Introduction:

 

Every day, a wide spectrum of customers accesses weather information collected by a vast network of volunteer cooperative observers.  Approximately 11,000 volunteer weather observers participate in the United States each day and nearly 200 of these volunteers are located in New Mexico. The data are used for many reasons, not the least of which is to provide  the scientific community with a better, more thorough understanding of local climate regimes and long-term trends in weather. Other important uses include engineering projects, energy budgeting and use, construction, architectural design work, farm and ranch management, water and land management, recreation, environmental studies, litigation, and insurance, just to name a few. The data are utilized frequently, both by the public and private sectors.

 

All coop stations report daily maximum and minimum temperatures as well as precipitation.  In addition, some stations report soil temperature, maximum and minimum relative humidity, pan evaporation and wind.  These reports, commonly referred to as Remote Observation Station Automation or ROSA reports, are available on our website along with examples of how to decode  them.

 

While temperature and precipitation are the most common elements,  wind observations from select coop sites in New Mexico are of particular interest to this author, especially those that capture nocturnal downslope wind events that occur in the lee of New Mexico's central mountain ranges during the cool season.   The Edgewood/Cedar Grove (CCEN5) station, located in southern Santa Fe County,  is ideally situated just east of the Sandia Mountain range and near the base of a smaller topographical feature, South Mountain, to examine these downslope wind events.  The purpose of this study is twofold.  First, the author will provide background information on the wind rose and explain how it can be used to interpret wind data. Secondly,  a downslope wind event that was observed at CCEN5 on February 12, 2005 will be examined.

Background:

 

A wind rose is a diagram that depicts the distribution of wind speed and direction for a particular location over a set period of time. Wind roses can represent conditions for a single month, a single year or a multi-year average.  Wind roses have been traditionally used to represent wind conditions at airports with first-order, hourly observations.  However, some coop stations in New Mexico report wind at least once a day using WxCoder II, and wind roses can be generated from these reports.  Edgewood/Cedar Grove (CCEN5) reports wind observations daily at 0700 and 1900 LST and peak winds as they occur.    

Using WRPLOT View software (ver. 5.0, Lakes Environmental Software, 2005), nearly 1400 wind observations from CCEN5 were analyzed between September 1, 2004 and August 31, 2005 .  The resultant wind rose plots are depicted in Figs. 1-4 to the right. The length of the rose petal or “spoke” is directly proportional to the frequency of time that the wind blows from a particular direction. Moreover, each “spoke” is sub-divided to show the relative frequency of various wind speed categories.  Wind roses summarize a considerable amount of wind frequency information into a single graphic.

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Results:

 

More often than not, when the wind blows at CCEN5, it will be from the north through northwest (Figs. 1 and 2).  In fact, between Sep. 1, 2004 and Aug. 31, 2005, when the standard observation was made at 0700 LST, the wind blew from only 3 directions half of the time (47 percent) - north, north-northwest and northwest.  Rarely if ever did it blow from the south or southwest.  Interestingly,  calm conditions (observed wind speed less than 1 mph) were only observed on 40 out of the 365 days at 0700 LST while approximately 35 days were calm at 1900 LST.  This suggests that nighttime/early morning winds at CCEN5 are equally if not more significant than typical afternoon/early evening winds.  Indeed, when Figs. 3 and 4 are compared it is clear that peak winds greater than or equal to 40 mph occur more frequently between 0000 and 0600 LST than they do between 1200 and 1800 LST.  Moreover,  strong nighttime winds are predominantly from the north or northwest while strong daytime winds are predominately from the west. 

Figure 1. CCEN5 wind rose at 0700 LST  from 1 SEP 2004 to 31 AUG 2005.

Figure 2. Same as Fig. 1 except at 1900 LST.

Figure 3. Same as Fig.1 except for  peak nighttime winds between 0000 and 0600 LST.

Figure 4. Same as Fig.3 except for  peak daytime winds between 1200 and 1800 LST.

The image above is a Skew-T/Log P diagram for the Albuquerque Sunport valid at 5am MST 13 February 2005.  Note the layer of 40-45 knot winds blowing from the northwest between 500 mb and 650 mb or roughly 15,000 to 18,000 feet MSL.  Also evident is the presence of a stable layer just above 700 mb or where the temperature had increased with height.   The image to the left depicts a 500 mb analysis valid for  the same time.  Past observations and experience suggest that  localized  downslope winds develop in the wake of an upper level trough when large scale downward motion coupled with a deepening stable layer promote development of mesoscale mountain waves.  Moreover, CCEN5's geographical location makes it especially vulnerable to downslope winds.