Fog Occurrence Research Articles

Fog Prediction for Road Traffic Safety in a Coastal Desert Region

Modern weather prediction models use relatively high grid resolutions as well as sophisticated parametrization schemes for microphysical and other subgrid-scale atmospheric processes. Nonetheless, with these models it remains a difficult task to perform successful numerical fog forecasts since many factors controlling a particular fog event are not yet sufficiently simulated. Here we describe our efforts to create a mechanism that produces successful predictions of fog in the territory located on the north coast of the Arabian Peninsula. Our approach consists in the coupling of the one-dimensional PAFOG fog model with the three-dimensional WRF 3.0 (Weather Research and Forecast) modelling system. The proposed method allows us to construct an efficient operative road traffic warning system for the occurrence of fog in the investigated region. In total 84 historical situations were studied during the period 2008–2009. Moreover, results of operative day-by-day fog forecasting during January and February 2010 are presented. For the investigated arid and hot climate region the land-sea breeze circulation seems to be the major factor affecting the diurnal variations of the meteorological conditions, frequently resulting in the formation of fog.

MTSAT 적외채널과 AMSR 마이크로웨이브채널의 결합을 이용한 한반도 주변의 해무 탐지

Brightness temperature (BT) difference between sea fog and sea surface is small, because the top height of fog is low. Therefore, it is very difficult to detect sea fog with infrared (IR) channels in the nighttime. To overcome this difficulty, we have developed a new algorithm for detection of sea fog that consists in three tests. Firstly, both stratus and sea fog were discriminated from the other clouds by using the difference between BTs <TEX>$3.7{\mu}m$</TEX> and <TEX>$11{\mu}m$</TEX>. Secondly, stratus occurring at a level higher than sea fog was removed when the difference between cloud top temperature and sea surface temperature (SST) is smaller than 3 K. In this process, we used daily SST data from AMSR-E microwave measurements that is available even in the presence of cloud. Then, the SST was converted to <TEX>$11{\mu}m$</TEX> BT based on the regressed relationship between AMSR-E SST and MTSAT-1R <TEX>$11{\mu}m$</TEX> BT at 1733 UTC over clear sky regions. Finally, stratus was further removed by using the homogeneity test based on the difference in cloud top texture between sea fog and stratus. Comparison between the retrievals from our algorithm and that from Korea Meteorological Administration (KMA) algorithm, shows that the KMA algorithm often misconceived sea fog as stratus, resulting in underestimating the occurrence of sea fog. Monthly distribution of sea fog over northeast Asia in 2008 was derived from the proposed algorithm. The frequency of sea fog is lowest in winter, and highest in summer especially in June. The seasonality of the sea fog occurrence between East and West Sea was comparable, while it is not clearly identified over South Sea. These results would serve to prevent the possible occurrence of marine accidents associated with sea fog.

On the fog variability over south Asia

An increasing trend in fog frequencies over south Asia during winter in the last few decades has resulted in large economical losses and has caused substantial difficulties in the daily lives of people. In order to better understand the fog phenomenon, we investigated the climatology, inter-annual variability and trends in the fog occurrence from 1976 to 2010 using observational data from 82 stations, well distributed over India and Pakistan. Fog blankets large area from Pakistan to Bangladesh across north India from west to east running almost parallel to south of the Himalayas. An EOF analysis revealed that the fog variability over the whole region is coupled and therefore must be governed by some large scale phenomenon on the inter-annual time scale. Significant positive trends were found in the fog frequency but this increase is not gradual, as with the humidity, but comprises of two distinct regimes shifts, in 1990 and 1998, with respect to both mean and variance. The fog is also detected in ERA-Interim 3 hourly, surface and model level forecast data when using the concept of “cross-over temperature” combined with boundary layer stability. This fog index is able to reproduce the regime shift around 1998 and shows that the method can be applied to analyze fog over south Asia. The inter-annual variability seems to be associated with the wave train originating from the North Atlantic in the upper troposphere that when causing higher pressure over the region results in an increased boundary layer stability and surface-near relative humidity. The trend and shifts in the fog occurrence seems to be associated with the gradual increasing trend in relative humidity from 1990 onwards.

Improved microphysical parametrization of drizzle and fog for operational forecasting using the Met Office Unified Model

AbstractIn order to improve the forecasting of fog, visibility and drizzle within the Met Office Unified Model, three parametrizations are included in high‐resolution, limited‐area versions of the model. These are (i) improved rainfall speeds; (ii) linking aerosol to the autoconversion scheme; and (iii) a taper curve for cloud droplets near the surface. The results show a better representation of low‐visibility conditions in winter. There is a general reduction in drizzle and a better representation of drizzle depending on whether the air mass is clean or polluted. Summertime occurrences of fog are also reduced and are now under‐forecast, which will require a further investigation to correct. The package described in the paper has been made operational in the 4 and 1.5 km operational forecast versions of the Unified Model during the spring and summer of 2011, showing an improvement in model performance and a reduction in model error. This demonstrates the ability to improve operational NWP forecasts by coupling cloud and aerosol processes. Copyright © 2012 British Crown copyright, the Met Office. Published by John Wiley &amp; Sons Ltd.

Quantification of cloud water interception along the windward slope of Santa Cruz Island, Galapagos (Ecuador)

The Galapagos Archipelago is nearly devoid of freshwater resources, but during six months of the year, a fog layer covers the windward slopes of the main islands. In order to investigate the hydrological importance of this phenomenon, a monitoring network was set up on Santa Cruz Island, at the center of the archipelago. Meteorological parameters were monitored together with throughfall and stemflow at two stations: a first in a secondary forest at the lowest fringe of the fog layer (400ma.s.l.), and a second in shrub lands of the Galapagos National Park, at the center of the fog layer (650ma.s.l.). Cloud water interception was quantified from the wet canopy water budget, based on a modified Rutter-type canopy interception model. This methodology allowed the estimation of fog interception for short time intervals (15min) and avoided the subjective separation into individual rainfall events. Fog was found to be a negligible water input at the lower site, but was estimated at 26±16% of incident rainfall at the higher site. Wind was shown to enhance fog interception, but this alone could not explain the difference in fog catch between the two sites. Higher liquid water content and more frequent fog occurrence contributed to the difference as well. This study highlights that the presence of fog may induce a marked increase of net precipitation, but this effect is restricted to the summit areas exposed to winds, located in the center of the cloud belt.

Micro-climatic study and trend analysis of fog characteristics at IGI airport New Delhi using hourly data (1981-2005)

Indira Gandhi International (IGI) airport, New Delhi where near about 675 flights on an averagedepart and arrive daily, is highly susceptible to dense fog occurrences during the winter season. In the present paper, anattempt has been made for development of an intensity based fog climatological information system for December andJanuary based on hourly visibility data of 25-years (1981-2005) recorded at IGI airport. Variations and trends if any werealso analyzed along with their extreme years and dates of occurrences. Data since 1964 were also used to find climaticjumps in the trend which includes various higher visibilities of no fog conditions. Besides various vital fog climatologicalinformation generated through the present study for use in aviation, the most important finding is the alarming increasingtrend of the dense fog (&lt; 200m) occurrences in both the months up to as high as 10-20 times from 1960s in contrast tounusual drastic reduction of higher visibility hours to as low as one thirtieth to one fiftieth of hours which were observedin 1960s. Thus, finally making IGI airport, a unique airport in the world which hardly experiences good visibilityconditions (&gt;5000m) in both the months. By considering the unexpected huge annual growth of 30% in both air trafficand passengers that India including IGI has presently been experiencing against the global average of 6%, such visibilitytrend also confirms that present flight disruptions and passengers sufferings in winter will be aggravated more severely indays to come unless CAT-III ILS implemented fully. Finally, we have computed further number of consecutive hours,spell periodicity, most favorable climatological timing of fog onset and fog dispersal based on various intensities for usein aviation and fog forecasting.

Enhanced trimethylamine-containing particles during fog events detected by single particle aerosol mass spectrometry in urban Guangzhou, China

Trimethylamine (TMA) plays an important role in atmospheric chemistry, yet its pathway towards aerosol is not clear. We report in this study the measurement of TMA in submicron particles using a single particle aerosol mass spectrometer (SPAMS) in urban Guangzhou, China for the period of 30 April through 22 May, 2010. The number fraction of TMA-containing particles relative to total detected particle number dramatically increased within the occurrence of fogs. It changed from an average of ∼7% for clear days to ∼35% for the fog events. For particles with size larger than 0.5 μm, the fraction could even account up to ∼60%. Averaged relative intensity of TMA was correlated well with relative humidity, indicating the important role of aerosol water content in the gas-to-particle partitioning of TMA. During the fog events, number based size distribution of TMA-containing particles shifted towards larger mode, peaking at droplet mode (0.5–1.2 μm), corresponding to the build-up of aerosol mass, suggesting the significant mass transfer of TMA and other semi-volatile species from gas to aerosol. Results also reveal that most of TMA-containing particles also contained signals from ammonium, nitrate and sulfate. A much larger fraction of TMA-containing particles was found to contain nitrate during the fog events than on clear days. During the fog events, nitrate and sulfate were more strongly associated with TMA compared to ammonia, which was oppositely observed on clear days. These results indicate that gas-to-particle partitioning of TMA in urban Guangzhou occurs preferentially during fog processing. The study could help improve our understanding in fog processing and potential roles of TMA-containing particles in urban Guangzhou.

Predictores ambientales de la riqueza de especiesde plantas del bosque húmedo de montaña de México

El bosque humedo de montana, tambien conocido como bosque mesofilo de montana o cloud forest, esta definido y limitado por la presencia persistente de nubes y neblina que, ademas de la lluvia, capturan las gotas de agua que se condensan sobre la vegetacion. Se reconoce ampliamente que la humedad atmosferica es el principal factor ambiental que promueve la persistencia y distribucion de este tipo de vegetacion en el mundo. Sin embargo, se carecen de analisis correlativos entre riqueza de especies del bosque humedo de montana y los factores ambientales a escalas amplias. Es de suponer que tal riqueza tiene su base en la hipotesis correlativa con variables ambientales contemporaneas, principalmente con la triada altitud-clima-suelo. El objetivo fue analizar e identificar que variables ambientales tienen asociacion significativa y cuales son las mas importantes en la prediccion de la riqueza de especies del bosque humedo de montana. Los sitios con bosque humedo de montana se ubicaron en 61 celdas de 1o latitud × 1º longitud. Mediante sistemas de informacion geografica se generaron capas ambientales que incluyeron variables bioclimaticas, de estacionalidad climatica, un modelo digital de elevacion y variables edaficas. Los resultados indican que 58% de la variacion en la riqueza de especies del bosque humedo de montana se relaciona con cinco variables: la precipitacion pluvial de los meses húmedos del ano, la altitud, la evapotranspiracion real anual, la estacionalidad de la precipitacion y el carbono organico en los suelos. En el bosque humedo de montana de Mexico existe una asociacion consistente entre la riqueza de especies y las expresiones categóricas de la relacion agua-energia.

Development of intensity based fog climatological information system (daily and hourly) at IGI airport, New Delhi for use in fog forecasting and aviation

The main objective of the present paper is to make a microclimatological study of occurrence of fog of different intensities at Indira Gandhi International (IGI) airport, New Delhi which includes their date-wise climatological probabilities and their corresponding total number of hours of occurrence for 62-days of peak winter from 1st December to 31st January by using hourly visibility data for the period of 1981-2005. Their hourly climatology has been discussed separately for both months using same data for understanding their diurnal variations. Both the computations have been done to find most vulnerable periods with exact dates and timings when both duration and intensity of the fog are very high and hazardous for aviation. Corresponding 10-days and 3-hourly climatology of cumulative fog occurrences are computed to identify a period when fog related flight diversion risk is highest. For better understanding of their variability, dates of extreme hours of occurrences of a particular fog type amongst occurrences of all dates for the period during both months have also been documented. These climatological informations can be used by various airlines for planning flight operation and action for establishment of fog dissipation mechanism. Finally, fogprobability matrices of various intensities based on these climatological data have been presented with dates in first column and hours in the first row for all 62 days of December and January and for all 24 hours of each day giving date and hour wise climatological probability of their occurrences which can be used at IGI as climatological tool for forecasting of fog of various intensity and expected climatological period.

A method for simple and accurate estimation of fog deposition in a mountain forest using a meteorological model

[1] To apply a meteorological model to investigate fog occurrence, acidification and deposition in mountain forests, the meteorological model WRF was modified to calculate fog deposition accurately by the simple linear function of fog deposition onto vegetation derived from numerical experiments using the detailed multilayer atmosphere-vegetation-soil model (SOLVEG). The modified version of WRF that includes fog deposition (fog-WRF) was tested in a mountain forest on Mt. Rokko in Japan. fog-WRF provided a distinctly better prediction of liquid water content of fog (LWC) than the original version of WRF. It also successfully simulated throughfall observations due to fog deposition inside the forest during the summer season that excluded the effect of forest edges. Using the linear relationship between fog deposition and altitude given by the fog-WRF calculations and the data from throughfall observations at a given altitude, the vertical distribution of fog deposition can be roughly estimated in mountain forests. A meteorological model that includes fog deposition will be useful in mapping fog deposition in mountain cloud forests.

Analysis of the formation of fog and haze in North China Plain (NCP)

Abstract. North China Plain (NCP) is one of the most populated and polluted regions in China. During the recent years, haze and fog occur frequently and cause severely low visibility in this region. In order to better understand the impact of aerosol particles on the formation of haze and fog, a long-term record of haze and fog occurrences in the past 56 yr (from 1954–2009) over NCP is analyzed. The results show that there are rapid changes in the occurrences of haze and fog over NCP. The occurrences of haze and fog were low during 1970–1980, and reached a maximum during 1981–1998. After 1999, the occurrences of haze and fog slightly decreased. There was a nonlinear relationship between the occurrences of haze and fog. When the occurrence of haze was lower than 40 days yr−1, the occurrence of fog was strongly proportional to the occurrence of haze. However, when the occurrence of haze was high (larger than 75 days yr−1), the occurrence of fog was not sensitive to the occurrence of haze. In order to better understand the relationship between the occurrences of haze and fog as well as the effect of aerosol particles on the formation of haze and fog, an in-situ field experiment was conducted during a period with a mixed occurrence of haze and fog. The analysis of the experiment suggests that there were considerably high aerosol concentrations during the measurement period with an averaged aerosol number concentration of 24 000 cm−3. The measurement also shows that a large amount of aerosol particles can act as condensation nuclei to enhance the formation of fog droplets. As a result, a large amount of fog droplets (&gt;1000 cm−3) with small size (5–6 μm) were observed during the fog period, resulting in extremely low visibility (less than 100 m).

The Continued Reduction in Dense Fog in the Southern California Region: Possible Causes

Dense fog appears to be decreasing in many parts of the world, especially in western cities. Dense fog (visibility <400 m) is disappearing in the urban southern California area also. There the decrease in dense fog events can be explained mainly by declining particulate levels, Pacific sea surface temperatures (SST), and increased urban warming. Using hourly data from 1948 to the present, we looked at the relationship between fog events in the region and contributing factors and trends over time. Initially a strong relationship was suggested between the occurrence of dense fog and the phases of an atmosphere–ocean cycle: the Pacific Decadal Oscillation (PDO). However, closer analysis revealed the importance to fog variability of an increasing urban heat island and the amount of atmospheric suspended particulate matter. Results show a substantial decrease in the occurrence of very low visibilities (<400 m) at the two airport stations in close proximity to the Pacific Ocean, LAX (Los Angeles International) and LGB (Long Beach International). A downward trend in particulate concentrations, coupled with an upward trend in urban temperatures were associated with the decrease in dense fog occurrence at both LAX and LGB. LAX dense fog that reached over 300 h in 1950 dropped steadily, with 0 h recorded in 1997. Since 1997, there has been a slight recovery with both 2008 and 2009 recording over 30 h of dense fog at both locations. In this study we examine whether the upturn is a temporary reversal of the trend. To remove the urban effect, we also included fog data from Vandenberg Air Force Base (VBG), located in a relatively sparsely populated area approximately 200 km to the north of metropolitan Los Angeles. Particulates, urban heat island, and Pacific SSTs all seem to be contributing factors to the decrease in fog in southern California, along with large-scale atmosphere–ocean interaction cycles. Case studies of local and regional dense fog in southern California point to the importance of strong, low inversions and to a lesser contributor, Santa Ana winds. Both are associated with large-scale atmospheric circulation patterns, which have changed markedly over the period of studied. These changes point to continued decreases in dense fog in the region.

Characteristics of high impact weather and meteorological disaster in Shanghai, China

This paper discusses the characteristics of high-impact weather events based on available data during 1960–2009, including the frequency and extreme value of rainstorm, typhoon, thunderstorm, strong wind, tornado, fog, haze and hot days in Shanghai, China. The frequency and spatial distribution of meteorological disasters and their impacts on both human and property during 1984–2009 are also discussed. Examination of the frequency indicates a decreasing trend in the occurrence of typhoon, thunderstorm, strong wind, tornado and fog, and an increasing trend in the occurrence of rainstorm, haze and hot days. The number of casualties caused by meteorological disasters appears to show a slight decreasing trend while the value of direct economic loss is increasing slightly during 1984–2009, and the number of collapsed or damaged buildings and the area of affected crops have no significant trend in Shanghai. These results can be attributed to the great efforts for prevention and mitigation of meteorological disasters made by Shanghai government in recent 60 years. With global climate change, urbanization and rapid economic development, Shanghai has become more vulnerable to high-impact weather and meteorological disaster, especially precipitation extreme, summer high temperature, haze and typhoon, so more strategies of mitigation and/or adaptation of natural disasters are quite useful and necessary for local government and the public in the future.

Large-Scale Environmental Influences on the Onset, Maintenance, and Dissipation of Six Sea Fog Cases over the Yellow Sea

Sea fog is typically formed and developed under a set of favorable environmental conditions, which are associated with the station pressure changes, sea level pressure, winds, temperature, water vapor supply, and sea surface temperature. Understanding of these environmental factors during the evolution of a sea fog episode is crucial for forecasting the occurrence and severity of sea fogs over the ocean and adjacent coastal areas. In this study, the large-scale environment variability of six fog events over the Yellow Sea was investigated. It was realized in the present study that the northwest Pacific Ocean high (NPH) is vital to fog formation over the Yellow Sea. In our study, six fog cases can be basically divided into two types: (1) pressure-weakening type, (2) pressure-strengthening type. The former type happened in spring and the latter type in summer. Prevailing southerly winds, accompanied with the well-positioned NPH, may supply a large amount of warm water vapor for the fog formation and maintenance. The intensity of the air temperature inversion is stronger in summer cases than that in spring ones. The wind direction change from south to north and the unstable lower atmosphere may lead to fog’s dissipation. This study may provide a comprehensive understanding of sea fog’s onset, maintenance, and dissipation over the Yellow Sea.

A Fuzzy Logic Fog Forecasting Model for Perth Airport

Perth Airport is a major airport along the southwest coast of Australia. Even though, on average, fog only occurs about twelve times a year, the lack of suitable alternate aerodromes nearby for diversion makes fog forecasts for Perth Airport very important to long-haul international flights. Fog is most likely to form in the cool season between April and October. This study developed an objective fuzzy logic fog forecasting model for Perth Airport for the cool season. The fuzzy logic fog model was based on outputs from a high-resolution operational NWP model called LAPS125 that ran twice daily at 00 and 12 UTC, but fuzzy logic was employed to deal with the inaccuracy of NWP prediction and uncertainties associated with relationships between fog predictors and fog occurrence. The outcome of the fuzzy logic fog model is in one of the four categories from low to high fog risk as FM0, FM5, FM15 or FM30, intended to map to approximate fog probability of 0, 5, 15 and 30%, respectively. The model was found useful in its 5 year performance in the cool seasons between 2004 and 2008 and required little recalibration if mist was treated as if it were also a fog event in the skill evaluation. To generate an operational fog forecast for Perth Airport, the outcome of the fuzzy logic fog model was averaged with the outcomes of two other fog forecasting methods using a simple consensus approach. Fog forecast so generated is known as the operational consensus forecast. Skill assessment using frequency distribution diagram, Hansen and Kuiper skill score, and Relative Operating Characteristic curve showed that the operational consensus forecast outperformed all three individual methods. Out of the three methods, the fuzzy logic fog model ranked second. It performed better than the other objective method called GASM but worse than the subjective method which relied on forecaster’s subjective assessment. The skills of the fuzzy logic fog model can be further improved with the tuning of fuzzy functions. In addition, similar models can be customised for other airports. The study also suggested the use of the simple consensus approach to enhance forecasting skills for other stations or weather phenomena if there were two or more independent forecasting methods available.

Fog Simulations Based on Multi-Model System: A Feasibility Study

Accurate forecasts of fog and visibility are very important to air and high way traffic, and are still a big challenge. A 1D fog model (PAFOG) is coupled to MM5 by obtaining the initial and boundary conditions (IC/BC) and some other necessary input parameters from MM5. Thus, PAFOG can be run for any area of interest. On the other hand, MM5 itself can be used to simulate fog events over a large domain. This paper presents evaluations of the fog predictability of these two systems for December of 2006 and December of 2007, with nine regional fog events observed in a field experiment, as well as over a large domain in eastern China. Among the simulations of the nine fog events by the two systems, two cases were investigated in detail. Daily results of ground level meteorology were validated against the routine observations at the CMA observational network. Daily fog occurrences for the two study periods was validated in Nanjing. General performance of the two models for the nine fog cases are presented by comparing with routine and field observational data. The results of MM5 and PAFOG for two typical fog cases are verified in detail against field observations. The verifications demonstrated that all methods tended to overestimate fog occurrence, especially for near-fog cases. In terms of TS/ETS, the LWC-only threshold with MM5 showed the best performance, while PAFOG showed the worst. MM5 performed better for advection–radiation fog than for radiation fog, and PAFOG could be an alternative tool for forecasting radiation fogs. PAFOG did show advantages over MM5 on the fog dissipation time. The performance of PAFOG highly depended on the quality of MM5 output. The sensitive runs of PAFOG with different IC/BC showed the capability of using MM5 output to run the 1D model and the high sensitivity of PAFOG on cloud cover. Future works should intensify the study of how to improve the quality of input data (e.g. cloud cover, advection, large scale subsidence) for the 1D model, particularly how to eliminate near-fog case in fog forecasting.

Estudo sobre frentes quentes que ocorrem no sul do Brasil

Este trabalho apresenta o comportamento de duas frentes quentes que afetaram o tempo no estado do Rio Grande do Sul. Imagens de satélite e vários campos meteorológicos foram analisados para ambas as situações. Na primeira situação o sistema deslocou-se da Argentina e foi atingindo gradualmente as diversas regiões do estado, de oeste para leste. Durante a passagem deste sistema foi registrada a ocorrência de precipitação; em algumas estações do litoral, foram observadas nuvens cumulonimbus. Durante aproximadamente 24 horas o estado do Rio Grande do Sul esteve sob a influência do sistema. No segundo caso, uma frente quente atingiu o referido estado, enquanto o vórtice ao qual ela estava associada, ainda encontrava-se no Oceano Pacífico. Para este segundo caso, foram construídas seções verticais de temperatura potencial e observou-se uma inclinação das superfícies isentrópicas, a qual está relacionada com movimento ascendente. A análise de mesoescala mostrou que existia uma região com pressões mais altas no nordeste do estado e pressões baixas no oeste/noroeste. Neste segundo caso, também foram observadas nuvens cumulonimbus em várias estações do estado, incluindo algumas na parte oeste do estado. Foi registrada também a ocorrência de nevoeiros nas duas situações.

Relationships between rainfall, fog and throughfall at a hill evergreen forest site in northern Thailand

AbstractThis study used hourly data of rainfall, water captured by a sheltered fog gauge and wind speed as collected at a montane forest site in northern Thailand during nearly 3 years to test the efficiency of the rain‐protected passive fog gauge as a predictor of fog occurrence. To separate possible contributions by wind‐driven rain (WDR) from fog, the maximum rate of water input to the fog gauge during rainless periods (Fogmax) was derived as a function of wind speed. During periods with rain and fog, the fog gauge often produced values above the Fogmax line, suggesting contributions by WDR. The specific conditions of rainfall intensity and wind speed under which this happened were identified and the corresponding data were excluded from the fog data‐set for subsequent reanalysis. Based on the recalculated data‐set, inter‐annual and seasonal variations as well as the diurnal pattern of fog occurrence at the studied forest are described. Fog‐induced canopy drip during rainless periods was only 19·3 mm over the 3 years, being less than 0·5% of total throughfall and ∼33% of the corresponding catch by the fog gauge (58 mm). However, the fog gauge captured nearly 18 times more water (1033 mm) during all times when WDR could reasonably be excluded. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Rainfall, fog and throughfall dynamics in a subtropical ridge top cloud forest, National Park of Garajonay (La Gomera, Canary Islands, Spain)

AbstractMixed tree‐heath/beech forest is a type of subtropical montane cloud forest found on wind‐ and fog‐exposed ridges in the Canary Islands. With a dry season of 5 months and an annual precipitation of 600–700 mm, the extra water inputs through fog interception assume particular importance in this environment. Measurements were made of rainfall, fog occurrence, wind speed and direction as well as of throughfall (TF) in a ridge top cloud forest located near the centre of the National Park of Garajonay on the island of La Gomera. Measured amounts of incident rainfall were corrected for wind‐induced losses around the gauge and for topographic effects. Amounts of fog water as collected by a 0·25‐m2 fog screen were corrected for changes in effective screen surface collection area depending on wind direction. No such corrections were taken into account in most if not all previous studies of rainfall and fog water inputs in the Canary Islands. TF fractions differed between events with rain‐only (87% of wind‐corrected rainfall), fog‐only (∼6% of wind‐corrected fog) and mixed precipitation (110%). It is concluded that the fog screen was more efficient at capturing fog water than the forest canopy, whereas previous wetting of the canopy and bryophytes by rain may have caused the higher TF fraction found on days with rainfall and fog. Copyright © 2010 John Wiley &amp; Sons, Ltd.

A comparison of the performance of three types of passive fog gauges under conditions of wind‐driven fog and precipitation

AbstractUnderstanding of the ‘typical’ amounts of fog intercepted by different types of cloud forests is hampered by a lack of comparative information on local fog climatology. Usually some kind of ‘fog gauge’ is used to characterize fog occurrence and amounts. Moreover, wind‐driven fog and precipitation are difficult to measure separately and reported measurements of ‘fog’ often represent a combination of the two. In this paper, the term ‘occult precipitation’ (HP) is used to represent fog in combination with the horizontal component of wind‐driven precipitation (WDR). Collection efficiencies of three widely used types of passive fog gauges, viz. a wire harp (WH) screen, a modified cylindrical gauge (MJU, Juvik‐type) and a tunnel gauge (TTG, Daube‐type), were derived by comparing the volumes of water collected by the respective gauges with horizontal cloud water fluxes (CWFs) derived from wind speed (u) and the fog liquid water content (LWC) as measured by a cloud particle spectrometer during conditions of fog at a windward cloud forest site in northern Costa Rica. Under conditions of fog‐only, the collection efficiencies of the three gauges were linearly related to the horizontal CWF as measured by the gauges themselves. Therefore, additional information on wind speed, droplet size and fog LWC was not needed. During conditions of HP, relative collection efficiencies were derived by comparing the volumes collected by the respective gauge types. The modified Juvik gauge had an efficiency close to 100%, independently of the wind speed and direction, whereas the efficiency of the WH depended critically on the wind speed. The tunnel gauge had an efficiency comparable to that of the Juvik gauge, with some additional catch occurring under conditions of low precipitation angles due to the reclined frontal surface of the gauge. Copyright © 2010 John Wiley &amp; Sons, Ltd.