Orographic Rainfall Enhancement Research Articles

Orographic enhancement of rainfall over the Congo Basin

AbstractThe Congo rainforest located in central equatorial Africa is an important, yet understudied part of the globe surrounded by complex orographic features. A primitive understanding of precipitation processes such as mesoscale convective dynamics magnifies uncertainties in the future climate projections of the hydrological cycle over the Congo. Furthermore, the effects of orography, which is an important forcing for convection and precipitation, are poorly resolved by climate models, and ill‐conceptualized over the Congo. To address this knowledge gap, perturbed orographic forcing experiments are conducted using the Weather Research and Forecasting (WRF) mesoscale numerical model in a high‐resolution convection‐permitting model setup. The model simulated selected dates in November 2014. The thunderstorms and rainfall simulated in the control run for the case study analyzed in this article compared reasonably well to satellite‐derived brightness temperature and rainfall data. The results from this case study show that the dynamical impact of increasing the height of the East African Highlands is the blocking of the lower‐tropospheric tropical easterlies. This weakening of the lower‐tropospheric zonal winds increases the windshear over the Congo Basin resulting in slower propagating, more intense mesoscale convective systems with enhanced rainfall.

Meteorological conditions leading to the 2015 Salgar flash flood: lessons for vulnerable regions in tropical complex terrain

Abstract. Flash floods are a recurrent hazard for many developing Latin American regions due to their complex mountainous terrain and the rainfall characteristics in the tropics. These regions often lack the timely and high-quality information needed to assess, in real time, the threats to the vulnerable communities due to extreme hydrometeorological events. The systematic assessment of past extreme events allows us to improve our prediction capabilities of flash floods. In May 2015, a flash flood in the La Liboriana basin, municipality of Salgar, Colombia, caused more than 100 casualties and significant infrastructure damage. Despite the data scarcity, the climatological aspects, meteorological conditions, and first-order hydrometeorological mechanisms associated with the La Liboriana flash flood, including orographic intensification and the spatial distribution of the rainfall intensity relative to the basin's geomorphological features, are studied using precipitation information obtained using a weather radar quantitative precipitation estimation (QPE) technique, as well as from satellite products, in situ rain gauges from neighboring basins, quantitative precipitation forecasts (QPFs) from an operational Weather Research and Forecasting Model (WRF) application, and data from reanalysis products. The La Liboriana flash flood took place during a period with negative precipitation anomalies over most of the country as a result of an El Niño event. However, during May 2015, moist easterly flow towards the upper part of La Liboriana caused significant orographic rainfall enhancement. The overall evidence shows an important role of successive precipitation events in a relatively short period and of orography in the spatial distribution of rainfall and its intensification as convective cores approached the steepest topography. There were three consecutive events generating significant rainfall within the La Liboriana basin, and no single precipitation event was exceptionally large enough to generate the flash flood, but rather the combined role of precedent rainfall and the extreme hourly precipitation triggered the event. The results point to key lessons for improving local risk reduction strategies in vulnerable regions with complex terrain.

Evaluation and Intercomparison of High-Resolution Satellite Precipitation Estimates—GPM, TRMM, and CMORPH in the Tianshan Mountain Area

With high resolution and wide coverage, satellite precipitation products like Global Precipitation Measurement (GPM) could support hydrological/ecological research in the Tianshan Mountains, where the spatial heterogeneity of precipitation is high, but where rain gauges are sparse and unevenly distributed. Based on observations from 46 stations from 2014–2015, we evaluated the accuracies of three satellite precipitation products: GPM, Tropical Rainfall Measurement Mission (TRMM) 3B42, and the Climate Prediction Center morphing technique (CMORPH), in the Tianshan Mountains. The satellite estimates significantly correlated with the observations. They showed a northwest–southeast precipitation gradient that reflected the effects of large-scale circulations and a characteristic seasonal precipitation gradient that matched the observed regional precipitation pattern. With the highest correlation (R = 0.51), the lowest error (RMSE = 0.85 mm/day), and the smallest bias (1.27%), GPM outperformed TRMM and CMORPH in estimating daily precipitation. It performed the best at both regional and sub-regional scales and in low and mid-elevations. GPM had relatively balanced performances across all seasons, while CMORPH had significant biases in summer (46.43%) and winter (−22.93%), and TRMM performed extremely poorly in spring (R = 0.31; RMSE = 1.15 mm/day; bias = −20.29%). GPM also performed the best in detecting precipitation events, especially light and moderate precipitation, possibly due to the newly added Ka-band and high-frequency microwave channels. It successfully detected 62.09% of the precipitation events that exceeded 0.5 mm/day. However, its ability to estimate severe rainfall has not been improved as expected. Like other satellite products, GPM had the highest RMSE and bias in summer, suggesting limitations in its way of representing small-scale precipitation systems and isolated deep convection. It also underestimated the precipitation in high-elevation regions by 16%, suggesting the difficulties of capturing the orographic enhancement of rainfall associated with cap clouds and feeder–seeder cloud interactions over ridges. These findings suggest that GPM may outperform its predecessors in the mid-/high-latitude dryland, but not the tropical mountainous areas. With the advantage of high resolution and improved accuracy, the GPM creates new opportunities for understanding the precipitation pattern across the complex terrains of the Tianshan Mountains, and it could improve hydrological/ecological research in the area.

Scoping a field experiment: error diagnostics of TRMM precipitation radar estimates in complex terrain as a basis for IPHEx2014

Abstract. A diagnostic analysis of the space–time structure of error in quantitative precipitation estimates (QPEs) from the precipitation radar (PR) on the Tropical Rainfall Measurement Mission (TRMM) satellite is presented here in preparation for the Integrated Precipitation and Hydrology Experiment (IPHEx) in 2014. IPHEx is the first NASA ground-validation field campaign after the launch of the Global Precipitation Measurement (GPM) satellite. In anticipation of GPM, a science-grade high-density raingauge network was deployed at mid to high elevations in the southern Appalachian Mountains, USA, since 2007. This network allows for direct comparison between ground-based measurements from raingauges and satellite-based QPE (specifically, PR 2A25 Version 7 using 5 years of data 2008–2013). Case studies were conducted to characterize the vertical profiles of reflectivity and rain rate retrievals associated with large discrepancies with respect to ground measurements. The spatial and temporal distribution of detection errors (false alarm, FA; missed detection, MD) and magnitude errors (underestimation, UND; overestimation, OVR) for stratiform and convective precipitation are examined in detail toward elucidating the physical basis of retrieval error. The diagnostic error analysis reveals that detection errors are linked to persistent stratiform light rainfall in the southern Appalachians, which explains the high occurrence of FAs throughout the year, as well as the diurnal MD maximum at midday in the cold season (fall and winter) and especially in the inner region. Although UND dominates the error budget, underestimation of heavy rainfall conditions accounts for less than 20% of the total, consistent with regional hydrometeorology. The 2A25 V7 product underestimates low-level orographic enhancement of rainfall associated with fog, cap clouds and cloud to cloud feeder–seeder interactions over ridges, and overestimates light rainfall in the valleys by large amounts, though this behavior is strongly conditioned by the coarse spatial resolution (5 km) of the topography mask used to remove ground-clutter effects. Precipitation associated with small-scale systems (< 25 km2) and isolated deep convection tends to be underestimated, which we attribute to non-uniform beam-filling effects due to spatial averaging of reflectivity at the PR resolution. Mixed precipitation events (i.e., cold fronts and snow showers) fall into OVR or FA categories, but these are also the types of events for which observations from standard ground-based raingauge networks are more likely subject to measurement uncertainty, that is raingauge underestimation errors due to undercatch and precipitation phase. Overall, the space–time structure of the errors shows strong links among precipitation, envelope orography, landform (ridge–valley contrasts), and a local hydrometeorological regime that is strongly modulated by the diurnal cycle, pointing to three major error causes that are inter-related: (1) representation of concurrent vertically and horizontally varying microphysics; (2) non-uniform beam filling (NUBF) effects and ambiguity in the detection of bright band position; and (3) spatial resolution and ground-clutter correction.

Kernel-Based Mapping of Orographic Rainfall Enhancement in the Swiss Alps as Detected by Weather Radar

In this paper, we develop a data-driven methodology to characterize the likelihood of orographic precipitation enhancement using sequences of weather radar images and a digital elevation model (DEM). Geographical locations with topographic characteristics favorable to enforce repeatable and persistent orographic precipitation such as stationary cells, upslope rainfall enhancement, and repeated convective initiation are detected by analyzing the spatial distribution of a set of precipitation cells extracted from radar imagery. Topographic features such as terrain convexity and gradients computed from the DEM at multiple spatial scales as well as velocity fields estimated from sequences of weather radar images are used as explanatory factors to describe the occurrence of localized precipitation enhancement. The latter is represented as a binary process by defining a threshold on the number of cell occurrences at particular locations. Both two-class and one-class support vector machine classifiers are tested to separate the presumed orographic cells from the nonorographic ones in the space of contributing topographic and flow features. Site-based validation is carried out to estimate realistic generalization skills of the obtained spatial prediction models. Due to the high class separability, the decision function of the classifiers can be interpreted as a likelihood or susceptibility of orographic precipitation enhancement. The developed approach can serve as a basis for refining radar-based quantitative precipitation estimates and short-term forecasts or for generating stochastic precipitation ensembles conditioned on the local topography.

Winter daily precipitation variability over Cumbria, Northwest England

Daily precipitation totals at 55 sites were used to investigate geographic variability in winter (DJF) rainfall over Cumbria, NW England, over an 11-year period. Winter is the wettest season (>800 mm in the mountainous Lake District), with rainfall mechanisms closely linked to North Atlantic forcing. The Lamb weather type catalogue was used to identify rainfall distributions under different wind directions. Precipitation magnitude over Cumbria is much more sensitive to a change in wind direction than the geographic pattern in rainfall, with southwesterly (easterly) winds producing the highest (lowest) spatially averaged daily rainfall totals of 8.2 mm (0.6 mm). S-mode principal components analysis was used to identify the main patterns of precipitation variability. Three principal components (PCs) were retained as being statistically significant (cumulative explained variance for unrotated PCs = 84.3%), with a correlated PC structure (direct oblimin rotation) best describing the spatial variance in rainfall. PC 1 has a very high index of strength (variance measure = 40.9), indicating that there is one dominant rainfall pattern. PC 1 shows a gradient between wetter conditions in southwest Cumbria and over the central Lake District and drier conditions in NE Cumbria, and is usually caused by active zonal west to southwest flows. Almost of equal importance to PC 1 is PC 3 (variance measure = 39.3), which has a more uniform rainfall distribution than PC 1 and is usually caused by fronts stalling over the region. PC 2, which shows an east to west decline in rainfall totals, is much less important than PCs 1 and 3 (variance measure = 18.6). PC 2’s rainfall pattern can be caused by easterly flows with high pressure over Scandinavia and low pressure over the Continent, or by strong southwesterly flows, with depressions often centred over Scotland. Finally, cluster analysis was carried out to identify precipitation regions for all days and for each wind direction. Clusters were found to be largely stable to changes in wind direction, with stations in the central Lake District often clustered together, thus highlighting the importance of orographic enhancement of rainfall in this region.

Rain on small tropical islands

[1] A high-resolution rainfall climatology based on observations from the Tropical Rainfall Measuring Mission's Precipitation Radar (PR) instrument is used to evaluate the influence of small tropical islands on climatological rainfall. Islands with areas between one hundred and several thousand km2 are considered in both the Indo-Pacific Maritime Continent and Caribbean regions. Annual mean climatological (1997–2007) rainfall over each island is compared with that over the surrounding ocean region, and the difference is expressed as a percentage. In addition to total rainfall, rain frequency and intensity are also analyzed. Results are stratified into two 12 h halves of the diurnal cycle as well as eight 3 h periods, and also by a measure of each island's topographic relief. In both regions, there is a clear difference between larger islands (areas of a few hundred km2 or greater) and smaller ones. Both rain frequency and total rainfall are significantly enhanced over larger islands compared to the surrounding ocean. For smaller islands the enhancement is either negligibly small, statistically insignificant, or, in the case of Caribbean rain frequency, negative. The enhancement in total rainfall over larger islands is partly attributable to greater frequency and partly to greater intensity. A diurnal cycle in island enhancement is evident in frequency but not intensity, except over small Caribbean islands where the converse is true. For the larger islands, higher orography is associated with greater rainfall enhancements. The orographic effect is larger (percentagewise) in the Caribbean than in the Maritime Continent. Orographic precipitation enhancement manifests more strongly as increased frequency of precipitation rather than increased intensity and is present at night as well as during the day. The lack of a clear diurnal cycle in orographic enhancement suggests that much of the orographic rainfall enhancement is attributable to mechanically forced upslope flow rather than elevated surface heating.

Assessing satellite-based precipitation estimates in the Southern Appalachian mountains using rain gauges and TRMM PR

Abstract. A study was performed using the first full year of rain gauge records from a newly deployed network in the Southern Appalachian mountains. This is a region characterized by complex topography with orographic rainfall enhancement up to 300% over small distances (<8 km). Rain gauge observations were used to assess precipitation estimates from the Precipitation Radar (PR) on board of the TRMM satellite, specifically the TRMM PR 2A25 precipitation product. Results show substantial differences between annual records and isolated events (e.g. tropical storm Fay). An overall bias of −27% was found between TRMM PR 2A25 rain rate and rain gauge rain rates for the complete one year of study (−59% for tropical storm Fay). Besides differences observed for concurrent observations by the satellite and the rain gauges, a large number of rainfall events is detected independently by either one of the observing systems alone (rain gauges: 50% of events are missed by TRMM PR; TRMM PR: 20% of events are not detected by the rain gauges), especially for light rainfall conditions (0.1–2mm/h) that account for more than 80% of all the missed satellite events. An exploratory investigation using a microphysical model along with TRMM reflectivity factors at selected heights was conducted to determine the shape of the drop size distribution (DSD) that can be applied to reduce the difference between TRMM estimates and rain gauge observations. The results suggest that the critical DSD parameter is the number concentration of very small drops. For tropical storm Fay an increase of one order of magnitude in the number of small drops is apparently needed to capture the observed rainfall rate regardless of the value of the measured reflectivity. This is consistent with DSD observations that report high concentrations of small and/or midsize drops in the case of tropical storms.

Relationship between Orographic Enhancement of Rainfall Rate and Movement Speed of Radar Echoes: Case Study of Typhoon 0709

It is well known that rainfall rate is enhanced over mountains due to orographic uplifting. It would be beneficial to rainfall nowcasting if the intensity of the orographic enhancement could be estimated using simple parameters. In the present study, we found a clear relationship between orographic enhancement of the rainfall rate and the movement speed of radar echoes in a case study of rainfall over mountains in the southwestern area of the Kanto District in Japan during Typhoon 0709, by using rainfall data derived from X-band polarimetric radar. The increasing rate of rainfall rate per unit altitude (ΔR/ΔH) showed a positive correlation (r = 0:95) with the movement speed of radar echoes (V) when V > 10 m s-1. Such a correlation suggests that V is an effective parameter for quickly estimating the orographic enhancement of rainfall, however, more case studies are required before it can be used in practical applications.

Geologic, geomorphic, and meteorological aspects of debris flows triggered by Hurricanes Frances and Ivan during September 2004 in the Southern Appalachian Mountains of Macon County, North Carolina (southeastern USA)

In September 2004, rain from the remnants of Hurricanes Frances and Ivan triggered at least 155 landslides in the Blue Ridge Mountains of North Carolina. At least 33 debris flows occurred in Macon County, causing 5 deaths, destroying 16 homes, and damaging infrastructure. We mapped debris flows and debris deposits using a light-detecting and ranging digital elevation model, remote imagery and field studies integrated in a geographic information system. Evidence of past debris flows was found at all recent debris flow sites. Orographic rainfall enhancement along topographic escarpments influenced debris flow frequency at higher elevations. A possible trigger for the Wayah and fatal Peeks Creek debris flows was a spiral rain band within Ivan that moved across the area with short duration rainfall rates of 150–230 mm/h. Intersecting bedrock structures in polydeformed metamorphic rock influence the formation of catchments within structural–geomorphic domains where debris flows originate.

A meso-scale three-dimensional dynamical model of orographic rainfall

A mesoscale three-dimensional (3-D) dynamical model is presented to diagnose orographic rainfall, with particular reference to the Western Ghats (WG) in India. This model has two parts, namely, a dynamical part and a thermodynamical part. In the dynamical part the vertical velocity induced by a mesoscale elliptical orographic barrier has been computed using the perturbation technique. In the thermodynamical part rainfall intensity (RFI) has been computed using the computed vertical velocities, with the help of continuity of moisture and mass. The computed RFI has been compared with observed RFI as well as with that computed by 2-D model. The present study shows that during the southwest monsoon season (SWMS), orographic rainfall enhancement in the WG area appears to be solely due to the vertical shear of the basic flow and its variation with height. Stability appears to have very little influence on it. The spatial distribution of RFI across the barrier shows that there are four regions of maximum rainfall, one primary on the windward side behind the peak of the barrier and three secondary on the leeward side. The symmetry in the locations of these secondary rainfall maxima appears to be critically dependent on the component of basic flow parallel to the major ridge axis of the barrier.

Rainfall Amount, Intensity, Duration, and Frequency Relationships in the Mae Chaem Watershed in Southeast Asia

Abstract A dense tipping-bucket rain gauge network was established in the Mae Chaem watershed in the mountains of northwestern Thailand as part of the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment-Tropics (GAME-T). Investigations of rainfall amounts, intensities, durations, and frequencies in the rainy season revealed strong orographic rainfall enhancement in the region. The larger amount of high-altitude rainfall was attributed to duration and frequency rather than intensity. Despite large rainfall variations, similar patterns were found in the two study years, 1998 and 1999.

Rainfall-frequency mapping for Greece

For the design of hydrotechnical projects in ungauged watersheds, the flood flow is estimated by various methods, which demand the estimation of rainfall of particular critical duration and return period. For medium-sized and large basins, the storms causing flood flows have usually duration larger than 24 hours. In this study daily rainfall data from 24 meteorological stations for the period 1950 to 1981 were used. These stations are evenly distributed over Greece. From these data, the rainfall depths for various durations were computed, i.e. 1 to 7 days. The Extreme Value I (Gumbel) theoretical distribution had the best fit to the data from other theoretical distributions and it was fitted to the maximum annual rainfall depths for various durations. As a result, the Depth-Duration_Frequency relationships for each station were estimated and mapped for Greece. Moreover, the analysis showed that the rainfall of various durations and return periods represents a certain percentage of the mean annual precipitation for hydrologicaly homogeneous areas of Greece. The homogeneous areas were identified through factor analysis of monthly precipitation data from 37 meteorological stations. The results of this study can be used for the estimation of rainfall at ungauged sites in medium and large watersheds but they should be applied with caution in mountainous areas. For the estimation of rainfall in these areas the climatic conditions of the region and the orographic enhancement of rainfall should be considered.

The Effect of Rainfall Duration and Intensity on Orographic Rainfall Enhancement in a Mountainous Area: A Case Study in the Mae Chaem Wateshed, Thailand.

Under the umbrella of GAME-T (GEWEX Asian Monsoon Experiment-Tropics), an on-going project is monitoring rainfall in the Mae Chaem watershed in Thailand. The main purpose of this study is to estimate the temporal and spatial distribution of rainfall in the mountainous area of the upper Chao Phraya River basin, in order to validate rainfall-runoff models. Thirteen rain gauges were installed at altitudes from about 380 to 2, 500m above sea level (a.s.l.) in the Mae Chaem watershed, which has an area of 3, 853km2. Data from these rain gauges allowed us to analyze the rainfall record with a fine time resolution (one second) from June to November 1998. In this watershed, it is clear that the amount of rainfall varies with elevation throughout the rainy season. We used two methods to investigate its effect on orographic rainfall enhancement, and calculated the duration and mean intensity of rainfall using two formulae. The effects of orographic rainfall enhancement vary with the time interval or mean time used for the calculations. It is clear that the amount of rainfall is correlated with the duration of rainfall, regardless of the method of calculation, whereas the amount of rainfall is not correlated with the mean rainfall intensity. In this mountainous area, the change in the amount of rainfall with altitude is correlated with the duration of rainfall and is independent of the mean rainfall intensity.

孤立した島の風下におけるバンド状対流雲システムの降水強化過程

During the Baiu period in 1996 the enhancement of rainfall in a slow southward-moving band-shaped convective cloud system was observed in the downwind side of Yaku-shima Island by the PPI-radar of Tanega-shima Meteorological Observatory and in some cases the dual-Doppler radar system of Nagoya University. Common atmospheric conditions in these cases were strong low-level west-southwesterly winds and stability which led to a Froude number between 0.3 and 0.7. Mainly on the basis of a case study on the radar-echo band observed on June 22 and numerical simulation on airflows modified by the orographic effect of Yaku-shima Island, the orographic enhancement of rainfall in the downwind side of the island is considered to have been caused by the combination of the following processes. (1) Convective clouds in the slow southward-moving system developed in the zone of low-level horizontal convergence intensified due to winds modified by the orographic effect of the island and they produced intensive rainfall in the downwind side of the island. (2) New convective clouds formed and developed in the zone of low-level horizontal convergence over the sea north of Yaku-shima Island, which was caused by the confluence of ambient winds around the island and winds modified by its orographic effect. The line of convective clouds which formed to the southeastern side of the slow southward-moving system mentioned in (1) produced rainfall in its downwind side. (3) In the slow southward-moving system, precipitation particles in the new cloud line are provided to convective clouds by ambient winds which turned around the southern end of the island. This process was observed as the merging of the two band-shaped radar-echoes and would have caused the efficient formation of precipitation in the downwind side of the island.

Distribution of precipitation and wet deposition around an Island mountain in South‐West Poland

AbstractA comparison has been made between measurements of precipitation intensity and chemical composition and a three‐dimensional model of airflow and orographic rainfall enhancement. the ‘seeder‐feeder’ effect is shown to be of importance in rainfall distribution, wet deposition of pollutants and the modification of the chemical composition of rainfall over an isolated island mountain. the results show increases in rainfall of between 30% and 83% over the hill, with the smallest increase corresponding to the presence of a strong inversion layer. the peak rainfall enhancement occurred between 0.5 and 2 km downstream of the summit.The measured concentrations of major ions in the feeder‐cloud droplets were greater than those in seeder rain by factors of between two and five. the scavenging of polluted cap cloud by rain drops resulted in an increase in wet‐deposition rate by factors of between three and five, with the peak deposition occurring approximately 2 km downstream of the hill summit.

Airflow over the Kii Peninsula and its Relation to the Orographic Enhancement of Rainfall.

In order to study the orographic effect on the rainfall distribution in the Kii peninsula, airflow over a three-dimensional mountains is simulated by use of a non-hydrostatic model and compared with the observed distribution of rainfall. Numerical experiments for simplified orography show that low-level updraft areas have two peaks in weak wind cases. The first peak is on the windward slope of the mountain, while the secondary peak is over the sea, which is associated with the low-level blocking by the mountain. Locations of both updraft areas shift upstream as wind velocity decreases, while in cases of stronger wind or less atmospheric stability, no secondary peak appears over the sea. The characteristic curvature of the orography of the Kii peninsula intensifies the low-level blocking in the case of southeasterly environmental wind. Heavy rainfall events in the Kii peninsula in 1985 are analyzed and it is confirmed that 1) the rainfall amount in each event at Owase tends to be higher than that at Shionomisaki, and 2) heavy rains occur at Owase when low-level synoptic wind is east-southeasterly to southerly. In numerical experiments for real orography of the Kii peninsula, low-level updrafts are found around Owase and the southern part of the Kii peninsula in easterly to southerly winds. The positions are well coincident with the positions of two maxima in mean annual rainfall distribution shown by Sakakibara and Takeda (1973). No secondary updraft occurs in the case of southwesterly to westerly winds. Numerical simulations using atmospheric conditions observed in heavy rain events in 1985 are conducted. The locations of simulated low-level updrafts correspond well with the observed heavy rain areas around Owase and the southern part of the Kii peninsula. In some cases, middle-level updraft areas associated with mountain waves are computed aloft above the observed heavy rain areas to the northeast of Owase. The experimental results are consistent with the empirical relation between the location of heavy-rain areas and wind speed: the sea-shore region in weak-wind type and the region deeper inland in strong-wind type. It is also suggested that low-level convergence due to the blocking effect of the mountains affects the orographic enhancement of the convective rain band which is often seen over the sea in the windward side of the Kii peninsula.

A Three‐Dimensional Model of Airflow and Orographic Rainfall Enhancement

AbstractA three‐dimensional model of airflow over hills has been used in conjunction with a two‐dimensional model of orographic rainfall enhancement. This development has allowed the accurate production of rainfall contour maps in areas of complex terrain. A comparison has been made, using a variety of atmospheric stability profiles, between the wind fields and rainfall distributions over a bell‐shaped hill and those over a two dimensional ridge of similar height. the results show that, in conditions of either stably stratified air or with an inversion layer aloft, three‐dimensional airflow effects can lead to much reduced rainfall over a bell‐shaped hill. It is shown that in such situations the use of a two‐dimensional approximation can introduce significant errors. an example is presented of the rainfall distribution in complex hill terrain over south Kintyre on the western coast of Scotland.

A three-dimensional model of airflow and orographic rainfall enhancement

A three-dimensional model of airflow and orographic rainfall enhancement

Orographic rainfall enhancement in the mountains of the Lake District and Snowdonia

Abstract A two-dimensional model of the seeder-feeder effect has been compared with rainfall data from a network of daily-read rain gauges located in the Lake District and the Snowdonian mountains. Two days in the winter of 1980, when persistent heavy orographic rainfall occurred, have been considered. The rain gauge data indicated that rainfall near the hill summits was enhanced by a factor of between 3.2 and 7.6. It was found that the model was capable of predicting accurately the areas of maximum rainfall but tended to underpredict the magnitude of the orographic rainfall enhancement. This was attributed to orographic effects on the seeding rainfall rates. Over certain topography three-dimensional airflow effects were thought to play an important role in determining the locations of the rainfall maxima. Using typical values for sulphate aerosol loading and the concentration of sulphate in seeding rain, the model predicted a greater deposition of sulphate over the hills accompanying the rainfall enhancement. Sulphate deposition was enhanced by a factor of approximately 4 relative to the coastal rates.