properties are distinctly associated with the characteristics of
three synoptic systems governing rain events in the region: the Cyprus low, the low
to the east, and the Active Red Sea Trough (CL, LE and ARST, respectively; see
Sect. 3.2). ARST events have shorter duration and their rain cells are
characterized by higher intensities, smaller areas, longer lifetimes, and
lower velocities compared to the two Mediterranean lows. The low cell
velocities are likely due to the more continental nature of this system
and to the smaller pressure gradients
, while the higher intensities could be related to higher
surface temperatures (observed in the clustering results, not shown) leading
to greater atmospheric instability .
The average directions of rain cells are southwest to west for CL events and
west to northwest for LE events. In ARST events both modes are common. These
results are explained by the location of the surface low in CL and LE systems
and by the cyclonic geostrophic wind (Fig. 3); a CL located west of the study
area is usually identified with southwesterly to westerly winds at low
levels, while northwesterly wind directions are more dominant when the low
is located at the East.
In both Mediterranean lows, southwestern cell directions are associated with
higher cell velocities, while northwestern directions with lower cell
velocities (Fig. 6). The lower cell velocities could be related to the slower
movement of the Mediterranean low when located above land and to the larger
distance of the center of the low from the study area (Fig. 3a and b). The lower
average cell velocities of the LE events is therefore explained by a larger
portion of northwestern cell directions. Furthermore, average cell velocity
components of Mediterranean low events are in better agreement with low level
(750–850 hPa) wind components, while ARST with higher levels (700 hPa; Fig. 7). These findings agree with the synoptic understanding that ARST
events are usually identified with medium-level clouds and Mediterranean lows
with low-level clouds. Unlike the Mediterranean lows, which have the
Mediterranean Sea as their major moisture supplier, the moisture essential
for the development of convective rain cells in ARST events must be
transported at the medium levels from remote southern origins, since a dry
easterly wind flow is found at the lower levels .
Advection (velocity and direction) distributions of dominant rain
cells for (a) non-flash-flood events (490 cells, 144 events),
(b) flash-flood events (220 cells, 29 events), and
(c) high-magnitude flash-flood events (111 cells, 15 events). High
magnitude is defined based on median values of measured flash floods in Darga
and Teqoa used in this analysis.
Variation within the study region
Rain-cell properties vary in space. Variations along the north–south axis of
the study area, characterized by a sharp decrease in mean annual rainfall
(Fig. 8g), an increase in distance from the Mediterranean Sea's shoreline
(Fig. 8f), and a change of topography (Fig. 8e) were examined. The relative
frequency of high-intensity rainfall increases with the reduction of annual
rainfall amounts , and
orographic effects lead to enhanced rainfall generation . Both phenomena are reflected in the
characteristics of the rain cells of Mediterranean lows: (i) mean rain
intensities are generally increasing with the degradation of mean annual
rainfall southward towards drier regions. Though the increase in rainfall
intensities southward is seen explicitly in the Mediterranean lows, the
increasing dominance of the ARST rain cells towards the south is likely to
play a significant role in the latitudinal increase in rainfall intensities
; (ii) rain cells are larger where topography is higher. As
stated by , clouds distancing themselves from the shoreline and
ascending the mountains, tend to become more uniform and continuous than over
the coastal plain, thus increasing their size. When the terrain features are
low enough, preexisting clouds that move over them produce maximum
precipitation on the upwind side of the barrier. As the precipitating cloud
is advected to the lee side, the precipitating capacity is weakened by the
downslope air motion . Our analyses included rain cells with
centroids located east of the water divide, i.e., on the lee side of the
mountain range (Fig. 1). Nevertheless, in many cases the cell area includes
precipitation also on the west side of the water divide and thus the total
effect obtained is the cell area increase along with the rain intensity
weakening over the mountainous areas.
Flash-flood-related characteristics
A few studies had focused on the contribution of the spatial and temporal
characteristics of rain cells to flash-flood generation in arid environments.
reported that slow storm velocities contribute to
flash-flood generation and underlined the importance of the
areal coverage of the storm core for runoff generation in a semi-arid
catchment. suggested that floods, though mainly related
to the total rainfall, were eventually triggered by intense bursts of rain.
Our results support these previous findings, showing that flash-flood-related
and non-flash-flood-related rain cells differ in size, areal coverage,
velocity, lifetime over the catchment, and rain intensity.
Other studies wished to analyze the impact of the spatial and temporal
characteristics of rain cells on flash-flood magnitude by using case studies
of real storms or model simulations
. While some
argued about the importance of rainfall intensity distribution
, a common conclusion concerned the
movement of the storm: slower storms directed downstream of the catchment seem
to produce flash floods of higher magnitudes . Our results confirm this effect: rain cells moving
downstream with a direction close to the orientation of the principal axis of
the catchments (Fig. 1) at low average velocities (<12 m s-1) are
related to higher peak discharges.
Due to the small sample of flash-flood events (29 events), the influence of
the synoptic type on flash-flood generation was not taken into consideration.
Nevertheless, results presented in Sect. 3 suggest that ARST rain cells
should have larger flash-flooding potential, due to the lower velocities,
longer lifetime, and higher intensities; for example, 55 % of the ARST
rain cells analyzed had velocities lower than 12 m s-1) against only
33 and 46 % for CL and LE. Out of them, the fraction with mean areal rain
intensities higher than 10 mm h-1) are 63, 39, and 46 % for ARST,
CL, and LE cells, respectively. In fact, 6 out of the 7 flash floods generated
by ARST events were characterized by high magnitude. Other studies underlined
the localized and intense nature of ARST storms and their high flash-flooding
potential .
Some of the rain-cell properties associated with flash floods are tied with
the catchment properties. For example, large catchments might be less
influenced by ARST rain cells due to their smaller size. The more
northwesterly cell directions associated with LE events and part of the ARST
events might present better flooding conditions relative to other cell
directions, due to the northwest to southeast orientation
(∼ 315∘) of the Darga and Teqoa catchments (Fig. 1), but might
not have any advantage in the case of a different catchment orientation. This
seems to be reflected in the increased representation of LE in
flash-flood-related events in Darga and Teqoa (55 %) in comparison with
the general frequency of LE events in the entire study area (44 %).
Furthermore, found that the frequency of flash floods in
the Negev (south of our study area) generated by ARST events is slightly
higher (38 % in comparison with 24 % found in our study) than Syrian
low events (33 %), which are most likely the equivalent to our LE events.
These differences may arise from the different sample sets analyzed by
(only floods >5 years recurrence interval were taken)
and from the more northeastern location of the Dead Sea region than the
Negev, and may indicate the importance of the LE events in
generating flash floods in the Dead Sea region, especially in the northern
parts of our study area.
Conclusions
This study provides a climatology of spatial and temporal
properties of radar-derived convective rain cells over the dry area of the
Dead Sea (Eastern Mediterranean region). These properties are examined in
relation to the governing synoptic system and to flash-flood generation and
magnitude. The study offers a statistical approach to relate rainfall
properties (e.g., the properties of the most contributing cell) to catchment
response. The main findings of the study are as follows:
Convective rain cells are on average 92 km2 in size, move at a velocity
of 12.8 m s-1 from west to east, and live for 18.1 min.
ARST events are characterized by the shortest event duration, highest
cell mean areal rain intensity, smallest cell area, longest cell lifetime,
and lowest cell velocity.
The area of rain cells generated by Mediterranean lows is positively
correlated with the topographic height in the northern part of the study
area, the number of cells is negatively correlated with the distance from the
shoreline, and the mean rain intensities are negatively correlated with mean
annual rainfall.
High mean annual rainfall in the northern mountainous part of the study
area results from a large number of rain cells with low velocities and large
area rather than cells of high rain intensities. Rain cells related to
flash-flood events are characterized by larger area, lower velocity, and
longer lifetime over the catchment.
Rain cells with lower velocities (generally < 12 m s-1) and
of north to northwestern origins, directed downstream with the main catchment
axis, lead to high magnitude flash floods.
Results from this study add insights and quantitative information to previous
studies in the Dead Sea region and in other arid regions worldwide. This
advocates the robustness of the methods applied and the adequacy of the radar
data used to represent rainfall over the study area. The distributions of the
convective rain-cell characteristics extracted in this work can be used for
stochastic simulations of convective rain storms and serve as input for
hydrological models and for flash-flood nowcasting systems.
Rainfall data from a C-band weather radar were provided by
the company E.M.S. Mekorot for exclusive use and elaborated by Marra and
Morin (2015). Streamflow data were provided by the Israel Hydrological
Service, and are available upon request:
http://www.water.gov.il/Hebrew/ProfessionalInfoAndData/Pages/default.aspx.
Atmospheric variables extracted from the ERA-Interim reanalysis (Dee et
al., 2011) are available at the following link:
http://apps.ecmwf.int/datasets/data/interim-full-daily.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Environmental
changes and hazards in the Dead Sea region (NHESS/ACP/HESS/SE inter-journal
SI)”. It is not associated with a conference.
Acknowledgements
The study was partially funded by the Dead Sea Drainage Authority,
the Israel Water Authority, the Israel Science Foundation (grant
no. 1007/15), the NSF-BSF grant (BSF 2016953) and the Lady Davis Fellowship
Trust (project: RainFreq). This work is a contribution to the HyMeX program.
The authors thank Maya Bartov, Moshe Armon and Uri Dayan for their assistance
in validating the automatic classification of the synoptic
systems.
Edited by: Florian Pappenberger
Reviewed by: two anonymous referees
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