Seeder-feeder Mechanism Research Articles

A SHORT-TERM RAINFALL PREDICTION TAKING INTO CONCIDERATION NONLINEAR EFFECT OF NON-OROGRAPHIC RAINFALL ON ORGRAPHIC RAINFALL

A nonlinear effect ofnon-orographic rainfall on orographic rainfall is introduced into a short-term rainfall predictionmethod which uses Tatehira's orographic rainfall model. In the previous paper, orographic rainfall is assumed tobe simply proportional to non-orographic rainfall. In the seeder-feeder mechanism, which is themost importantpart in the generation of orographic rainfall, ratio of capturingcloud drops by raindrops (by non-orographic rainfall) c is taking a vital role. However, c is not proportional to the non-or0graphic rainfall intensity. In this paper, therefore, c is assumed to be anonlinear function in terms of the non-orographic rainfall and a method ofsolving simultaneous equationscomposed of the nonlinear function and theconservation equation of clouddrop is proposed. As a result, it is found that, when the nonlinear effect is taken into account, computedorographic rainfall becomesmuch smaller and that assumed depth of atmospheric layer ofhundred meters is not enough. Therefore, the computation method is further modified so thatorographic rainfall can be generated frommulti layers.

Small‐scale variability of orographic precipitation in the Alps: Case studies and semi‐idealized numerical simulations

AbstractThis study presents high‐resolution numerical simulations in order to investigate the small‐scale precipitation variability in a north Alpine region with an extraordinarily high density of operational rain‐gauge stations. Some of the simulations have been conducted with realistic analysis data, considering two heavy‐precipitation cases that occurred in July and August 2005. Both cases were related to cyclonic northeasterly flow and exhibited a small‐scale precipitation distribution differing substantially from the climatological mean. Most notably, a station lying on the southwestern flank of a major mountain massif recorded precipitation amounts far exceeding all surrounding stations in both cases, although its precipitation climatology does not show outstanding values. Despite some quantitative differences, all real‐case simulations exhibit a precipitation pattern consistent with the observed one. They indicate that the small‐scale precipitation variability, including the unusual local maximum, was dominated by the classical seeder–feeder mechanism combined with downstream drift of the precipitation hydrometeors.In addition, a large number of semi‐idealized simulations have been performed, combining realistic topography with synthetic large‐scale flow conditions. Their main purpose is a systematic investigation of the factors controlling the structure and amplitude of the small‐scale precipitation variability. It is found that the ambient wind direction has the largest impact on the small‐scale precipitation patterns. Selecting a 700 hPa wind direction that differs by about 60° from the actual one yields a precipitation pattern that is completely uncorrelated with the observed data. The pattern correlation with the observed data is also degraded when the ambient wind speed is moved away from its real value, but the impact is not as large as for the wind direction. A less systematic impact is found for temperature, or freezing level, but the most realistic precipitation pattern is still obtained when the specified values approximately match those observed in the real cases. Interestingly, when choosing large‐scale flow conditions that approximately match the real ones, the pattern correlation with the observed rain‐gauge data becomes in both cases at least as high as for the best real‐case simulation. Copyright © 2007 Royal Meteorological Society

Radar evidence of orographic enhancement due to the seeder feeder mechanism

AbstractThere are many cases where observed orographic enhancement has been plausibly attributed to the seeder feeder mechanism. To date, however, there have been few examples of radar data that clearly illustrate the concept of process, in the manner that the data presented here do. In this case study, vertical pointing radar data collected on the windward side of New Zealand's Southern Alps show two separated precipitating systems, with shallow rainfall enhanced by snowfall from aloft. Simple analysis of the synoptic conditions and rainfall data showed that this interaction did result in orographic enhancement. Further evidence of a seeder feeder event is given by simultaneous scanning radar data, which show a marked increase in raining area at the onset of the snowfall. Copyright © 2005 Royal Meteorological Society

AN INVESTIGATION OF MONSOON RAINFALL OVER A TROPICAL MOUNTAIN IN SOUTHEAST ASIA USING REGIONAL CLIMATE MODEL

Since 1997, rainfall has been measured by using tipping-bucket rain gauges in a mountainous area as a part of GAME-Tropics (the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME)). The fact that a larger amount of rainfall was recorded at a high altitude was attributed to duration and frequency rather than intensity. After investigating orographic precipitation using a regional climate model, the duration and frequency of rainfall at high altitudes were found to be caused by two mechanisms: 1) convective clouds over mountains are activated in the evening, and 2) stratus formed by radiative cooling or heating at the tops of clouds and low-level orographically triggered convective clouds cause a seeder-feeder mechanism. The results of a numerical simulation were quite consistent with the measurements acquired from the tipping-bucket rainfall gauges.

Spatial variations of wet deposition rates in an extended region of complex topography deduced from measurements of 210Pb soil inventories

The radionuclide 210Pb derived from gaseous 222Rn present in the atmosphere becomes attached to the same aerosols as the bulk of the main pollutants sulphur and nitrogen. When scavenged from the atmosphere by precipitation, the 210Pb is readily attached to organic matter in the surface horizons of the soil. Inventories of 210Pb in soil can thus be used to measure the spatial variations in wet (or cloud) deposition due to orography averaged over many precipitation events (half-life of 210Pb is 22·3 year). Measurements of soil 210Pb inventories were made along a transect through complex terrain in the Scottish Highlands to quantify the orographic enhancement of wet deposition near the summits of the three mountains Ben Cruachan, Beinn Dorain and Ben Lawers, which, respectively, lie at distances of approximately 30, 55 and 80 km from the coast in the direction of the prevailing wind. The inventory of 210Pb on the wind-facing slopes of Ben Cruachan shows an increase with altitude that rises faster than the precipitation rate, which is indicative of seeder-feeder scavenging of orographic cloud occurring around the summit. Results for Beinn Dorain show a smaller rise with altitude whereas those for Ben Lawers give no indication of a rise. It is concluded that the seeder–feeder mechanism in regions of complex topology decreases in effectiveness as a function of distance inland along the direction of the prevailing wind.

High resolution studies of rainfall on Norfolk Island, Part III: A model for rainfall redistribution

A dense network of high resolution rain gauges has provided data on spatial scales of about 1.5 km and time scales of 15 sec on Norfolk Island, where the topography is dominated by a 300 m hill that is found to modify the spatial distribution of rainfall depending on wind direction. A correlation method is used to track coherent storm motion so as to identify the dominant wind direction and speed. Transects are selected across the island in the wind direction and the rainfall interpolated along each transect. It is found that there is often a decrease in rainfall upwind of the hill, followed by an increase downwind of the peak. The ratio of downwind to upwind catches increases linearly with wind speed. These observations do not accord with the often accepted explanation using a seeder–feeder mechanism. A simple wind drift model is proposed, in which raindrops follow trajectories modified by the perturbed wind flow over the hill. The transects are approximated by bell-shaped hill profiles. Analytic solutions for potential flow over a cylinder are then used to find a streamline with a bell shape similar to a topography transect through the hill and aligned with the wind direction. This streamline is used as the solid surface boundary, thereby giving a direct analytic flow field for the bell-shaped hill. A streamfunction is found for drop motion, allowing rainfall variations to be predicted. A parameterisation is developed that explains the shape and magnitude of the observed rainfall variations along a transect. Agreement between the rudimentary model and observations is found possible to within a few percent. This indicates that the wind drift process is viable as the dominating mechanism for determining rainfall distribution in the presence of low hills when low level moisture is absent.

台風9307の通過によって、大隅半島に発生した豪雨の解析

On the night of 9 August 1993, Typhoon 9307 passed near the southern part of Japan and caused heavy rainfall and many land-slide disasters across the Ohsumi Peninsula. A notable feature of this event is that the rainfall amount was very large on the lee side of the mountains. The 9-hour rainfall exceeded 150 mm at the greater part of the stations on the lee side, while it was only 80 mm at the station on the windward side. All the land-slide disasters occurred on the lee side. The heavy rainfall was mainly produced between the eyewall and the rainband of the typhoon. Radar echoes showed strong reflectivity over the mountains during the rainfall event. The rainfall increasing process was studied by using a two-dimensional numerical model. The results suggested that the seeder-feeder mechanism contributed to the rainfall enhancement on the lee side. Namely, the precipitating particles from the upper cloud grew over the mountains by accreting cloud water produced by the orographic lifting, and the grown particles were carried to the lee side by the strong wind of the typhoon. In addition, it was also suggested that the downdraft of the mountain wave made an additional contribution to the rainfall enhancement by transporting many ice particles to the lower layer.

The effect of orography on wet deposition in an industrial area

Two field experiments were carried out to observe the variation in wet deposition of ions caused by orography in a polluted region of the UK. the site chosen was Winter Hill which lies on the edge of the industrialised Mersey valley. Rain and cloud samples were collected daily and chemically analysed. Synoptic and meteorological data were used to classify individual rain events as frontal or convective. For the frontal events the altitude related increase in wet deposition was found to be entirely caused by the seeder-feeder mechanism (the scavenging of cap cloud by rain from above). the concentration of marine ions in the water scavenged from the cap cloud was found to be between five and six times that in the seeder rain whereas for anthropogenically produced ions it was about twice as great. the efficiency of the seeder-feeder mechanism is correspondingly affected. Convective events accounted for less than a quarter of the rainfall volume. However, this contained over half of the deposition of marine ions in the valley. Deposition of anthropogenically produced ions in convective events was small probably due to a lack of convection over land. A small decrease in deposition with altitude was observed for these events although this is expected to be a highly local effect. It is concluded that reliable predictions of the wet deposition of non-marine ions can be made on the basis of the seeder-feeder effect for northern and western regions of the UK. Elsewhere in the UK and for much of continental Europe wet deposition from convective rain should be taken into account.

Influence of air pollution on the foliar nutrition of conifers in Great Britain

The nutritional status of needles from Sitka spruce, Norway spruce and Scots pine in a total of 108 stands was assessed. There was little evidence of nutritional deficiency, although potassium levels were frequently quite low. Analysis of some heavy metals (lead and copper) failed to reveal any likely toxicity problems. Sulphur, nitrogen and iron levels in/on the foliage were all related to various measures of sulphur and nitrogen pollution, determined using improved deposition models that take into account cloud deposition and the seeder-feeder mechanism. The analysis strongly suggested that direct air pollution has a greater effect on sulphur, nitrogen and iron foliar analyses than indirect pollution (wet deposition). The relationships were identified for levels of pollution that were generally lower than those seen in traditional gradient studies.

Orographic rainfall over low hills and associated corrections to radar measurements

The horizontal and vertical distribution of orographic rainfall over ridges of height approximately 150 m was resolved by a dense rain gauge network and rainfall radar. Orographic enhancements of up to about 2 mm h −1 were found in conditions typical of wintertime warm fronts and warm sectors in the UK. The enhancement magnitude (averaged over a few hours) was found to depend partly upon the surface wind and the average background rainfall rate. The growth of orographic precipitation was observed in cloud layers of varying depth; in one case, significant enhancement occurred within cloud with a total depth of no more than 1500 m. Doubt is cast upon the applicability of some existing models of the seeder-feeder mechanism to cases such as this. In this and other respects, our conclusions differ from earlier findings. The observations highlight the difficulty in estimation of surface rainfall rate by radar if the beam is several hundred metres or more above the surface. If no attempt is made to correct for precipitation growth beneath the beam, serious underestimation of rainfall is likely, even in lowland regions with hills of about 100 m height.

A refined model of the influence of orography on the mesoscale distribution of extreme precipitation

Abstract This paper briefly summarizes the results of a four-year research project carried out for the Federal Ministry of Agriculture and Forestry in Vienna, Austria. The main objective was the development of a deterministic model for the estimation of the distribution of extreme precipitation in complex terrain. The orographic component of precipitation is conceptually divided into a part directly generated by dynamic uplift and a part originating from convective activity. The model for the first type of precipitation enhancement is based on the theory of linear two-dimensional air flow over mountains. A dynamically consistent flow field and explicit simulation of the seeder-feeder mechanism are the main improvements compared to an earlier version of this model (Haiden et al., 1990). The assessment of convective precipitation is based on a new parameterization using results of an analytical model of thermally induced upslope flow. A comparison of model results with observations shows reasonable agreement regarding the structure of the precipitation fields.

On the modelling of warm orographic rain by the seeder-feeder mechanism

A two-dimensional model of the seeder–feeder mechanism for the orographic enhancement of rain is presented. It satisfactorily reproduces observed rainfall rate distributions and is helpful in studying some aspects of the physics of orographic rain. The modelling approach is based on the principle of continuity of the water substance and on the two-dimensional theory of small perturbations of airflow over topographical ridges of modest dimensions. The model is significantly different from those found in the literature and is used to clarify two major points. Firstly, it emphasizes that there is a range of hill dimensions for which the seederfeeder mechanism plays a major role. That is, the model predicts that a change of regime occurs for larger hills (half length > 20 km) where the seeder-feeder mechanism is no longer dominant because of the possibility of rain development within the feeder cloud and also because the horizontal scale of rain drift becomes small compared with that of the topography. Secondly, it is also suggested that the difference of wind curvature below and above a low-level jet may be important in determining the intensity of orographic rain. The physical process behind the latter finding is closely linked to partial reflection of upward propagating gravity waves which may lead to ‘resonance’ of the orographically induced vertical velocity.

On the modelling of warm orographic rain by the seeder‐feeder mechanism

AbstractA two‐dimensional model of the seeder–feeder mechanism for the orographic enhancement of rain is presented. It satisfactorily reproduces observed rainfall rate distributions and is helpful in studying some aspects of the physics of orographic rain. The modelling approach is based on the principle of continuity of the water substance and on the two‐dimensional theory of small perturbations of airflow over topographical ridges of modest dimensions. The model is significantly different from those found in the literature and is used to clarify two major points. Firstly, it emphasizes that there is a range of hill dimensions for which the seederfeeder mechanism plays a major role. That is, the model predicts that a change of regime occurs for larger hills (half length > 20 km) where the seeder‐feeder mechanism is no longer dominant because of the possibility of rain development within the feeder cloud and also because the horizontal scale of rain drift becomes small compared with that of the topography. Secondly, it is also suggested that the difference of wind curvature below and above a low‐level jet may be important in determining the intensity of orographic rain. The physical process behind the latter finding is closely linked to partial reflection of upward propagating gravity waves which may lead to ‘resonance’ of the orographically induced vertical velocity.

The influence of altitude on wet deposition comparison between field measurements at great dun fell and the predictions of a seeder-feeder model

The influence of topography on rainfall rate and chemical composition has been investigated at Great Dun Fell in northern England. The measurements at eight different altitudes between 250 and 850 m above sea-level on the western slopes of Great Dun Fell (GDF) show, in the presence of a cap cloud on the hill and a west or southwest wind flow, a marked increase in both rainfall amount and concentrations of major ions with altitude. Three case studies of the variation in rainfall rate and chemical composition with altitude when the seeder-feeder mechanism was operating are described and compared with model predictions. Associated measurements of the cap cloud microphysics and chemistry were also made. The case studies show close agreement between model predictions and measured values for changes in chemical composition and wet deposition with altitude. This pattern of wet deposition is a frequent occurrence at GDF, but exceptions do occur, for example when the upstream flow is blocked or the wind speed is small, no increase in the concentration or rainfall amount was observed.

A model of wet deposition to complex terrain

A two dimensional model of the seeder-feeder mechanism of orographic rainfall enhancement has been developed. The model has been extended to include the deposition of aerosol material incorporated into the orographic feeder cloud by nucleation scavenging. Parameterizations of any changes in the concentration of SO 4 2− in the cloud due to chemical reactions have also been included. The model is used to predict the rainfall enhancement and SO 4 2− deposition over terrain consisting of two parallel ridges oriented perpendicular to the wind. A wide range of spatial scales has been used of up to 150 km. It is found that the patterns of rainfall enhancement and deposition are strongly dependent on the spatial scales, the atmospheric structure and the cloud chemistry.

A model of the orographic enhancement of snowfall by the seeder-feeder mechanism

A model has been constructed of the orographic enhancement of snowfall by the seeder-feeder mechanism. It is found that the collection of water from the feeder cloud is much more efficient than with rainfall under similar conditions, producing significantly larger enhancements of up to three times the seeder cloud precipitation rate. Over long hills growth is mostly by vapour diffusion, with riming dominating over shorter hills. It is found that the peak snowfall enhancement is strongly dependent on hill height, wind speed and feeder cloud depth over a long hill. These sensitivities are much weaker or are absent over short hills due to the large effects of wind drift.

Air motion and precipitation growth in a major snowstorm

A mesoscale analysis of the snowstorm of 8–9 January 1982 has been carried out using data from the U.K. weather radar network, the geostationary satellite Meteosat and other more conventional sources. More than 60cm of snow accumulated over S.E. Wales and neighbouring parts of England in association with an intense frontal zone. Much of the snow occurred during a period when synoptic forcing and orographic modulation were essentially steady. The depth of cloud and moist air was not great in the region of heavy snowfall in south Wales, but an orographically enhanced circulation on the mesoscale nevertheless led to efficient growth of precipitation through the seeder-feeder mechanism. The seeder cloud was generated within a shallow current of air which ascended above the front and which originated from low levels, probably mostly from the boundary layer, on the warm side of the front. Precipitation was initiated within this current repeatedly along a well-defined and remarkably stationary line just upwind of the south Wales coast. As the rising current travelled inland towards the E.N.E., convective generating cells embedded in it produced precipitation particles which fell into the underlying cold easterly flow. Considerable growth occurred as these particles descended through orographically enhanced feeder cloud, much of the growth occurring in the lowest kilometre or so.