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An Investigation of Fog Patterns in the Lower Columbia Basin of Oregon and Washington. - Darren Van Cleave - . Motivation.

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An Investigation of Fog Patterns in the Lower Columbia Basin of Oregon and Washington

- Darren Van Cleave -

slide2

Motivation

The lower Columbia Basin of Oregon and Washington is routinely stricken with persistent inversion-triggered fog in the winter months. Although this fog poses a major traffic hazard to various freeways and airports in the area, no thorough investigation of these fog tendencies has been performed. Thus, the purpose of this study is to examine the various conditions that lead to fog formation in that area.

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Prior Research

This project is a continuation of a study begun in the summer of 2005 by Jon Mittelstadt (Science and Operations Officer with the National Weather Service in Pendleton, Oregon) and Darren Van Cleave. Unfortunately, this initial effort was hampered by a lack of time and resources. As mentioned, there have been no other fog studies pertaining to the Lower Columbia Basin. However, this research is similar to studies conducted by Mark Burger1 of fog patterns in the San Joaquin valley of California. This study uses concepts of pressure heights developed in the Burger study, as well as fog persistence concepts used in a study by Jonathan Slemmer2 of fog patterns in the Salt Lake City area. Lastly, this study uses probability figures similar to a study by Wayne Shaffer3 of fog patterns in the Knoxville, Tennessee area.

1. http://www.wrh.noaa.gov/wrh/03TAs/0309/index.html

2. http://www.wrh.noaa.gov/wrh/04TAs/0401/SLCFog.htm

3. http://www.srh.noaa.gov/mrx/research/Fog/fogTYS.htm

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Data

- ASOS

Surface observations were taken from ASOS (Automatic Surface Observing Systems) records dating back to 1998 for three cities in the Lower Columbia Basin: Pendleton (Oregon), Pasco (Washington), and Walla Walla (Washington). All of these ASOS stations are situated at airports. Because large-scale fog in the Lower Columbia Basin is experienced primarily from November through the first week of February, the data set was limited to those days. Special observations were eliminated, and the hourly observations were used exclusively.

- NCEP

Readings of 700 millibar pressure heights were taken from NCEP (National Centers for Environmental Prediction) reanalysis data. The heights were derived from a geographical point near Pasco, which gave a solid representation for the Lower Columbia Basin.

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Methodology

This study utilized exploratory data analysis in each aspect of the research. For simplicity, a fog event was defined as any time in which an ASOS station reported fog without reference to any precipitation (for example, a report of fog/rain would be ignored). Persistence figures were calculated for the three stations. Probabilities of several meteorological aspects were calculated for time periods prior to fog events, and the results were expressed in pie graphs. Lastly, graphs of 700 millibar pressure heights versus time were rendered with fog events noted at the respective times.

For some of the figures, composites were used in place of individual station readings (for the pressure-heights figures, composites of the three sites were developed by noting times in which fog occurred at all three stations simultaneously). The use of composites was justified given the fact that all three sites have meteorological similarity, and also because large-scale fog events will most often generate fog at all three locations. Additionally, the goal was to obtain a representation of the Lower Columbia Basin as a whole, and not just the individual sites.

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Methodology – Data Problems

For reasons unknown, the ASOS data archives had occasional periods of missing data. On average, the sites had 65 missing observations, which is only 0.33 of one percent of the entire length of over 19,000 observations since 1998. Unfortunately, the algorithms for several of the exploratory data analyses demanded observations for every hour. To compensate, missing data points were filled in with observations from the previous hour (a special thanks to Kathy LeBlanc for assistance in this matter). If the gap was two or more hours, data from before and after the gap was used. A more statistically sound means of compensation such as a Kalman filter would have been preferable. However, this was not practical since the observations included discrete events such as precipitation and fog formation.

Example of Data Problems:

Missing 3 Hours

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Fog Persistence in Pasco

Pasco has the least fog of the three cities due to its low elevation. As an inversion in the Lower Columbia Basin strengthens, the fog bank tends to rise, leaving areas which are nearest to the Columbia River under a stratus deck. This deck of low clouds enshrouds the nearby hills and creates problems for roads which traverse them. Thus, while the Pasco airport may not register as much fog as the other cities, it is still affected by prolonged events.

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Fog Persistence in Walla Walla

Walla Walla experiences more fog events as well as longer event lengths than the two other locations. This is largely a result of the surrounding topography. The city lies in a relatively flat area directly on the edge of the Blue Mountains, with the flat Columbia Basin on the other side. The lack of surrounding hills causes the fog deck to remain nearer to the ground for longer periods of time. Only under very strong inversions will the fog deck ascend the Blue Mountains. Therefore, Walla Walla has less occurrences of low-cloud fog events than Pasco, and thereby experiences lengthier surface fog events.

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Fog Persistence in Pendleton

The characteristics of fog events in Pendleton lie somewhere between those of Pasco and Walla Walla. It is interesting to note that the rate of decrease in the length of events in Pendleton slows considerably after 10 hour events, and this holds until the 16 hour events are reached. Thus, a persistence forecast seems plausible after a fog event has lasted 10 hours. However, the effectiveness of that conclusion is questionable given the small number of events that last over 10 hours.

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Wind Probabilities

Fog in the Lower Columbia Basin requires weak mixing (light winds) in the twelve hours prior to its formation. These pie charts depict the average wind speeds for twelve hours prior to all fog events which lasted at least six hours. When the three sites are averaged, light winds (between two and 6 miles-per-hour) are present 75.6% of the time.

Walla Walla

Pendleton

Pasco

wind direction probabilities
Wind Direction Probabilities

This figure depicts fog events which lasted eight or more hours. As evidenced in the figure, fog development is dominated by northerly winds (95.1% of the time) in the twelve hours prior to an event. This is due to advection of fog from the north. At the onset of an event, fog forms closest to the Columbia River (in the center of the basin) and prevailing northwesterly winds advect it towards the Blue Mountains. If the fog bank can reach the foothill canyons of the Blue Mountains, the diurnal upslope winds quickly carry it up the canyons.

precipitation probabilities
Precipitation Probabilities

Fog formation in the Lower Columbia Basin often requires at least one night of clear skies to allow for strong radiational cooling of the surface. This composite charts shows the most recent precipitation for all fog events. For the majority of the events (69.5%), there was no precipitation recorded in the previous twenty-four hours. A much smaller portion (18.3%) of the events had precipitation in the past twelve hours, and even less (12.3%) experienced precipitation in the previous twenty-four hours.

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Fog Formation (Visible Satellite vs. 700 Millibar Height)

These figures show the relationship between 700 millibar heights and fog formation

11/27/02

11/28/02

11/29/02

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Fog Formation vs. 700 Millibar Height

This figure plots times in which all three stations reported fog onto 700 millibar pressure heights for the same period (the heights are detrended).

Note that the vast majority of the events occur at or near peaks in the pressure heights. This clearly demonstrates the relationship between 700 millibar pressure heights and fog formation.

fog formation vs 700 millibar height
Fog Formation vs. 700 Millibar Height

This figure shows fog events in Pendleton plotted against the 700 millibar heights. In this figure, only the events which last greater than five hours and have no precipitation in the previous twenty-four hours are shown.

Despite the consistent association between pressure heights and fog formation, pressure heights are difficult to use as a predictive tool due to the discrete nature of fog formation. One possibility not covered in this study is to use pressure heights to predict the length or intensity of fog events.

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Conclusions

  • Persistence It is obvious that Walla Walla receives much more fog than Pendleton and Pasco, both in terms of the number of events and the length of events. For Walla Walla and Pendleton, there is a period between ten and sixteen hours of fog event length in which the probability of fog lasting for another hour significantly increases, and so a persistence forecast may be possible for those times. - Probabilities For the Lower Columbia Basin as a whole, light winds between two and six miles-per-hour (75.6% of the time) are needed to enable weak mixing within the twenty-four hours prior to a fog event. Northerly breezes are necessary (95.1% of the time), usually to advect the fog into an area from the north and northwest. Dry conditions within the twenty-four hours prior to fog events prevail (69.5% of the time).- Pressure Heights Pressure heights of 700 millibars are clearly linked to fog formation in the Lower Columbia Basin. However, their usefulness as a predictive tool is doubtful for three reasons: the discrete nature of fog, the occurrence of anomalous events (not all major events occur during a pressure ridge), and the fact that the existence of a stratus deck is not taken into consideration.