Orographic Rainfall Research Articles

Extreme Orographic Rainfall in the Eastern Andes Tied to Cold Air Intrusions

Long-duration localized heavy rainfall (> 6 hours, < 10 km2) was recorded by raingauges on October 4, 2013 at mid-elevations (~1,500 m) in the Peruvian Andes, which was not apparent in infrared or microwave measurements from either geostationary or polar orbiting satellites. This spring event is investigated here through a numerical modeling study using the Weather and Research Forecasting (WRF) model, and examined in the context of the climatology of Cold Air Intrusions (CAIs) along the eastern flanks of the tropical Andes. The model results suggest significant precipitation enhancement from intense shallow convection at the CAI frontal boundary amplified by orographic lifting as it propagates northward latched to the slopes of Andes. Analysis of CAI mesoscale dynamics was conducted using four decades of European Center Medium-Range Weather Forecasts (ECMWF) reanalysis, Tropical Rainfall Measurement Mission (TRMM) data products, and rain-gauge observations with emphasis on characterizing year-round CAI frequency, CAI interactions with Andes topography, and their impact on orographic precipitation climatology. The data show a robust enhancement of the diurnal cycle of precipitation during CAI events in all seasons, and in particular increases in surface rainfall rate during early morning at intermediate elevations (~ 1,500 m), that is the eastern Andes orographic maximum. This link between CAI frequency and rainfall suggests that they play an important role in maintaining the Andes to Amazon year-round terrestrial connectivity through runoff production and transport by the river networks.

Resolving the weakening of orographic rainfall over India using a regional climate model RegCM 4.5

In this study, the weakening of the orographic rainfall for the period 2000–2011 during the Indian summer monsoon (JJA) is investigated using the regional climate model RegCM 4.5. Analysis reveals that when sulfate aerosol feedback is included in the model run, the simulated temperature shows a cooling of the continental upper troposphere by 0.3 °C. At the same time, the sulfate aerosol loading in the atmosphere had induced cooling of 0.2 °C of the lower troposphere over the Indian region. Consequently, the simulated zonal wind fields show a weakening at the upper and lower tropospheric levels, particularly where the semi-permanent systems that drive the monsoon circulations are located. It is suggested that the weakening of zonal winds has caused a suppression of the orographic rainfall along the west coast of India (Western Ghats), as evident from the observations. Concurrently, the simulations indicate a sulfate aerosol induced reduction in orographic monsoon rainfall over the southern Western Ghats, whereas their effect is insignificant in controlling the orographic rainfall in the northern Western Ghats. Moreover, our analysis indicates a decline in rainfall over the Himalayan foothills when the sulfate aerosol feedbacks are included, which corroborates well with the observations.

Evaluation of TAMSAT satellite rainfall estimates for southern Africa: A comparative approach

Developing countries with a paucity of direct rainfall measuring systems on the ground, satellite remote sensing is increasingly being relied upon as a means through which national governmental response and future strategies to manage natural disasters, such as drought and flooding are decided upon. The aim of this study was to assess the performance of the Tropical Applications of Meteorology Using Satellite Data and Ground-Based Observations (TAMSAT) monthly rainfall dataset for southern Africa by comparing it to two widely used stable global rainfall datasets, the Global Precipitation Climatology Centre (GPCC) and the Global Precipitation Climatology Project (GPCP) datasets. Using rainfall records for the period 1984–2010, four statistical measures namely, the correlation coefficient, the Root Mean Square Error (RMSE), relative bias, mean absolute deviation and the Nash Sutcliffe Efficiency index were calculated to facilitate pair-wise comparisons. This inter-product comparison study found a good agreement between TAMSAT satellite rainfall and the two global rainfall products with regard to the spatial and temporal representation of monthly rainfall over the region. However, it was found that TAMSAT consistently provides lower monthly rainfall estimates compared to both GPCC and GPCP rainfall estimates over southern Africa. The relative bias between TAMSAT and the two global rainfall products (GPCC and GPCP) was much lower (≤10%) during the austral summer months (November–March) compared to winter when the relative bias surpassed 70%. These findings indicate TAMSAT is geared more towards convective precipitation brought about by the inter-tropical convergence zone in summer. The observed large discrepancy between TAMSAT estimates and those from two mature global rainfall products during winter raises a key concern regarding the use of TAMSAT as a tool for monitoring rainfall patterns and volumes across the southern reaches of the African continent. Overall, the findings of this study demonstrate the diminished ability for TAMSAT to “see” non-convective precipitation from frontal systems as well as orographic rainfall due to its reliance upon the observation of the cold cloud duration and convection characteristics of tropical rainfall systems. This calls for the need to further optimise the TAMSAT algorithm using gauge data so that TAMSAT is better able to characterise different precipitation events in those regions where convective rainfall is far from the only major source of rainfall.

Termite assemblages (Blattaria, Isoptera) in two montane forest (Brejo de Altitude) areas in northeastern Brazil

Abstract: Wet enclaves of montane forests in Caatinga domain, locally known as "Brejos de Altitude", are associated with plateau at altitudes greater than 500 m a.s.l. and to orographic rainfall. Termite assemblage structures were studied in two areas of montane forest (Brejo de Altitude) located in the municipalities of Bezerros and São Vicente Ferrer, Pernambuco State, Brazil. Sampling was performed in 65 x 2 m transects, totaling 300 m2/per area; the densities of nests in each area were estimated using six 1300 m2 plots. A total of 35 morphospecies were recorded that belonged to 21 genera and three families, with 133 encounters. The soil-feeders feeding group predominated, followed by wood-feeders species. A total of five species constructing conspicuous nest were recorded in the two areas. The mean density of active conspicuous nests was 2.6 ± 6.3 nests/ha (mean ± sd) in Bezerros, and 21.8 ± 21.4 nests/ha in São Vicente Ferrer. Termite richness in the study areas were within the amplitude ranges recorded in other montane forests. Thus, the results presented here, combined with data from literature, reinforce need additional studies of the termite fauna in montane forest areas, once the "Brejos" are currently under high ecological pressure and their preservation is urgent.

Palaeohydrology of lowland rivers in the Murray-Darling Basin, Australia

This study derives a new function describing the relationship of channel bankfull discharge (Qbf) to channel width in modern rivers of the Murray-Darling Basin (MDB) of southeastern Australia and applies this to dated palaeochannels of seven rivers to quantify late Quaternary discharge history in this important basin. All rivers show high MIS3 and MIS2 Qbf, declining in the Holocene. The Qbf of modern MDB rivers is correlated with total catchment precipitation but comparison with palaeochannel Qbf estimates shows that while enhanced runoff efficiency is necessary to account for much larger late Pleistocene palaeochannels, either lower or higher precipitation rates could have prevailed. A strong association between relative palaeo- Qbf enhancement and temperature suggests a temperature-mediated mechanism controlling river discharge, such as the fraction of precipitation stored as snow and thawing in spring, the enhancement of orographic rainfall, or CO2 feedbacks with vegetation cover. Significantly enhanced MIS3 Qbf requires an additional mechanism, such as increased rainfall. These findings are consistent with others that increased moisture availability was associated with past colder climates, although this was not necessarily the result of enhanced precipitation.

Large catchment-scale spatiotemporal distribution of soil organic carbon

There are a dearth of studies globally examining soil organic carbon (SOC) at large catchment scales. Here we assess the catchment scale climate and geomorphic controls on SOC. The study was conducted on the east coast of New South Wales, Australia and focussed on a 562 km2 (Krui River) catchment sampled in 2006 and 2015 and a 606 km2 (Merriwa River) catchment sampled in 2015. Both catchments have similar soils, topography and landuse. There was no significant difference in SOC between 2006 and 2014 data sets, indicating that SOC was temporally stable over the intervening 8 years (Krui catchment), despite the seasonal variability in climate such as rainfall. SOC concentration was also shown to have no significant difference between Krui catchment and Merriwa catchment, indicating that SOC is spatially stable for catchments of similar land-use, climate and geomorphology. SOC concentrations from all three data sets were compared to a range of terrain attributes. Similar with other studies, elevation, as a surrogate for orographic rainfall, was found to have the strongest significant control on SOC % at the large catchment scale. Confirmation of the use of elevation as a surrogate for rainfall was made by comparing SOC with rainfall obtained from a network of weather stations across the study sites. Relationships between SOC and other terrain attributes found inconsistent relationships. The findings demonstrate that for catchments with similar soils, topography and climate that SOC can be reliably predicted using simple topographic variables. The methods here provide a robust tool which can be used for SOC assessment at other sites as well as assist in understanding SOC distribution and controls for longer term and regional scales.

Terrain-Trapped Airflows and Orographic Rainfall along the Coast of Northern California. Part II: Horizontal and Vertical Structures Observed by a Scanning Doppler Radar

Abstract This study documents the mean properties and variability of kinematic and precipitation structures associated with orographic precipitation along the coast of Northern California in the context of terrain-trapped airflows (TTAs). TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. Seven land-falling winter storms are examined with observations from a scanning X-band Doppler radar deployed on the coast at Fort Ross, California. Additional information is provided by a 915-MHz wind-profiling radar, surface meteorology, a GPS receiver, and balloon soundings. The composite kinematic structure during TTA conditions exhibits a significant horizontal gradient of wind direction from the coast to approximately 50 km offshore and a low-level jet (LLJ) that surmounts a weaker airflow offshore corresponding to the TTA, with a zone of enhanced precipitation evident between ~5 and 25 km offshore and oriented nearly parallel to the coastline. Conversely, the composite kinematic structure during NO-TTA conditions exhibits a smaller offshore horizontal gradient of wind direction and precipitation structures are generally enhanced within km of the coastline. Interstorm variability analysis reveals significant variations in kinematic structures during both TTA and NO-TTA conditions, whereas significant variations in precipitation structures are only evident during TTA conditions. The interstorm analysis also illustrates more clearly how LLJ vertical structures evident during NO-TTA conditions exhibit ascent along the coast and over the coastal mountains, which is in contrast to TTA conditions where the ascent occurs offshore and over the TTA.

A dynamical model for diagnosing orographic rainfall across Assam–Burma hills in India

An attempt has been made to diagnose the orographic rainfall associated with internal gravity waves excited by the Assam–Burma hills (ABH) in India, during south west monsoon season (SWMS). To develop this model ABH has been synthesised by two 3-D elliptical barriers, separated by a valley of some finite distance. In this paper, a three dimensional frictionless, laminar, adiabatic, boussinesq, non-rotating and inviscid moist airflow with realistic vertical variation of wind and temperature, has been considered. Here also the basic flow consists of two components such as zonal wind component U(z) and meridional wind component V(z). The perturbation technique is used to linearise the governing equations of the airflow and to obtain perturbation vertical velocity ( $$w^{\prime}$$ ) the quasi-numerical approach have been used. The perturbation vertical velocity ( $$w^{\prime}$$ ) is used to compute the rainfall intensity (RFI) over ABH during SWMS. Also, we examine the sensitivity of the computed RFI on the length (d) of the valley. Finally, the computed results have compared with Gridded Rainfall Data set, generated by India meteorological Department (IMD), India.

Characteristics of vertical velocities estimated from drop size and fall velocity spectra of a Parsivel disdrometer

Abstract. Vertical air velocities were estimated from drop size and fall velocity spectra observed by Parsivel disdrometers during intensive field observations from 13 June to 3 August 2016 around Mt. Jiri (1915 m a.s.l.) in the southern Korean Peninsula. Rainfall and wind velocity data measured by Parsivel disdrometers and ultrasonic anemometers, respectively, were analyzed for an orographic rainfall event associated with a stationary front over Mt. Jiri on 1 July 2016. In this study, a new technique was developed to estimate vertical air velocities from drop size and fall velocity spectra measured by the Parsivel disdrometers and investigate characteristics of up-/downdrafts and related microphysics on the windward and leeward sides of the mountain. To validate results from this technique, vertical air velocities between the Parsivel disdrometers and anemometers were compared at different locations and were shown in quite good agreement with each other. It was shown that upward motion was relatively more dominant on the windward side and even during periods of heavy rainfall. In contrast, downward motion was more dominant on the leeward side during nearly the same periods of heavy rainfall. Occurrences of upward and downward motion were digitized as percentage values as they are divided by a total count of occurrences during the entire period. On the windward (leeward) side, the percentages of upward (downward) motion were much larger than those of downward (upward) motion. The mean rainfall intensity on the leeward side was stronger than on the windward side, suggesting that most of the rainfall on the leeward side was relatively more affected by the downward motion. With the estimated vertical air velocities, histogram characteristics of rainfall parameters were also examined between the windward and leeward sides.

Tectonic control of erosion in the southern Central Andes

Landscape evolution modeling and global compilations of exhumation data indicate that a wetter climate, mainly through orographic rainfall, can govern the spatial distribution of erosion rates and crustal strain across an orogenic wedge. However, detecting this link is not straightforward since these relationships can be modulated by tectonic forcing and/or obscured by heavy-tailed frequencies of catchment discharge. This study combines new and published along-strike average rates of catchment erosion constrained by 10Be and river-gauge data in the Central Andes between 28°S and 36°S. These data reveal a nearly identical latitudinal pattern in erosion rates on both sides of the range, reaching a maximum of 0.27 mm/a near 34°S. Collectively, data on topographic and fluvial relief, variability of rainfall and discharge, and crustal seismicity suggest that the along-strike pattern of erosion rates in the southern Central Andes is largely independent of climate, but closely relates to the N–S distribution of shallow crustal seismicity and diachronous surface uplift. The consistently high erosion rates on either side of the orogen near 34°S imply that climate plays a secondary role in the mass flux through an orogenic wedge where the perturbation to base level is similar on both sides.

Role of Orography, Diurnal Cycle, and Intraseasonal Oscillation in Summer Monsoon Rainfall over the Western Ghats and Myanmar Coast

Rainfall over the coastal regions of western India [Western Ghats (WG)] and Myanmar [Arakan Yoma (AY)], two regions experiencing the heaviest rainfall during the Asian summer monsoon, is examined using a Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) dataset spanning 16 years. Rainfall maxima are identified on the upslope of the WG and the coastline of AY, in contrast to the offshore locations observed in previous studies. Continuous rain with slight nocturnal and afternoon–evening maxima occurs over the upslope of the WG, while an afternoon peak over the upslope and a morning peak just off the coast are found in AY, resulting in different locations of the rainfall maxima for the WG (upslope) and AY (coastline). Large rainfall amounts with small diurnal amplitudes are observed over the WG and AY under strong environmental flow perpendicular to the coastal mountains, and vice versa. Composite analysis of the boreal summer intraseasonal oscillation (BSISO) shows that the rain anomaly over the WG slopes lags behind the northward-propagating major rainband. The cyclonic systems associated with the BSISO introduces a southwest wind anomaly behind the major rainband, enhancing the orographic rainfall over the WG, and resulting in the phase lag. This lag is not observed in the AY region where more closed cyclonic circulations occur. Diurnal variations in rainfall over the WG regions are smallest during the strongest BSISO rainfall anomaly phase.

Vertical structure of orographic precipitating clouds observed over south Asia during summer monsoon season

Orography profoundly influences seasonal rainfall amount in several places in south Asia by affecting rain intensity and duration. One of the fundamental questions concerning orographic rainfall is nature of the associated precipitating clouds in the absence of synoptic forcing. It is believed that these clouds are not very deep, however, there is not much information in the literature on their vertical structure. The present study explores the vertical structure of precipitating clouds associated with orographic features in south Asia using data collected with the precipitation radar on board the Tropical Rainfall Measuring Mission satellite. Two types of precipitating clouds have been defined based on cloud echo top height, namely, shallow echo-top cloud and medium echo-top cloud. In both, radar reflectivity factor is at least 30 dBZ at 1.5 km altitude, and tops of shallow and medium echo-top clouds lie below 4.5 km and between 4.5 and 8 km, respectively. The Western Ghats contains the highest fraction of the shallow echo-top clouds followed by the adjacent eastern Arabian Sea, while the Khasi Hills in Meghalaya and Cardamom Mountains in Cambodia contain the least fraction of them. Average vertical profiles of shallow echo-top clouds are similar in different mountainous areas while regional differences are observed in the medium echo-top clouds. Below 3 km, precipitation liquid water content in medium echo-top clouds is the highest over the Western Ghats and the eastern Arabian Sea. The average precipitation liquid water content increases by \(0.16\,\hbox { gm m}^{-3}\) for shallow echo-top clouds between 3 and 1.5 km altitude, while the corresponding increase for medium echo-top clouds is in 0.05–0.08 \(\hbox { gm m}^{-3}\) range.

Contrasting Features of Monsoon Precipitation Around the Meghalaya Plateau Under Westerly and Easterly Regimes

AbstractPrecipitation features around the Meghalaya Plateau, northeast India, during summer are investigated using a 17 year (1998–2014) high‐spatial‐resolution Tropical Rainfall Measuring Mission precipitation radar data set. Precipitation around the plateau fell into two distinct regimes based on the low‐level wind direction that fluctuates on intraseasonal time scales over Bangladesh, windward of the plateau: a westerly regime (WR) and an easterly regime (ER). Under the WR, strong low‐level onshore southwesterlies across Bangladesh encounter the plateau, and localized strong low‐level southerlies running parallel to the Arakan Mountains (i.e., the barrier jet) also blow toward the plateau, concentrating convective unstable air onto its southern slopes. The low‐level wind fields and large‐scale upper level divergent fields promote frequent and intense orographic rainfall along the southern slopes due to forced uplift, generating high precipitation. In contrast, under the ER, strong southeasterlies that blow along the Gangetic Plain without encountering the plateau and subsidence inhibit upward motion around the plateau, resulting in low precipitation. Diurnal variations in precipitation significantly affect the daily precipitation around the plateau under both regimes. High rainfall frequency persists over the southern slopes between 2100 and 1200 LT of the next day under the WR, whereas modest rainfall frequency occurs between 0000 and 0600 LT under the ER, with a daytime minimum and nocturnal maximum in both regimes. The atmospheric boundary layer processes over Bangladesh regulate the wind speed and vertical structure of the low‐level wind toward the plateau, with deceleration during daytime and acceleration at night (i.e., nocturnal jet) that result in the nocturnal rainfall maximum.

Dominant Synoptic Disturbance in the Extreme Rainfall at Cherrapunji, Northeast India, Based on 104 Years of Rainfall Data (1902–2005)

AbstractThe characteristics of active rainfall spells (ARSs) at Cherrapunji, northeast India, where extreme high rainfall is experienced, and their relationships with large-scale dynamics were studied using daily rainfall data from 1902 to 2005 and Japanese 55-Year Reanalysis from 1958 to 2005. Extreme high daily rainfalls occur in association with ARSs. The extremely large amounts of rainfall in the monsoon season are determined by the cumulative rainfall during ARSs. ARSs start when anomalous anticyclonic circulation (AAC) at 850 hPa propagates westward from the South China Sea and western North Pacific, and covers the northern Bay of Bengal. The AAC propagates farther westward and suppresses convection over central India during ARSs at Cherrapunji, and continues for 3 to 14 days. Consequently, a northward shift of the monsoon trough during the “break” in the Indian core region occurs. The westerly wind, which prevails in the northern portion of the AAC, transports moisture toward northeast India and enhances moisture convergence over northeast India with southerly moisture transport from the Bay of Bengal, and greatly intensifies the orographic rainfall. In the upper troposphere, the Tibetan high tends to extend southward with the onset of ARSs. A linear relationship can be seen between the length and total rainfall of an ARS. Longer ARSs tend to result in greater total rainfall. AACs with a greater zonal scale tend to produce longer and more intense ARSs. This study provides evidence for the effect of western North Pacific AACs on the Indian summer monsoon.

Further Improvement of the Heavy Orographic Rainfall Retrievals in the GSMaP Algorithm for Microwave Radiometers

AbstractAn orographic/nonorographic rainfall classification scheme has been introduced for the operational algorithm of the Global Satellite Mapping of Precipitation (GSMaP) for passive microwave radiometers. However, problems of overestimations and false alarms of heavy orographic rainfall remain unresolved. This is because the current scheme selected lower constant thresholds of orographic rainfall conditions for global application and used values of orographically forced upward motion w derived from near-surface atmospheric data. This study improves the conceptual model of the warm-rain process for considering the strength of the upstream flow of the low-level troposphere. Under a weak upstream current, rain reaches the foothills of the windward mountain slope because of sufficient time for condensation and precipitation enhancement by the topography. Conversely, under a strong upstream current, precipitation enhancement occurs nearer to the mountain peak. This is because the upstream current flows so quickly that there is insufficient time for enhancement of precipitation over the foothills of the windward mountain slope. After implementing a variable threshold for w that depends on the mean horizontal low-level wind, the area of orographic enhancement of rain was detected reasonably well in cases of both strong and weak winds. To improve the accuracy of estimates of orographic rainfall, an adjustment to the rain estimation was introduced using a lower-frequency channel. The biases of the rainfall estimate for the adjusted scheme from the Tropical Rainfall Measuring Mission Precipitation Radar were improved for the cases considered here as well as for the Asian region of heavy orographic rainfall over land.

Terrain-Trapped Airflows and Orographic Rainfall along the Coast of Northern California. Part I: Kinematic Characterization Using a Wind Profiling Radar

AbstractThis study develops an objective method of identifying terrain-trapped airflows (TTAs) along the coast of Northern California and documenting their impact on orographic rainfall. TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. A 13-winter-seasons dataset is employed, including observations from a 915-MHz wind profiling radar along the coast at Bodega Bay (BBY, 15 m MSL) and surface meteorology stations at BBY and in the coastal mountains at Cazadero (CZD, 478 m MSL). A subset of rainy hours exhibits a profile with enhanced vertical shear and an easterly wind maximum in the lowest 500 m MSL, roughly the same depth as the nearby coastal terrain. Both flow features have a connection to TTAs along the coast of Northern California. Based on the average orientation (320°–140°) and altitude of nearby topography, mean wind direction in the lowest 500 m MSL () between 0°–140° is used as the initial criterion to identify TTA conditions. Application of this threshold yields a CZD/BBY rainfall ratio of 1.4 (3.2) for TTA (NO TTA) conditions. More detailed analysis of the relationship between and orographic rainfall reveals that an upper threshold of 150° more precisely divides the TTA and NO-TTA regimes. A sensitivity analysis and comparison with a TTA documented in a previous case study show that the best TTA identification criteria correspond to with a duration of at least 2 h. This objective identification method is applied to seven case studies in Part II of the present study.

Simple and approximate estimations of future precipitation return values

Abstract. We present estimates of future 20-year return values for 24 h precipitation based on multi-model ensembles of temperature projections and a crude method to quantify how warmer conditions may influence precipitation intensity. Our results suggest an increase by as much as 40–50 % projected for 2100 for a number of locations in Europe, assuming the high Representative Concentration Pathway (RCP) 8.5 emission scenario. The new strategy was based on combining physical understandings with the limited information available, and it utilised the covariance between the mean seasonal variations in precipitation intensity and the North Atlantic saturation vapour pressure. Rather than estimating the expected values and interannual variability, we tried to estimate an upper bound for the response in the precipitation intensity based on the assumption that the seasonal variations in the precipitation intensity are caused by the seasonal variations in temperature. Return values were subsequently derived from the estimated precipitation intensity through a simple and approximate scheme that combined the 1-year 24 h precipitation return values and downscaled annual wet-day mean precipitation for a 20-year event. The latter was based on the 95th percentile of a multi-model ensemble spread of downscaled climate model results. We found geographical variations in the shape of the seasonal cycle of the wet-day mean precipitation which suggest that different rain-producing mechanisms dominate in different regions. These differences indicate that the simple method used here to estimate the response of precipitation intensity to temperature was more appropriate for convective precipitation than for orographic rainfall.

Orographic rainfall hot spots in the Andes‐Amazon transition according to the TRMM precipitation radar and in situ data

AbstractThe Andes‐Amazon transition, along the eastern Peruvian Andes, features “hot spots” with strong precipitation. Using 15 years of Tropical Rainfall Measuring Mission PR data we established a robust relation between terrain elevation and mean surface precipitation, with the latter peaking around 1000 m above sea level (asl), coinciding with the moisture flux peak of the South American Low Level Jet (SALLJ). There is strong diurnal variability, with afternoon (13–18 LT) convection in the Amazon plains, while on the eastern slopes (1000–2000 m asl), after the forcing associated with the thermal heating of the Andes subsides, convection grows during the night and surface precipitation peaks around 01–06 LT and organizes into mesoscale convective systems (MCSs). These then displace downslope to an terrain elevation of 700 m asl with stratiform regions spreading upslope and downslope and then decay during the remainder of the morning. The large MCSs contribute with at least 50% of daily rainfall (60% of the 01–06 LT rainfall). On synoptic scales, the large MCSs are more common in stronger SALLJ conditions, although subtropical cold surges are responsible for 16% of the cases.

Stable isotope and noble gas constraints on the source and residence time of spring water from the Table Mountain Group Aquifer, Paarl, South Africa and implications for large scale abstraction

Large scale groundwater abstraction is increasingly being used to support large urban centres especially in areas of low rainfall but presents particular challenges in the management and sustainability of the groundwater system. The Table Mountain Group (TMG) Aquifer is one of the largest and most important aquifer systems in South Africa and is currently being considered as an alternative source of potable water for the City of Cape Town, a metropolis of over four million people. The TMG aquifer is a fractured rock aquifer hosted primarily in super mature sandstones, quartzites and quartz arenites. The groundwater naturally emanates from numerous springs throughout the cape region. One set of springs were examined to assess the source and residence time of the spring water. Oxygen and hydrogen isotopes indicate that the spring water has not been subject to evaporation and in combination with Na/Cl ratios implies that recharge to the spring systems is via coastal precipitation. Although rainfall in the Cape is usually modelled on orographic rainfall, δ18O and δ2H values of some rainfall samples are strongly positive indicating a stratiform component as well. Comparing the spring water δ18O and δ2H values with that of local rainfall, indicates that the springs are likely derived from continuous bulk recharge over the immediate hinterland to the springs and not through large and/or heavy downpours. Noble gas concentrations, combined with tritium and radiocarbon activities indicate that the residence time of the TMG groundwater in this area is decadal in age with a probable maximum upper limit of ∼40years. This residence time is probably a reflection of the slow flow rate through the fractured rock aquifer and hence indicates that the interconnectedness of the fractures is the most important factor controlling groundwater flow. The short residence time of the groundwater suggest that recharge to the springs and the Table Mountain Group Aquifer as a whole is vulnerable to climate change and reductions in regional precipitation. Any plans for large scale abstraction to supplement the City of Cape Town water supply would need to factor this into models of maximum sustainable yield.

Spatial Contrast of Geographically Induced Rainfall Observed by TRMM PR

Abstract In this study, the spatial variability in precipitation at a 0.1° scale is investigated using long-term data from the Tropical Rainfall Measuring Mission Precipitation Radar. Marked regional heterogeneities emerged for orographic rainfall on characteristic scales of tens of kilometers, high concentrations of small-scale systems (&amp;lt;10 km) over alpine areas, and sharp declines around mountain summits. In detecting microclimates, an additional concern is suspicious echoes observed around certain geographical areas with relatively low rainfall. A finescale land–river contrast can be extracted in the diurnal behavior of rainfall in medium-scale systems (10–100 km), corresponding to the course of the Amazon River. In addition, rainfall enhancement over small islands (0.1°–1°) was identified in terms of the storm scale. Even 0.1°-scale flat islands experience more rainfall than the adjacent ocean, primarily as a result of localized small or moderate systems. By contrast, compared with small islands, high-impact large-scale systems (&amp;gt;100 km) result in more rainfall over the adjacent ocean. Finescale hourly data represented the abrupt asymmetric fluctuation in rainfall across the coastline in the tropics and subtropics (30°S–30°N). Significant diurnal modulations in the rainfall due to large-scale systems are found over tropical offshore regions of vast landmasses but not over small islands or in the midlatitudes between 30° and 36°. Rainfall enhancement over small tropical islands is generated by abundant afternoon rainfall, which results from medium-scale storms that are regulated by the island size and inactivity of rainfall over coastal waters.