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Weather Observing Systems -‐ MTR 2410

Professor Scott Landolt

Spring 2013

Wind Direction

May 7th, 2013

Introduction:

Instrumentation is at the core of modern meteorological science. It is the basis for how we approach our study

of the atmosphere, attempt to understand it, and form an explanation for what is occurring in the world.

Beginning with data observations at the surface and extending to nearly all levels of the troposphere, it is

possible to form a 3-‐dimensional model of the natural processes occurring in the atmosphere. From there,

data can then be applied using a variety of models, conceptual solutions, and derived scientific parameters.

The result is the modern process of forecasting weather on earth.

For this study, two identical weather stations were sited and installed at the NCAR Marshall Study Site

(39°56'54.14"N, 105°11'41.81"W) on the evening of April 3, 2013. Instrumentation on each tower recorded air

temperature, relative humidity, atmospheric pressure, wind speed, wind direction, solar radiation, and

precipitation from 4/3-‐4/23 – 24hrs/day on 1-‐minute intervals.

The Marshall Study Site is located in Boulder County in Northeastern Colorado, and is dedicated primarily to

weather instrument testing and research. The site is generally secluded, free from tree cover, large obstacles

(such as buildings), and exists on a predominantly flat orographic profile. The elevation of the site is ~5630’.

The analysis contained here within focuses on recorded wind data – specifically, wind direction – from a R.M.

Young Aerovane wind monitor located on station #1. This instrument was chosen for study due to the unique

location of the study site and the time of year the study took place. April in Northeastern Colorado can be a

very interesting time to study weather phenomenon. Often April is characterized by extremes in temperature

and precipitation/type. Although the mean high temperature for Boulder, CO is ~63F during the month of

April, the month is also the second snowiest of the year. During this time of year, the region is still susceptible

to cold large-‐scale weather systems associated with the PFJ, despite warming diurnal cycles. As a result, rapid

changes in temperature, wind, and the threat for heavy snowfall can be common in April. Wind direction in

Northeast Colorado is essential for diagnosing sensible weather, and is one of the main dynamic forces

leading to measurable precipitation.

During the 20-‐day monitoring, a period of ~40hrs (April 8-‐9) was selected as a “case study,” a particularly

interesting interval for weather in Northeastern Colorado. The study period presents a classic case of early

spring weather in Northeastern Colorado, and a dramatic transition from warm/dry conditions to cold/snowy.

Using the wind data collected in conjunction with data from the other (7) instruments, this report will attempt

to diagnose what occurred during the period. However, in order to accurately diagnose weather phenomena,

we must question and compare the data provided for validity.

Tower 1 / Tower 2 wind direction – comparison and correlation

[Figure 1] shown below plots Tower #1 wind direction data vs. the same data from Tower #2. Despite the

missing data from Tower 2 at the beginning of the plot, what follows is (2) sets of recorded data from (2)

different stations that match up very well. As wind direction changes, the trend from each station follows a

very similar path. Thus, we can conclude both sensors were sited and installed correctly using the same

criteria, and both sensors were most likely from the same location (which they were).

[Figure 1] Wind direction data Tower 1 and Tower 2 vs time

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DirecOon (deg)

(month/day)

Tower 1

Tower 2

y = 0.7829x + 35.721 R = 0.786922

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DirecOon (deg)

DirecOon (deg)

To look further into how data from the (2) stations matches up, correlation analysis is necessary. [Figure 2]

below shows a correlation from Tower 1 and Tower 2 for the entire period of observation. With the exception

of a few data pairs, the two sets of data line up very well. Discrepancies occur mainly during northerly winds,

and are most likely the result of challenges in plotting wind direction using a 360 degree system.

The “R” value is 0.786922 for the correlation, slightly lower than what we would like to see. However it is

important to recognize the variability that might occur when pairing wind direction data. Discrepancies are

likely to occur as the instrument measures wind direction down to 0.1 degrees. Small variabilities like these

would be expected due to the response limitations of the instrument, slight calibration errors (human error),

and the fact that the (2) sensors are ~40ft away from each other. Therefore, I do not believe the relatively

“low” R value is cause for concern or the result of bias error.

[Figure 2] Tower 1/Tower 2 correlation plot

Concerning other sources of error visible in the (2) figures above, static errors such as noise raises an

interesting question. Although we can see noise in [Figure 3] below (and also in [Figure 4]), I do not believe it is

an indication of error. Noise may very well be the nature of wind observations. Even when sensible weather

observations report a consistent wind direction, that wind is still changing constantly, although by a very small

amount. The sensitivity of the instrument (down to 0.1 degrees) is the primary cause. Wind direction is often

simplified into directions such as “north, east, south or west.” However it’s important to recognize a “north”

wind is anywhere between ~338-‐360, 0-‐22 degrees. A northerly wind may be reported slightly differently by

two different sensors due to minor changes in topography or upstream components. This would be a form of

exposure error, but I think this would always be present in wind direction data.

Further sources of error are not visible, however. A strong correlation suggests the absence of dynamic errors

like hysteresis, and visible analysis of the figures confirms this. Drift is not evident in the data, and could not be

accurately diagnosed as the time sample is too short of a period.

Analysis

Throughout the ~40hr case study period, Tower 1 experienced wind from nearly all directions [Figure 3].

However, beginning around 6500 minutes, winds turned NW/N/NE for the remainder of the period. Between

late fall and early spring in Northeastern Colorado, a wind shift like this would be indicative of a cold front

passage. Typically, surface winds blow parallel to the advancing cold front before the FROPA. A Canadian or

Pacific system dropping south into Northeastern Colorado (or backing in as they often do) would produce

W/SW prefrontal winds. This is visible in [Figure 3] from 6200-‐6500 minutes. Winds continue to veer up until

the FROPA which occurs around 6600 hours – although, we’ll analyze the time of the FROPA later on.

Post cold-‐frontal winds in Northeastern Colorado generally blow perpendicular to the front and are usually of

the Northerly variety. They are a good indication of cold air advection (isotachs crossing isotherms at/near a

right-‐angle) as a cold air-‐mass filters into the region. For the period following 6500 minutes, north winds turn

predominantly northeast for the remainder of the period. Therefore, we can consider the period from 6200-‐

6500 “prefrontal,” and the remaining period “postfrontal.” Next, data from the other (7) sensors over the

same time period will be used to support this hypothesis.

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DirecOon (deg)

minutes since first observaOon

Tower 1

[Figure 3] Tower 1 wind direction (case study period)

[Figure 4] below displays wind speed and max gusts from the case study period. In general, speed and gusts

are below 6m/s for most of the period. However during a particular period of interest (~7000-‐7200 minutes),

winds increase dramatically nearing 14m/s. The highest reported speed is 13.82m/s at 7070 minutes, and the

highest reported gust is 15.24m/s at the same time. After this period winds decrease, however, and do not

return to the lower values seen before their maximums.

Typically with cold fronts, winds slowly build in speed in advance of the front. Often, the greatest speeds are

observed during the FROPA as one air-‐mass is displaced by another. Following the frontal passage, winds

decrease but often remain strong until the core of the air-‐mass reaches the area. [Figure 4] below is consistent

with this analysis. Wind speed for the remainder of the period rises and falls regularly, but generally remain

above 3m/s.

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speed (m/s)


Wind Speed

Wind Speed Max

[Figure 4] Tower 1 wind speed/gusts (case study period)

[Figure 5] below displays ambient temperature and dewpoint for the case study period. During the 40-‐hour

period, temperature was highest at 6933 minutes (16.17C) and lowest at 8491 minutes (-‐10.18C) -‐ a range of

more than 26C. Similar to previous figures, a significant change occurred just before 7000 minutes. From

there, ambient temperature is shown to fall dramatically throughout the rest of the period – with the

exception of a slight increase near 8100 minutes (an explanation for this to follow).

Secondarily, the dewpoint experiences a similar cycle throughout the period, which is expected as the two

values are related. However the most notable change is what occurs near 7200 minutes. Following this period,

the ambient temperature and dewpoint lines are very close, indicating a low dewpoint depression – and thus

– the air is nearing saturation. The lines are parallel and stay close throughout the period, slowly moving away

from each other. There is quite a bit of noise (static error) visible in both sensors directly preceding the frontal

passage, likely due to building and variable winds shaking the towers, which [Figure 4] supports.

Once again, [Figure 5] is an indication of a cold frontal passage. The sharp decrease in temperature and

increasing moisture levels are a perfect representation of a cold front, and perhaps falling precipitation (more

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Degrees (C)


Ambient Temperature (C )

Dewpoint (C )

on this later). Additionally, a temperature swing of more than 26C over less than 40hrs points to a strong cold

front, especially for April in Northeastern Colorado.

[Figure 5] Tower 1 ambient temperature/dewpoint (case study period)

[Figure 6] below represents radiation and relative humidity values for Tower 1 during the case study period.

Since radiation is a measure of solar energy (W/m2), it is important to consider daytime vs. nighttime periods

during analysis. However due to an error in the data provided (*inconsistent time/missing ~8hrs), these

daytime/nighttime periods cannot necessarily be trusted. Since radiation is the only measurement that is a

primary result of solar energy, analysis can continue despite this error. Additionally, there are a few things the

radiation data can tell us.

Following the hypothesized frontal passage (near 7000 min), radiation values drop to 0 W/m2 for an extended

period (through ~7800). This would be consistent with cloud cover following a FROPA. Additionally, falling

radiation values proceeding this time are also consistent with a building cloud deck. Following 7800min,

radiation values begin a steady climb, with a notable sharp increase beginning at ~8100min. Similarly, ambient

temperature experienced an increase during the same time. Thus, it is logical to conclude simply that the sun

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RadiaOon (w/m2)


Radiamon (W/m2) RH (%)

came out – warming the air and increasing radiation values. This is common following a cold front passage in

Northeastern Colorado, as clouds/precipitation are often short-‐lived.

Relative humidity data ranges from ~30-‐95% throughout the whole period. Minimum values can be found near

~7000min and maximums near ~7300min. This data is once again consistent with the hypothesis of a FROPA.

Following ~6900min, RH increases dramatically, and maintains a high level (> 75%) throughout the remainder

of the period. This would likely be the case following a cold front – as upsloping winds lift the air, condense

into clouds, and perhaps even produce precipitation. If precipitation occurred, RH values would likely remain

high through the period and even after precipitation stopped. [Figure 6] is consistent with this analysis.

[Figure 6] Tower 1 radiation/RH(case study period)

[Figure 7] below represents the final (2) measurements taken, atmospheric pressure and precipitation during

the case study period. Pressure readings range from 784.93mb at 6980min to 803.84mb at 8531min. Pressure

trend is decreasing before 6980min, and increasing throughout the rest of the period. Additionally, the highest

rates of increase and decrease can be found on either side of 6980min, which is consistent with the hypothesis

presented.

Proceeding a cold front passage, pressure is expected to fall – and most rapidly just before the FROPA.

Following the FROPA, pressure typically increases rapidly initially, and then continues to climb steadily. This is

because a frontal zone is an area of low pressure. Following the FROPA, high pressure begins to work into the

region, and in this case, a cold air-‐mass from the north. Because pressure, temperature, and volume are all a

function of one another (ideal gas law), it is likely that pressure data could be influenced by a variety of

factors. This appears to be the case as we can see quite a bit of noise and perhaps static deterministic errors

throughout the plot. It is worth noting, however, the general pressure trend is clear.

The only measurable precipitation during the case study period occurred at 7213min. 0.252mm of water was

recorded from the station’s tipping bucket gauge. Although 7213min is our best guess at when the

accumulation occurred, it’s not entirely clear when it began/ended precipitating. This is due to the nature of

tipping bucket gauges which do not record (or “tip”) until they sense a certain amount of precipitation. In this

case, we are at the limitations of the instrument’s sensitivity.

Lastly, the measured precipitation does confirm the hypothesis of what occurred during the case study period.

7213min is slightly after the projected FROPA from the other 7 instruments. This is consistent however.

Typically following a cold front passage in Northeastern Colorado, we must “build” a cloud deck and then cool

the lower levels of the atmosphere through evaporative cooling before precipitation can occur. The case-‐study

period was not a heavy precipitation event, but it was measureable nevertheless.

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(mm/hr) Pressure (mb)


Pressure

Precipitamon

[Figure 7] Tower 1 pressure/precipitation (case study period)

Supplemental Analysis / Final Results

[Figures 8 & 9] below are windroses comprised of Tower 1 wind speed and direction data from the case study

period, superimposed onto an image of Marshall Study Site (courtesy of Google Earth). The use of a windrose is an

excellent way to conceptualize these two measurements. Different colors represent respective wind speed

values, while their vector represents the direction the speed came from. Additionally, the thicker the color

block, the more (higher percentage as a whole) the wind speed came from that direction. It is useful in

determining prevailing winds, trends, and what direction the highest/lowest winds came from.

The windrose matches previous data and the analysis above well, as we see our highest wind speeds out of

the N/NW – most likely when the FROPA occurred. Additionally, [Figure 9] provides a nice aerial view of the

site, and enables visualization of what occurred during the case study. Topography of the foothills is also

visible. With this particular storm system, upslope winds were primarily northerly, without a strong easterly

component. This could be a good indication of why heavy precipitation did not occur in Boulder County.

0.252mm

[Figure 8] Tower 1 windrose (case study period)

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