Radar and raingauge observations of orographic rain over south Wales

Hill, F. F.; Browning, K. A.; BADER, MJ

doi:10.1256/smsqj.45311

Cited by 53 publications

(48 citation statements)

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“…Daily weather forecasts produced by the 4 km model were extracted from the operational archive and cases of significant orographic rain were selected as those times when at least 100 (or 25 in the Lake District due to the smaller area) model grid-boxes with surface elevation above 300 m experienced a total precipitation rate of greater than 4 × 10 −4 kg m −2 s −1 (equivalent to about 34 mm day −1 or 1.4 mm h −1 ). From these times an attempt was made to identify cases of warm sector flow by applying selection criteria based on the synoptic settings listed in Table 1 of Hill et al (1981), who analysed observations during eight cases of orographic rain enhancement over South Wales. These criteria were that the low-level (850 hPa) flow: (1) was from the southwest quadrant, (2) had a wind-speed greater than 14 m s −1 , and, (3) had a wet bulb potential temperature of greater than 282.5 K. These types of cases were sought as they are the principle cause of flooding in England (Holgate, 1973), for which it is therefore most important to be able to correctly predict the amount and location of orographic rain.…”

Section: Selection Of Casesmentioning

confidence: 99%

“…Douglas and Glasspoole (1947) found that the heaviest orographic rainfalls are associated with the strong southwesterly winds in the warm sector of a depression, just ahead of the cold front, which bring warm, moist air of subtropical origin to the UK. Hill et al (1981) found that rain was heavy and continuous over the hills in the warm sector, but that the heaviest rain over the hills was usually associated with the passage of preexisting mesoscale precipitation areas. Holgate (1973) found that it was this type of prolonged heavy rainfall rather than thundery activity that is the principle cause of flooding for the hills of North Wales and North-West England.…”

Section: Introductionmentioning

confidence: 99%

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Sensitivity of orographic precipitation enhancement to horizontal resolution in the operational Met Office Weather forecasts

Smith

Vosper

Field

2012

Meteorological Applications

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Rain gauge observations show that when averaged over a large number of cases of frontal systems passing over the UK, strong orographic rain enhancement occurs on the lee slopes of the first hills encountered by the southwesterly flow in the warm sector. The operational forecasts using 1.5 km grid spacing produced realistic looking mean rainfall patterns over the Lake District and Wales, with an area-averaged rain accumulation error of less than 2%. Model-level rain rates increase with decreasing altitude consistent with the seeder-feeder mechanism. Increasing the horizontal grid spacing in the operational weather forecast model decreases the amount of rain produced over the hills, thereby reducing forecast accuracy. The area-averaged rain accumulations are 11-24% smaller than observed at 12 km grid spacing and 33-48% smaller than observed at 40 km grid spacing.In additional simulations of the 15 January 2011 case over the Lake District at 1.5 km grid spacing, replacing the orography with that used by the 12 and 40 km models reduced the area-averaged rain accumulations by 10 and 23% respectively. These changes were due to reduced cloud water and ice mixing ratios over the lower hills resulting in slower increases in rain rate with decreasing altitude. It is demonstrated that neglecting the horizontal advection of falling rain drops results in too much rain falling on the windward slope and not enough falling on the lee slopes.

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Section: Selection Of Casesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Sensitivity of orographic precipitation enhancement to horizontal resolution in the operational Met Office Weather forecasts

Smith

Vosper

Field

2012

Meteorological Applications

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“…The seeder-feeder effect was identified in the late 1970s and early 1980s as the mechanism responsible for orographic rain enhancement over the hills of South Wales in the UK (e.g. Browning et al, 1975;Hill et al, 1981), with hills sometimes becoming self-seeding due to the release of potential instability. Radar observations during eight case studies analyzed by Hill et al (1981) showed that 80% of the rain enhancement occurred in the lowest 1.5 km above the hills.…”

Section: Introductionmentioning

confidence: 99%

A parametrization of subgrid orographic rain enhancement via the seeder–feeder effect

Smith

Field

Shipway

et al. 2015

Quart J Royal Meteoro Soc

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Numerical models used for low‐resolution weather and climate simulations are unable to produce enough orographic rain enhancement via the seeder–feeder effect during the passage of frontal systems when the orography is not sufficiently well represented. This article describes the development of a simple parametrization scheme to remedy this problem. The scheme estimates the increase in the grid‐box mean cloud liquid‐water mixing ratio that would result from vertical displacements produced by unresolved hills, accounting for regions where the actual surface is both higher and lower than the model surface. Using idealized two‐dimensional simulations of neutrally stratified flow over a Gaussian‐shaped ridge resolved to varying degrees, the new scheme was shown to increase the total (resolved plus parametrized) orographic water towards that produced over a well‐resolved hill, with good agreement for a wide range of hill sizes. This subgrid orographic water can be used by a microphysics parametrization scheme to enhance the growth of rain (or snow) via accretion. In simulations with a constant rain rate into the top of the domain, the surface orographic rain rate was increased over a small completely subgrid hill by up to 90%, giving much better agreement with the simulations over a well‐resolved hill. The scheme also increases the rain produced by flow over large subgrid mountains if the resolved cloud is able to produce some rain via autoconversion.

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“…These studies lent credence to the seeder-feeder theory, whilst also pointing to the additional role of potential instability accompanying some of these synoptic scenarios. Closer analysis of the evolution and three-dimensional structure of these orographically modulated events was enabled by the use of weather radar, as in the work of Hill et al (1981) for hills in south Wales, UK. By this time the observational results were being compared with early models of the seeder-feeder mechanism (Storebo, 1975;Bader & Roach, 1977;Gocho, 1978).…”

Section: Introductionmentioning

confidence: 99%

Orographic influences during winter precipitation events on the Avalon Peninsula, Newfoundland

Banfield

Hudak

Thomson

2000

Meteorological Applications

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Precipitation enhancement over a low coastal hill in winter is demonstrated for particular associations of synoptically determined onshore airflow and local geography encountered over southeastern Newfoundland. Four such cases, involving a mixture of pre‐warm‐frontal precipitation types at surface temperatures just below freezing, are analysed using comparative surface gauge records from sites at the coast and hill summit and detailed volume scan data from a Doppler radar. Whilst precipitation at the hill summit and upwind coast was of similar overall duration in each case, the surface rates on the hilltop exceeded those at the coast by 1.0–4.0 mm h−1 during different stages of the events. Analysis of the Doppler reflectivity patterns reveals that intensities are especially enhanced near the windward hill crest, supporting the observed association of greatest enhancement with a strong local upslope wind component exceeding 20 m s−1. In the majority of these cases the enhancement is maintained primarily by the ‘seeder–feeder’ mechanism, which appears to be accelerated during precipitation transitions and with the surface warm front 120–150 km distant; however, a preliminary phase of enhancement due to topographically induced uplift of stable pre‐frontal air is also recognised in one case. Copyright © 2000 Royal Meteorological Society

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Radar and raingauge observations of orographic rain over south Wales

Cited by 53 publications

References 0 publications

Sensitivity of orographic precipitation enhancement to horizontal resolution in the operational Met Office Weather forecasts

Sensitivity of orographic precipitation enhancement to horizontal resolution in the operational Met Office Weather forecasts

A parametrization of subgrid orographic rain enhancement via the seeder–feeder effect

Orographic influences during winter precipitation events on the Avalon Peninsula, Newfoundland

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