Local and Seasonal Meteorological differences change sound propagation.
Sound propagation is affected by atmospheric conditions. The FAA’s Environmental Assessment(EA) assumes a homogeneous atmosphere with a single temperature, humidity level and pressure. This doesn’t correlate with real-world conditions.
Southern California is subject to persistent meteorological effects, such as inversion layers and prevailing winds. Temperature and humidity are not a constant across all altitudes and not all climate zones follow standard atmospheric assumptions. Ignoring atmospheric variables when modeling noise across distances can result in errors of 5-20dB.
Sound is typically modeled as a sphere of sound energy- where noise propagates equally in all directions from its source. Noise attenuation and direction of propagation will differ depending upon meteorological conditions. Increasing temperatures, wind and overcast have a downward spreading effect which increases noise to the ground. Inversion layers can refract sound waves resulting in higher noise levels at the ground.
In the summer, from the ground, planes appear louder due to high pressure inversion and increasing temperatures. Persistent seasonal changes can cause a shift in noise impacts for months further separating modeled vs perceived noise.
Temperature and humidity vary according to height
An earlier comment [Average weather calculation does not represent Noise impacts from overflights] pointed out the inaccuracies in using a single average value for temperature and humidity across Southern California’s diverse climate. Temperature and humidity vary with altitude. A standard reference atmosphere differs in vertical temperature -3.5 °F/1000 ft, and this likely is used in the modeling software. According to San Diego weather balloon data and SCAQMD’s Sodar data, Southern California’s lower troposphere doesn’t follow a uniform progression of temperature change.
The use of ground-based measurements of temperature and humidity will be inaccurate when modeling sound levels unless the changing vertical atmospheric variables are taken into account.
In the atmosphere, temperatures normally decrease with an increase in height. An inversion layer occurs when temperatures increase as altitude increases. Portions of Southern California spend more time under an inversion layer than not. Inversions not only capture smog in southern California, they also effect sound propagation. Primary inversion layers can occur at heights between 900 and 13,000ft.
When planes fly above an inversion layer the aircraft noise reaching the ground is spread out farther at a lower intensity, when the aircraft is below the inversion layer the overall sound will be greater and spread over a farther distance. Arriving and departing aircraft cross the inversion layers which happen at different altitudes, and different intensity, depending upon time of day, season, and geographic location.
Southern California and the Los Angeles Air Basin are prone to inversion layers when warm air overlays a ground level layer of cooler air. Our coastal areas undergo a Marine Atmospheric Boundary Layer(MABL) as cool westerly winds from the ocean flow under the warm air from the land. The MABL are early day events that can occur during any month, but are most common during the dry season that runs from May through October.Thermal inversion layers and their effects are not included in the noise modeling but it would increase the accuracy. The Santa Ana winds are a regional subsidence inversion where warm air from the high deserts slide on top of cool marine air. Most significant is the summertime specific High Pressure Inversion: A high pressure system from the Western Pacific Hadley global circulation cell will sink warm, dry air on top of the marine layer, trapping it in an inversion.
Inversions vary seasonally, but are a dominant feature in California air basins. “June Gloom” is a common name for the annual inversion layers affecting the Los Angels Basin in late spring.
Southern California has prevailing directional winds of varying speeds. Wind refracts sound downward, distributes sound a farther distance downwind, and shortens distribution upwind. Wind can vary sound between 10-20 dB.
Modeling sound propagation requires factoring vector wind-speed, downwind conditions, and atmospheric stability to arrive at results that can have high-confidence values for accuracy.
The following graphs demonstrate the consistency of wind-speed and direction in six of the nine Southern California climate zones. The EA fails to incorporate the effects of the prevailing wind on overall sound levels.
Ground level reflections can increase the amount of sound that is transmitted to the listener. In Southern California the sound wave’s interaction with the ground will be impacted by the semi-arid climate and urban sprawl. The relatively flat ground, lack of grass and other sound absorbing vegetative ground cover will result in greater reflections. The suburban sprawl and predominately low lying buildings will add additional reflections to the direct source.
The model uses direct source of sound and doesn’t include any indirect reflections unless they were incidentally recorded with the aircraft’s noise profile.
Yearly DNL/CNEL aggregate vs seasonal aggregates
DNL/CNEL is a measures aggregate of noise events over the day. A yearly aggregate is an average of daily aggregates. Depending on the climate area within the study seasons can dramatically change the effect the atmosphere has on sound propagation. In order to represent noise over the year the DNL/CNEL values need to take into account our seasonal and regional atmospheric effects.
Noise perceived on the ground from the same aircraft flown during winter and summer will have a different spread and dB readings. Summertime aircraft will be perceived as louder. For this reason a single yearly DNL value will not properly reflect the seasonal range in noise differences.
DNL metrics should account for seasonal differences. DNL for specific locations should be expressed as a range, or use the worst case values as default.
Acoustic Modeling Constraints
The FAA’s Integrated Noise Model’s (INM) use of SAE AIR-1845 parameters and noise-power-distance (NPD) data enable the simulation of aircraft in a variety of thrust and 3D operational conditions. INM’s accuracy is dependent upon a matrix of empirical profiling of each aircraft configuration and use of formulas to extrapolate values.
Refraction of sound by vertical gradients of temperature and wind, inversion layers, separation of climates by horizontal segregation, scattering of sound by atmospheric turbulence and ground reflections are not readily within the abilities of the modeling software. Sound calculations depend upon direct line-of-sight and ignore indirect contributors to noise.
An objective of acoustic modeling is to accurately simulate the propagation of aircraft noise to ground and return results that would approximate those found in a ground study. Comparing no action against a proposed action, when there are differences in flight altitudes, will be prone to error. Accurate modeling is also particularly important when attempting to determine which geographic areas fit within the noise criteria of DNL 45 dB, 60 dB and 65+ dB.
To correctly model noise at the ground, atmospheric conditions such as thermal gradients, prevailing winds and vertical segmentation of the atmosphere should model the characteristics of the study area.
Averaging a daily aggregate, like DNL, over a year results in discarding important information about the range of noise that occurs. DNL should be reported for each day.
If the EA is to effectively communicate with the public about the expected noise impacts of the proposed action, and be used to support changes to the airspace, it needs to more accurately represent how noise will be distributed.
 Atmospheric Factors in the Propagation of Sound, Doug Sheadel, Air and Waste management Association (2008)
 Sound Propagation in the Atmospheric boundary layer, D. Keith Wilson, Acoustics Today, (Spring 2015)
 California Climate Zones and Bioclimatic Design, The Pacific Energy Center’s Guide(2006)
 Illustrations are after those in Richard P. Turco’s book, Earth Under Siege, and Cal State University Northridge online resources for Geography 103
 Impact of Climate Change on the Frequency and Intensity of Low-Level Temperature Inversions in California, Sam Iacobellis et al, California Air Resources Board (2010) http://www.arb.ca.gov/research/single-project.php?row_id=64774
 Noise Reduction by Vegetation and Ground, D. Aylor, Journal of the Acoustical Society of America, (1972)
 Review of Integrated Noise Model (INM) Equations and Processes, David W. Forsyth, NASA (2003), NASAICR-2003-21241 4
 South Coast Air Quality Management District operates 4 SODAR wind profiler sites in the Metroplex.