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9.1.1 Upper-Air Meteorological Variables

Meteorological variables measured/reported in upper-air monitoring programs include wind direction, wind speed, pressure, temperature, and humidity. With some exceptions (e.g., radiosonde measurements of pressure, temperature, and humidity), the upper-air data for these variables are based on indirect measurements; i.e., the desired variable is derived from measurements of other variables which are measured directly. This is a significant difference from the in situ measurements of these variables; i.e., when monitored in situ (such as from a meteorological tower) these variables are measured directly. This difference has significant implications for calibrations and audits of upper-air measurement systems (see Section 9.6).

Fundamentals related to upper-air monitoring of wind, pressure, temperature, and humidity are presented in the following. This is followed by information on estimating mixing heights and stability for use in dispersion modeling. Although the latter are often included in discussions of upper-air meteorological conditions, they are not really upper-air variables; a more accurate classification of mixing height would describe it as a boundary layer variable which can be derived from upper-air measurements. Stability, as defined for use in dispersion modeling, is a surface layer variable and is not necessarily related to or correlated with upper-air measurements.

Wind Upper-air wind speeds and wind directions are vector-averaged measurements. None of the measurement systems described in the following sections provide a means to measure winds as scaler quantities, as is done with cup and vane sensors mounted on aninstrumented tower. While tower-based measurements near the surface are easily obtained, there are very few instrumented tall towers that can provide vertical profiles of upper-air winds over the altitudes needed for some air quality applications.

Upper-air wind data comprise either path averages (radiosondes) or volume averages (remote sensors) rather than point measurements. For air quality programs, where the interest is mainly to characterize winds in the atmospheric boundary layer (ABL) and lower troposphere, radiosonde data are typically averaged over vertical layers with a depth of approximately 45 to 75 meters (m). Wind data provided by sodars are typically averaged over layers that are 5 to 100 m deep, while radar wind profiler data are usually averaged over 60 to 100 meter intervals. The altitude at which the winds are reported is assumed to be the mid-point of the layer over which the winds are averaged. Averaging periods for upper-air wind data also vary depending on the instrument system used. An individual wind data report from a radiosonde sounding system is typically averaged over no more than 30 to 120 seconds, representing averages of 60 to 700 meters. The averaging interval for winds measured by sodars and radar profilers is usually on the order of 15 to 60 minutes.

Upper-air wind data are needed to accurately characterize upper-air transport. For example, observing and resolving the vertical shear of the horizontal wind (both speed and directional changes with height) can be important for air quality model applications. Figure 9-1 shows a plot of upper-air winds measured by a radiosonde sounding system, along with simultaneous profiles of temperature, dew-point temperature, and potential temperature. The wind data are represented in the “wind barb” format, in which the direction of the wind is indicated by the orientation of an arrow's shaft (relative to true north, which is toward the top of the figure), and the wind speed is indicated by the number and length of barbs attached to the shaft. Note the change in wind speed and direction that is evident in the first few hundred meters of the sounding. In this case, below about 280 meters the winds are east-southeasterly. Above this level the winds veer (turn clockwise) with height to become southerly, southwesterly, then westerly. This is a simple example of a pattern that is common in upper-air measurements; in fact, much more complex wind shear conditions are often observed. Wind shear conditions can have important implications with respect to air quality, because of the different transport and turbulence conditions that can exist at different altitudes where air pollutants may be present.

Shear patterns such as those depicted in Figure 9-1 occur in part because of the frictional drag exerted on the atmosphere by the earth's surface. The atmospheric boundary layer isgenerally defined as the layer of the atmosphere within which the dynamic properties (i.e., winds) and thermodynamic properties (i.e., temperature, pressure, moisture) are directly influenced by the earth's surface. Factors that influence the vertical distribution of winds include

  • horizontal gradients in temperature (thermal wind effects),
  • the development of local temperature and pressure gradients in shoreline settings (land/sea-breeze circulations) and complex terrain environments (mountain-valley airflows)
  • vertical momentum transport  by turbulent eddies,
  • diurnal reductions in frictional stress at night that can lead to the formation of low-level jets.

Processes such as these are described in references [68] and [60]; examples of the effects of such circulations on air quality are described in reference [70].


Figure 9-1 Example wind and temperature profiles from a radiosonde sounding system.

Consequently, upper-air wind data are critical to air quality analysis and modeling efforts. The data are used for the assessment of transport characteristics, as direct input to Gaussian dispersion models, and in the initialization and application of meteorological models (that are used to prepare time-varying, three-dimensional meteorological fields for puff and grid-based air quality models).

Upper-air wind speeds are almost always reported in units of meters per second (ms -1 ) or knots (nautical miles per hour). Wind direction is reported as the direction from which the wind is blowing in degrees (clockwise) relative to true north. Altitude is usually reported in meters or feet and must be defined as corresponding to height above mean sea level or height above ground level. Radiosonde data are typically reported as height above mean sea level (msl), whereas wind data collected by the remote sensing systems are often reported as height above ground level (agl).

Some remote sensing systems described in these guidelines provide a measure of vertical velocity. To date, however, little use has been made of these data in air quality modeling or data analysis applications. Additional work is needed (possibly on a case-by-case basis) to determine the utility of these data for air quality applications.

Pressure Vertical profiles of atmospheric pressure are measured during radiosonde ascents. The remote sensing systems considered in this document do not measure pressure. Pressure data are critical for radiosonde soundings because they are used to calculate the altitude of the sonde (strictly speaking, the geopotential altitude). Differential global position systems (GPS) rawinsonde systems are being developed that will be able to measure the altitude of the sonde directly, but pressure data will still be needed to support many modeling and data analysis efforts. For air quality purposes, pressure data are used in the application of meteorological models, and as direct input to air quality models. Pressure is reported in units of millibars (mb) or hectopascals (hPa).

Temperature Upper-air temperature measurements are most commonly obtained using radiosonde sounding systems. Radiosonde temperature measurements are point measurements. These can be obtained every few seconds, yielding a vertical resolution of a few meters to about 10 m, depending on the rate of ascent of the balloon.

Temperature data can also be obtained using RASS. RASS temperature measurementsare volume averages, with a vertical resolution comparable to that of the wind measurements reported by the remote sensing systems (i.e., 50 to 100 m). RASS measures the virtual temperature (Tv ) of the air rather than the dry-bulb temperature (T). The virtual temperature of an air parcel is the temperature that dry air would have if its pressure and density were equal to those of a parcel of moist air, and thus Tv is always higher than the dry-bulb temperature. Under hot and humid conditions, the difference between Tv and T is usually on the order of a few (2 to 3) degrees C; at low humidity, differences between Tv and T are small. Given representative moisture and pressure profiles, temperature can be estimated from the virtual temperature measurements.

Temperature data are used widely in air quality analysis and modeling, including theapplication and evaluation of meteorological models, and as direct input to air quality models. The vertical temperature structure (stability) influences plume rise and expansion and thus the vertical exchange of pollutants. Temperature also affects photolysis and chemical reaction rates. Temperature is reported in degrees Celsius (°C) or Kelvins (K).

Moisture Like pressure, upper-air moisture measurements suitable for air qualityapplications are primarily obtained using radiosonde sounding systems. The sampling frequency and vertical and temporal resolution of the moisture data are the same as the other thermodynamic variables measured by these systems. Moisture is most commonly measured directly as relative humidity (RH), and is reported as percent RH or as dew-point temperature (Td) in °C (or frost point temperature). Dew-point depression, the difference between temperature and dew-point temperature (T - Td ), is also a commonly reported variable. Some radiosonde sounding systems measure the wet-bulb temperature instead, and determine RH and dew-point temperature through the psychrometric relationship.

Upper-air moisture profiles are used in the initialization and application of meteorological models, and as direct input to air quality models. Moisture data can be important to a successful meteorological modeling effort, because the accurate simulation of convective development (clouds, precipitation, etc.) depends on an accurate representation of the three-dimensional moisture field. Upper-air moisture data are also useful to the understanding of the formation and growth of aerosols, which grow rapidly at high relative humidity (90 to 100 percent).

Mixing Height For the purposes of this guidance, mixing height is defined as the height of the layer adjacent to the ground over which an emitted or entrained inert non-buoyant tracer will be mixed (by turbulence) within a time scale of about one hour or less (adapted from Beyrich [43] . This concept of a mixing height was first developed for characterizing dispersion in a daytime convective boundary layer (CBL). Since tracer measurements are impractical for routine application, alternative methods are recommended for estimating mixing heights based on more readily available data (Table 9-2). The Holzworth method [44] is recommended for use when representative NWS upper-air data are available. This procedure relies on the general theoretical principle that the lapse rate is roughly dry adiabatic (no change in potential temperature with height) in a well-mixed daytime convective boundary layer (CBL); the Holzworth method is described in Section 6.5.1. Other alternatives include using estimates of mixing heights provided in CBL model output (references [45] and [46]). Mixing heights derived from remote sensing measurements of turbulence or turbulence related parameters are discussed in the following.

Turbulence, or turbulence related measurements (e.g, backscatter measurements from a sodar or refractive index measurements from a radar wind profiler) though not surrogates for an inert tracer can sometimes be used to estimate mixing heights since, under certain conditions, such measurements correlate with the top of the mixed layer. In looking at these measurements, one attempts to determine depth of the layer adjacent to the surface within which there is continuous or intermittent turbulence; this is a non-trivial exercise since turbulence varies considerably, not only with height, but with time and location. This variability is dependent upon which processes control/dominate the production of turbulence near the surface; these processes are discussed in the following.

The production of turbulent eddies during the daytime is dominated (under clear sky conditions) by heating of the ground surface and (under overcast conditions) by frictional drag. Daytime vertical mixing processes can be vigorous (especially under convective -conditions) and can produce a well mixed or nearly uniform vertical concentration profile of an inert tracer. During the nighttime, there are several processes that contribute to the production of turbulence including wind shear (created near the ground by friction), variations in the geostrophic wind, and the presence of a low-level jet (wind shear both below and above the jet can enhance turbulence). Nighttime vertical mixing processes are typically patchy and intermittent, and not capable of producing a well-mixed uniform vertical concentration profile.

Table 9-2

Methods for Determining Mixing Heights



Inert tracerConsistent with the definition of mixing height as used in dispersion modeling.  Labor intensive, not practical for routine applications
A relatively robust technique for estimating the daytime (convective) mixing depth.  Limited by the non-continuous nature of rawnisode launches
Used for continuous monitoring of boundary layer conditions.  The range of a sodar, however, is limited; estimates of the mixing height are possible only when the top of the mixed layer within the range of the sodar.  A good tool for monitoring the nocturnal, surface-based temperature inversion - although different from the mixing height, the nocturnal inversion is equally important for modeling nocturnal dispersion conditions.
Radar wind profilerRefractive indexUsed for continuous monitoring of boundary layer conditions.
RASSVirtual TemperatureThe virtual temperature profile obtained using a RASS is used to estimate the convective mixing height in the smae manner that temperature data are used (limited to the range of the RASS ~ 1 km).

Wind turbulence parameters and/or acoustic backscatter profiles derived from sodar data can also be used to estimate mixing height. These data can be used for both daytime and nighttime conditions, but only when the top of the mixing height is within the range of the sodar. 

The refractive index structure parameter (Cn2 ) calculated from radar wind profiler reflectivity measurements can also be used to estimate mixing height [71]. During nighttime hours, however, the mixing height may be below the range of the radar wind profile.

The virtual temperature profile obtained using a RASS instrument can be used to estimate convective mixing height in the same manner that temperature data are used; this is possible only when the mixing height is within the range of the RASS.

TurbulenceSome sodars report wind turbulence parameters. In using these parameters, one must remember that sodars measure the vector components of the wind. Furthermore, there may be significant differences in time and space between the sampling of the components so that any derived variables using more than one component may be affected by aliasing. Thus, the derived turbulence parameters from sodars are generally not the same parameters that models expect for input. Numerous studies have been performed comparing sodar-based turbulence.statistics with tower-based turbulence statistics. Findings from these studies have generally shown that measurements of the standard deviation of the vertical component of the wind speed (w ) are in reasonable agreement , while the standard deviation calculations incorporating more than one component (e.g., ) are not [72]. It is therefore recommended that, unless models are designed to use sodar-type statistical parameters, the use of derived turbulence parameters be limited to single component calculations such as w . Note however that the utility of  w will depend upon the resolution of the sodar system.

9.1 Fundamentals  
      9.1.1 Upper-Air Meteorological Variables  
     9.1.2 Radiosonde Sounding System  
     9.1.3 Doppler Sodar 
     9.1.4 Radar Wind Profiler 
     9.1.5 RASS  
 9.2 Performance Characteristics  
     9.2.1 Definition of Performance Specifications  
     9.2.2 Performance Characteristics of Radiosonde Sounding Systems 
     9.2.3 Performance Characteristics of Remote Sensing Systems  
 9.3 Monitoring Objectives and Goals  
     9.3.1 Data Quality Objectives  
 9.4 Siting and Exposure
 9.5 Installation and Acceptance Testing 
9.6 Quality Assurance and Quality Control 
     9.6.1 Calibration Methods  
     9.6.2 System and Performance Audits  
     9.6.3 Standard Operating Procedures 
     9.6.4 Operational Checks and Preventive Maintenance  
     9.6.5 Corrective Action and Reporting  
     9.6.6 Common Problems Encountered in Upper-Air Data Collection 
 9.7 Data Processing and Management (DP&M) 
9.7.1 Overview of Data Products  
     9.7.2 Steps in DP&M 
     9.7.3 Data Archiving  
 9.8 Recommendations for Upper-Air Data Collection 

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