Aermet – Part 1 Course #423 Day 1 Afternoon

Aermet – Part 1 Course #423 Day 1 Afternoon
Air Pollution Dispersion Models: Applications with the AERMOD Modeling System Aermet – Part 1 Course #423 Day 1 Afternoon

Day 1 Afternoon: AERMET – Part 1

Overview This lesson introduces:
Minimum Meteorological Data Requirements Hourly Weather Observations Upper Air Soundings 1-Minute Wind Data Site-specific Data Data Representativeness Temporal Data Requirements Surface Characteristics Multistage Processing Three stage process Overview of the control file structure

Learning Objectives At the end of this session, you will understand:
AERMET data requirements Data representativeness and substitution General control file structure Multistage processing

AERMET Meteorological Data Requirements

AERMET Data Requirements
Minimum Requirements Hourly Weather Observations – National Weather Service (NWS) Observer-based vs. Automated Observations Automated Surface Observation System (ASOS) Commissioning Date – why is it important? Basic elements collected Sky conditions such as cloud height and cloud amount up to 12,000 feet, Surface visibility up to at least 10 statute miles, Basic present weather information such as the type and intensity for rain, snow, and freezing rain, Obstructions to vision like fog, haze, and/or dust, Sea-level pressure and altimeter settings, Air and dew point temperatures, Wind direction, speed and character (gusts, squalls), Precipitation accumulation, and Selected significant remarks including- variable cloud height, variable visibility, precipitation beginning/ending times, rapid pressure changes, pressure change tendency, wind shift, peak wind Prior to 1992, human observers were required to record meteorological observations. In 1992, the installation of automated systems began known as ASOS – Automated Surface Observation System. The Commissioning date is the date the ASOS was deemed operational at a site. This date is important because of how the wind data that are collected by ASOS are archived. ASOS winds are truncated rather than rounded to whole knots. To compensate for this known bias, AERMET adds 0.5 knot to each ASOS wind speed. AERMET stores a data table of commission dates for airport sites and makes this adjustment to the wind speed data when applicable. Listed are the basic meteorological paraameters collected by ASOS.

Minimum Requirements Hourly Weather Observations – National Weather Service (NWS) Minimum Data Elements to Run AERMET with NWS Data Wind speed and direction, nominally about 10 meters Temperature, about 2 meters Cloud cover Data Formats Recognized by AERMET CD-144 (80-character records primarily to comply with paper cards) SCRAM – EPA’s reduced format of CD-144 with only required elements (28 char) SAMSON, HUSWO – available from NCDC on CD; additional fields TD3280 – fixed and variable block – a format for magnetic tape ISHD, ISH, ISD – latest incarnation; international; includes special observations There is a minimum set meteorological data parameters required by AERMET to generate data required by AERMOD to perform dispersion modeling. These include a subset of hourly surface parameters as well as upper air sounding data. With regard to the hourly surface observations, the minimum data required include: wind speed and direction, temperature, and cloud cover. These are collected at ASOS sites administered by the National Weather Service (NWS) and the Federal Aviation Administration (FAA). In lieu of cloud cover, temperature difference (10m – 2m) and solar radiation can be used which is more typical of site specific data. AERMET can accept a number of formats used to store hourly surface observations. These include: CD-144: Used ‘overpunches’ to account for data longer than the allowable field length and for special characters; ended 12/31/95 SCRAM (1/1/1984 – 12/31/1992): Formerly available on EPA BBS and web site and from some sites that provided meteorological data for free SAMSON (1/1/1961 – 12/31/1990): Includes several radiation parameters HUSWO (1/1/1990 – 12/31/1995): Less ‘friendly’ than SAMSON TD3280: Variable block meant to save space on the magnetic tape ISHD, ISH, ISD (TD3505): Integreated Surface Hourly Data. Previous formats – 1 record, 1 hour; AERMET searches for the regular observation record and uses it but will use a special observation if the regular hourly obs is missing; will only use observation from half-past hour to on the hour Changes in the content and values over the various formats means AERMET does some internal conversions to get correct units and or values

Minimum Requirements Hourly Weather Observations – NWS, cont’d Notes on Formats CD-144, SCRAM, SAMSON, HUSWO, TD3280 report in local time; ISHD reports in Greenwich Mean Time (GMT) INSWO – NOT SUPPORTED - International Surface Weather Observations Availability of Data Format dependent CD-144, SCRAM – not available unless archived by 3rd party SAMSON, HUSWO – not available unless CD is available or archived TD-3280 – not available unless archived ISHD – available for download from NCDC – 1901 through present ftp://ftp.ncdc.noaa.gov/pub/data/noaa Pick year Pick station Abbreviated ISHD – NOT SUPPORTED More recent surface data are archived by the National Climatic Data Center (NCDC) in the ISHD (a.k.a ISH, ISD) format and is available free of charge from NCDC. The NCDC website provides documentation for the ISHD data including data history and inventory. Users should be aware there is also what is known as the “Abbreviated ISHD” which is also available free of charge from NCDC through their web page. AERMET is not able to process the Abbreviated ISHD format. The ‘full’ ISHD data, which AERMET is able to process, can be downloaded via the ftp site shown on the slide. Offline options remain and data can be ordered but payment is required. Program(s) exist to expand the abbreviated format to a format resembling the full format can be found; no guarantees that AERMET will correctly process data expanded from the abbreviated format

1-Minute ASOS Wind Data Studies indicated that more calm winds are associated with hourly reported ASOS data Annually, increased from a few percent with pre-ASOS, non-automated observations to 10%, 20%, and possibly more Biases concentration estimates in AERMOD since an hour with a calm wind is not processed 1-minute wind data archived at NCDC AERMINUTE developed to process 1-minute data compute a 1-hour average to supplement hourly winds Included in AERMET (stage 2) TD-6505 (wind); also available is TD-6506 (temperature) but not used 2000 – present SIDEBAR: How are the hourly values calculated from the ASOS observations? Refer to the ASOS Trainers Toolbox or User’s Manual There is an EPA support program called AERMINUTE to process what are known as 1-minute ASOS wind data. The use of AERMINUTE will be covered in a later session, but this slide provides a basic overview of the need for 1-minute ASOS data that can be used to replace the archived hourly wind data due to the number of calms that are often present in the hourly data.

Upper Air Data (Radiosondes, Rawinsondes) Instrumented package typically released twice-daily to obtain profiles of height, pressure, temperature, relative humidity, and winds Usually 0000 GMT and 1200 GMT; often seen about ±1 hour; special soundings may be launched at other times such as 0600 and 1800 Upper air data are typically collected twice a day at 0000 GMT and 1200 GMT. Special soundings may be collected at other times during the day such as 0600 and 1800 GMT. Profiles of height, pressure, temperature, relative humidity, wind speed, and wind direction are collected. The pictures shown are the balloon (left); the red material on the table is a parachute; and box (middle) houses the instrumentation. The picture on the right has been launched.

Upper Air Data (Radiosondes, Rawinsondes) 92 stations in North America; distance between stations in U.S. - ~200 km 10 stations in Caribbean This map identifies the locations of upper air stations in the U.S. Most states have one or two upper air stations, although many states in the Northeast and Kentucky have none, Texas has 7, and Alaska has 12

Upper Air Data (Radiosondes, Rawinsondes) Data Elements Required by AERMET Height, pressure, temperature; sounding winds are not used Data Formats Recognized by AERMET TD-6201: VB or FB Forecast Systems Laboratory Two versions – only difference is how missing data is indicated Availability CD: Radiosonde Data of North America from NCDC Online: present AERMET will accept upper air data in a couple different formats. These include: TD-6201: Variable block and fixed block – variable block may not be processed correctly, so choose fixed block. FSL versions: Missing data may be identified either with or -9999; AERMET can distinguish between them and does not need user input to correctly identify it within the data. Upper air data are available free of charge at the website shown. The Integrated Global Radiosonde Archive (IGRA) format is available through the NCDC web site, but this format is NOT recognized by AERMET

Upper Air Data, cont’d AERMET and Upper Air Sounding Times AERMET searches for an ‘early morning sounding’ for the day being processed For US stations this means a 1200 GMT sounding AERMET looks in the period from 1100 GMT to 1300 GMT for the sounding User can override this window and expand or move, e.g. 1000 – 1400 1300 – 1600 AERMET globalized to find appropriate upper air sounding to use AERMET requires an upper air sounding near the time of local sunrise. In U.S., this would be the upper air data collected at 1200 GMT which is 0700 local Eastern standard time and 0400 local Pacific standard time. AERMET prefers a sounding just before sunrise. The timing depends on the location and time of year, e.g., in the summer on the east coast of the U.S., the sun rises around 5:30 a.m. local time, whereas the soundings are released at 8:00 a.m. local time AERMET was originally designed for U.S. applications, so the 1200 GMT sounding was the sounding used for calculations. AERMOD system is now used worldwide so AERMET is now globalized to look for appropriate sounding for all locations on earth which sometimes means using the 0000 GMT sounding or a sounding from the previous evening (1200 GMT); see the AERMET Addendum for additional information

Upper Air Data, cont’d Mandatory and Significant Pressure Levels Mandatory: at specific pressure levels 1000 mb, 925 mb, 850 mb, 700 mb, 500 mb, 300 mb, 250 mb, 200 mb, 150 mb, 100 mb, and several above this level Significant: a pressure level with an important characteristic, such as surface termination level (as a result of balloon burst, floating balloon, pressure or temperature sensor failure, weak signal) tropopause, base and top of an inversion with a temperature change greater than 2.5°C or 20% RH below 300 mb When downloading upper air data and the choice is give to download mandatory, significant, or both, ALWAYS CHOOSE BOTH Why? Upper air sounding data can include data recorded at both mandatory and significant levels. Mandatory levels are the specific pressure levels listed on the slide. Significant levels are based some important characteristic such as those listed on the slide. Here are approximate heights associated with different pressure levels: 850 mb = 5,000 ft (Denver) = 1,500 m (approximate heights) 500 mb = 18,000 ft = 5,500 m 100 mb = 53,000 ft = 16,000 m Aircraft fly at about mb (30,000 – 39,000 ft) Not all significant levels identified in the slide will be in every sounding Why download both mandatory and significant levels? Too few levels leads to a less accurate estimate the mixing height from the sounding; this leads to errors in the convective velocity scale (w*) and other calculations that use the mixing height.

Pairing Hourly Weather Observations with Upper Air Data Collocated would be first choice, but there are many more surface stations than upper air stations If not collocated, consider proximity, terrain or exposure Pre-processed Data Many states pre-process NWS hourly observations and upper data and make the files available on their web site Paring: At a minimum, AERMET requires hourly surface data and upper air sounding data. Choosing the upper air station to pair with the surface data is important. The best pairing may not always be the two stations in closest proximity if the stations are not colocated. If a surface station is inland, pairing an inland upper air station would be preferred over an upper air station located on the coast (marine environment) and vice versa. Pre-processed Data: Often the files are specific to a county or region within the state – information may be found on the state’s web site as to which files(s) to use; if there is no guidance, consult with the state modeling contact.

Site-specific Meteorological Data Data collected by a facility Variables collected will often depend on purpose of data collection Often only 1 level of wind speed and direction, 1 level of temperature; sometimes solar or net radiation, σv No specific format for data storage User must inform AERMET what variables are present and how to read them Running AERMET with only site-specific data A refinement to using surface hourly airport data is to use site-specific data. The represenativeness of the site where the meteorological data were collected with regard to the the terrain and land cover around the source is an important consideration. Site-specific data should be more representative of the area modeled. More on representativeness is presented on the next slide. Site-specific data can be supplemented with NWS airport to fill missing hours of certain data elements. What is the difference between onsite and site specific? Onsite implies the data were collected on the facility’s property whereas site-specific is more general, allowing for the collection to be off-site but representative

Data Representativeness Important concept to understand and apply EPA Meteorological Monitoring Guidance states “…meteorological data should be representative of conditions affecting transport and dispersion in the area of interest as determined by the locations of sources and receptors.” When using NWS data in AERMOD, data representativeness can be thought of in terms of constructing realistic ABL similarity profiles and adequately characterizing the dispersive capacity of the atmosphere Case-by-Case and Variable-by-Variable Assessment Factors to consider include (but not limited to) Proximity Complexity of terrain Exposure Period of time Examples of meteorological data that are not representative: Data collected at a coastal location affected by a land/sea breeze circulation would generally not be appropriate for modeling air quality at an inland site located beyond the penetration of the sea breeze. Met data collected in a valley and applied in complex terrain. Four factors identified in 40 CFR 51 Appendix W (Guideline on Air Quality Models) are listed on the slide. Spatial representativeness of the data can be adversely affected by large distances between the source and receptors of interest and complex topographic characteristics of the area. Temporal representativeness is a function of the year-to-year variations in weather conditions. Since the spatial scope of each variable could be different, representativeness should be judged for each variable separately. For example, for a variable such as wind direction, the data may need to be collected very near plume height to be adequately representative, whereas, for a variable such as temperature, data from a station several kilometers away from the source may in some cases be considered to be adequately representative.

Meteorological Data Substitution Reference level – what is it? Manual Hourly Surface Upper Air (“morning”) Site-specific Automated Why didn’t AERMET run with NWS data only? AERMET’s Stage 3 METHOD keyword (concept to be discussed shortly) AERMINUTE output To perform several of the boundary layer calculations, AERMET requires winds and temperature at a near-surface height known as a ‘reference level’. For hourly site-specific data, this is usually the lowest level, but certain conditions do apply for winds and the lowest level may not necessarily be selected as the reference level within AERMET. (This height is NOT user selectable). See the AERMOD Model Formulation Document for details on the decision process within AERMET. For NWS data, which is only a single level, the reference level is the height of the hourly NWS observations, nominally 10 meters for winds and 2 meters for temperature. Without these ‘reference level’ values, calculations cannot be made. Manual Hourly Surface: If a critical hourly surface observation is missing, AERMET (as well as AERMOD) simply considers the hour missing, unlike AERMOD’s predecessor (ISCST3) in which all hours needed to be present. Conditions between two surface locations may be very different and there is no simple solution about what is appropriate to substitute. In pre-ASOS years when hourly weather was recorded by human observers, it was not uncommon for an ‘NWS’ station to operate only during the day (i.e., there would be no observations at night) or report every three hours (rather than hourly). With the introduction of ASOS, these scenarios are not likely to occur. There will likely still be missing hours, but not for extended periods unless the equipment has a problem. Upper Air: Of much more concern are missing upper air soundings because if the morning sounding is missing then AERMET makes NO CALCULATIONS for convective conditions. Scattered days should not be of great concern, but periods of missing soundings should be. NOTE: “morning” refers to the sounding AERMET will choose and depends on location, especially outside the US. Soundings from another station could be substituted if it is representative of the original location AND the regulatory authority agrees with the proposed substitution. Site-specific: Substitutions may occur within the site-specific data. One scenario is that data are collected at a second tower a short distance away (tens of meters) and was used as a backup in case instrumentation on the primary tower failed. NOTE: for data to be substituted from a secondary tower, the instrumentation and data will also have to pass quality assurance procedures just like the primary tower.

Length of Meteorological Data Record Regulatory requirements – Appendix W, Section a. five years of representative meteorological data preferable for the period to be consecutive years from the most recent, readily available period adequately representative and my be site-specific or nearby NWS b. 5 years NWS or 1 year site specific if more than 1 year site-specific, quality assured data are available, then it should be used (only up to 5 years) c. if emission limits are based on a specific year of met data, that year must be added to the period (5 years NWS or 1-5 years site-specific) d,e. long range transport (LRT) considerations (not AERMOD) f. where site-specific data are relied upon in complex wind situations, use of at least 1 year, or more if available, should be used; as before, quality-assured data must be used Consecutive years from the most recent, readily available 5 years – due to possibility of inadequate cloud cover observations (limitations of ceilometer to 12,000 feet) from ASOS, the most recent 5 years of OBSERVER-BASED data may be used NWS data are considered quality data, however site-specific data must be collected at an approved location under an approved quality assurance project plan. Data must be reviewed and judged of sufficient quality for dispersion modeling purposes. For long range transport (LRT) situations, there are similar requirements for the data record, except 3 years can be used if mesoscale data are used in conjunction with NWS or site-specific data within and near the modeling domain.

Length of Meteorological Data Record Requirements for the new probabilistic standards 1-hour NO2, 1-hour SO2, and 24-hour PM2.5 Appendix W: 5-years NWS or 1-year (up to 5 years if available) site-specific Ambient monitoring design values Standard based on three years data Does NOT preempt or alter the Appendix W requirement Various clarification guidance documents issued by EPA (e.g., Tyler Fox, August 23, 2010 – Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient Air Quality Standard) Use of 5-years NWS or 1 or more years site specific data “… serves as an unbiased estimate of the 3-year average for purposes of modeling demonstrations of compliance with the NAAQS. Modeling of “rolling 3-year averages,” using years 1 through 3, years 2 through 4, and years 3 through 5, is not required. Furthermore, since modeled results for SO2 are averaged across the number of years modeled for comparison to the new 1-hour SO2 standard, the meteorological data period should include complete years of data to avoid introducing a seasonal bias to the averaged impacts.” The last statement conflicts with guidance that partial site-specific years can be used. The guidance further states that “… an approach that utilizes the most conservative modeling result based on the first complete-year period of the available data record vs. results based on the last complete-year period of available data may be appropriate, subject to approval by the appropriate reviewing authority. “

Surface Characteristics Quick definition Albedo (noontime) (α) – surface reflectivity: 0 (100% reflected) to 1 (nighttime) (Daytime) Bowen ratio (B, Bo) – surface moisture: typically 0.1 for water to 10.0 for desert Surface roughness length (z0) – surface obstacles: meters over water to order of 1 meter in urban and forested areas Dual Characteristics Why are these characteristics important? AERSURFACE – shortly AERMET requires values for the following surface characteristics: noontime albedo, daytime Bowen ratio, and surface roughness length. These are characteristics of the surface that affect the boundary layer development and the calculations in AERMET and concentration estimates in AERMOD. The surface characteristics must be representative of the meteorology in the modeling domain Albedo is the fraction of total incident solar radiation reflected by the surface back to space without absorption. For fresh snow this value is around 0.9, non-dimensional Daytime Bowen ratio, an indicator of surface moisture, is the ratio of sensible heat flux to latent heat flux and is used for determining planetary boundary layer parameters for convective conditions driven by the surface sensible heat flux. It is also non-dimensional. Surface roughness length is related to the height of obstacles to the wind flow and is, in principle, the height at which the mean horizontal wind speed is zero based on a logarithmic profile. The surface roughness length influences the surface shear stress and is an important factor in determining the magnitude of mechanical turbulence and the stability of the boundary layer. Dimensions of meters (in AERMET), but you will often see it expressed in centimeters Sources of information on realistic values to be presented shortly when AERSURFACE is introduced If the meteorological data is not collected at the application site, i.e., use of NWS meteorology, the conditions at the NWS site may not be the same as at the application site. This is particularly true for wind speed which is affected by the surface roughness. AERMET is somewhat sensitive to changes in roughness length. AERMET provides a means to specify two sets of surface characteristics so the affects of surface roughness can be accounted for at both the NWS meteorological site and the application site. AERMET is less sensitive to albedo and Bowen ratio.

Multistage Processing
Three Separate Stages Historical Perspective Based on old MPRM Limited computer memory – 640 Kb max with about 512 Kb memory to run programs Force user to (hopefully) review data and reports after each processing stage and identify problems, if any Two executables Now only 1 executable but still 3 stages Stage 1 – extract surface and upper air data from “archive” format and perform basic QA QA only on on-site data Stage 2 – combine QA’d data into 1-day ‘chunks’ Not really meant for review but can help troubleshooting problems Stage 3 – develop the files needed by AERMOD “Surface” file – boundary layer parameters Profile file – parameters at one or more heights AERMET has 3 separate processing stages. One of the thoughts behind the multi-stage concept for MPRM and AERMET was that the user should stop and review the reports and data as it is processed in Stage 1. Another reason was that computer RAM (random access memory, not hard drive space) was limited and 512 Kb was about the largest an executable could be and still run. Although computers are much faster – and all three stages of AERMET can run 5 years of data in a few minutes – it is a very good idea to look at the reports (and data if necessary) to see if something is along the way. Although difficult to read, Stage 2 output was only meant to be an ‘interim’ file. It is an ASCII file and can be used to detect possible problems with the data if you understand the structure of the file. The AERMET user’s guide explains the structure. The two files output from Stage 3 for input to AERMOD along with the reports should be looked at carefully to be sure you are getting results that make sense. As we proceed through the exercises, we will give you some things to look for, both good and bad.

Control File Structure/Formats - Overview Pathway/keyword/parameter Pathways JOB – report files; also for just checking syntax (all stages) SURFACE – typically NWS hourly observations (stage 1) UPPERAIR – typically twice-daily NWS soundings (stage 1) ONSITE – site-specific data (stage 1) MERGE – stage 2 METPREP – stage 3 Keyword Many unique to each pathway Some common to SURFACE, UPPERAIR, ONSITE LOCATION, DATA, AUDIT, XDATES, RANGE, QAOUT Mandatory/Optional Repeatable/Non-repeatable Reprocessed The “control” or “input control” or “input” file are the same terms for the file that directs AERMET on how to process data. AERMET uses the pathway/keyword concept that is used throughout the AERMOD system. However, it deviates from both AERMOD and AERMAP in that you only need to specify a pathway once rather than on each record. Each record in the control file can be no longer than 132 characters, including spaces. The pathways and most keywords will be explained in later sessions. For now we only do a quick overview. The JOB pathway is used to identify two report files: MESSAGE, which is a compilation of all messages issued by AERMET processing and REPORT, which is a summary of the messages. Not all messages are incorporated in to the summary. Both of these keywords take a filename; there are no default filenames. The JOB pathway also has a third keyword: CHK_SYNTAX, which checks the syntax of each record without running AERMET. The SURFACE pathway is used to specify NWS hourly weather observations, including the latitude, longitude and time zone. The UPPERAIR pathway is used to specify NWS upper air data (soundings) and includes locational information the same as found on the SURFACE pathway.

Control File Structure/Formats - Overview Parameter For most keywords, one or more parameters Typically unique for each keyword Inform AERMET on how to process data for the particular keyword The parameters associated with the keywords are specific to the keyword and the data being processed. The appendices in the AERMET users guide provide detailed information on all the pathways, associated keywords, and parameters. We will go into more detail for several keywords during later sessions.

Summary In this session, we covered the following topics:
AERMET meteorological data, including NWS data 1-minute ASOS data Site-specific data Site characteristics Data representativeness Data substitution Temporal requirements for compliance demonstrations Multi-stage processing in AERMET Control file basics

Aermet – Part 1 Course #423 Day 1 Afternoon

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