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Rooftop Temperature Bias Study

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About 50 of the NWS's 11,700 Coop stations are located on the roofs of buildings.

Rooftop exposures has been known for years to create measurement biases as compared to the traditional ground-based measurement sites. For example, in 1942, one of the world's leading climatologist, Helmet Landsburg, wrote in his textbook "Physical Climatology," "conclusions on climate derived form records of roof stations may be no means representative of those at the ground level."

Since surface temperature measurements are intended to represent conditions where people live, work, commute, and grow their food there is interest in quantifying the biases associated with rooftop measurements. Although some site specific studies have been performed, no large scale attemps have ever been conducted to measure differences.

In the interest of adherence to exposure standards and represnetativeness of surface observations, the NWS commissioned a study of rooftop temperature biases, beginning in 1999 and scheduled for completion in 2000. The results will be published and used to determine future NWS policy of its own rooftop stations.

The proposal, attached below, is funded by NOAA's Environmental Data and Information Services.

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Proposal Title: A Comparison of Rooftop and Standard Ground-Based Temperature Measurements

Accuracy and continuity of surface air temperature measurements are critical for many NOAA activities including short term weather forecasting and warning, climate monitoring and the prediction and assessment of decadal to centennial climate change. Historically, most air temperature observations have been taken over a grassy surface at a height of approximately five feet above the ground. Non-standard instrument siting, such as rooftop installation, introduces uncertainty in both the accuracy and representativeness of temperature data within the NWS. The continued expansion and availability of real-time weather data from sources outside of NOAA which often use rooftop installations, and the growing opportunities within NOAA to include such data in forecasting, verification, research and climate applications, makes it imperative to understand what effects rooftop siting has on data quality, continuity and representativeness.

This project will study biases and uncertainties in temperature records caused by rooftop instrument locations. Both National Weather Service stations and non-NOAA sources will be investigated. Ground level to rooftop temperature differences will be studied to show what, if any, differences occur and whether these differences are significant, systematic, predictable and a function of current weather conditions or if differences are highly variable and inconsistent. Work will be performed at the National Climatic Data Center to better document where and when NWS weather stations have been situated on rooftops.

Data collection will take place during FY 1999. Most data analysis, assimilation and presentation of results will follow in FY 2000.

Project Objective:

To quantify biases that rooftop temperature measurements exhibit under different synoptic conditions and instrument exposures and to provide data users with the effects these  biases are having on the representativeness of temperature data

Project Description:

A. Introduction

Ground-based temperature observations have historically provided the vast bulk of temperature information for forecasting, climate change research, and public information and business application.

The standard exposure height for sensors is 5 feet above a ground surface representative of the surrounding environment. The intent of this standard is to provide representative measurements of air temperature where people live, work, play, and grow their food. Standards for accurate and representative weather observations and climatic data collection encourage consistent and relatively uniform instrument siting and ground-level exposure. However, there are numerous locations around the country where instruments are exposed on rooftops instead of on the ground. Rooftop sitings are usually the result of lack of a nearby ground-based site or security considerations

The NWS currently operates numerous rooftop stations nationwide. In the private sector, rooftop station exposures are growing rapidly nationwide. Several thousand such sites now are operating. In many cases, these stations include real-time data accessability and widespread public dissemination by the media.

Mixing rooftop observations with observations from standard ground-level weather stations can pose significant issues for weather forecasting and verification, weather and climate analysis, and important climate applications such as energy demand planning and forecasting by large public utilities.

Issues include: Do rooftop temperatures accurately represent the air temperatures that people experience at ground level? Does the representativeness vary depending on time of year, exposure of instruments on the roof, the type of roof (metal, stone, color, etc.), height of roof, latitude, elevation above sea level, and surrounding climate and vegetation?

Little quantitative work has been done to document the magnitude oSeptember 24, 2013ware of the potential impacts associated with rooftop observations. Rooftop weather stations (both NOAA and non-NOAA) continue to be accepted as official sources of weather observations. Rooftop temperature data published by NOAA can be included in climatic data sets without obvious flags that alert data users to this fact.

This project is an effort to quantify the bias that results from taking temperature measurements on rooftops versus the standard ground-based exposure for all four seasons and under different synoptic situations. This goal will be accomplished by making a large number of temperature comparisons between rooftop observations and nearby ground-based measurements.

Temperature differences will be evaluated as a function of varying weather conditions such as cloud cover, wind speed, sun angle (time of year). To the extent possible, the effects of building height, roof characteristics (area, color), location of the sensor on the roof (east corner, center, etc.) and surrounding objects and land surface characteristics will also be assessed.

The project is further defined by five goals:

i) To establish the relationship of temperature observations from rooftop to ground level locations.
ii) To document in NCDC metadata files the dates and time periods of NWS rooftop observations.
iii) To identify meteorological conditions associated with larger temperature effects of the rooftop locations.
iv) To assess the impact of rooftop locations on defining the climate (normals) of a location, and assess the potential impact of rooftop observations on long-term climate research.
v) To present study results at national meetings to inform a wide audience.

This project is a collaborative effort of several groups with a strong interest in understanding continuity, biases and data quality issues associated with rooftop temperature observations. Participants include Colorado State University, the NWS and NESDIS (NCDC) in NOAA, and a media participant (Bob Ryan, NBC, local television affiliate in Washington, DC).

The study will be conducted by collecting data from a large number of both rooftop and ground based stations for one year and then performing analyses of the data in the second year.

The results of this study will be used to evaluate to what extent rooftop temperature data are useful in weather and climate applications. If differences are small and/or relatively predictable and consistent, relationships to ground level measurements could be developed and applied. However, if differences are large and unpredictable, efforts should be made to terminate rooftop measurements.

B. Concept

The project includes two types of data comparisons:

i) comparison of rooftop observation to non-rooftop observations
ii) comparison of rooftop observation to field standard observations.

In the first type, the rooftop and non-rooftop observations will be from instruments that are in regular service as a part of a designated network. The first network includes sites operated by the NWS and will include ASOS and the Cooperative Observer Network (Coop). Observations will include daily maximum and minimum temperatures.

In the second type of comparison one field standard R.M. Young (RMY) aspirated temperature system will be placed beside the rooftop instrument and a second and third RMY will be placed at the surface near the building with the rooftop instrument. One RMY will be used to make an absolute temperature comparison with the rooftop instrument. The other two RMYs will be used to make observations at the surface nearby. One of the surface RMY systems will be placed on the North side of the building and one at a location to be chosen. The two at the surface will allow a definition of horizontal differences in surface temperature and the "North" temperature. The temperature on the North side of the building is of interest since many Coop and first order observations in the historic climate records came from such a location.

Comparisons and results from the NWS networks are of great importance to the NWS as we are currently evaluating the future of NWS rooftop observations. Comparisons with other networks are equally important since their observations are being made available publicly to a wide audience.

C. Research Plan

The research is described in this section is ordered by the five specific goals in section 1.

I) To establish the relationship of temperature observations from rooftop to ground level locations.

The first step is to identify the rooftop sites to be used in the study. At least 50 sites will be identified with one half in the NWS and one half outside the NWS. Historical data will be acquired for as many sites as possible. Several of the NWS sites have data for a few decades. Data will be collected from all sites during the first year of the study. In addition hourly data will be collected from the nearest ASOS location to each site.

Three types of analysis will be used in the data comparisons. The first is to form the accumulated sum of the difference in temperature between the rooftop instrument and the comparison instrument. The analytic model to be used is to assume a linear relationship between them.

In a graphical form of the accumulated temperature difference as a function of time, the slope of the line is the bias between the two observations. If the assumed relationship is correct, then the slope is steady and easily determined. If the relationship is not correct, then the slope varies with time and a variable relationship is identified.

The second analysis is to correlate the two temperatures and calculate the fraction of the variance which is explained by the correlation.

The third analysis is to prepare frequency distribution of the temperature difference and analyze them. A relationship with a stable slop will be characterized as a nearly gaussian distribution with small tails. If the distribution is skewed toward one side or has a limited number of large deviations for the mean, then we plan to try to identify the weather conditions which lead to large differences.

ii) To document in NCDC metadata files the dates and time periods of NWS rooftop observations.

The NCDC participation in the project is primarily to improve and digitize files to allow the project to verify the actual times and locations of rooftop instruments in the NWS for Coop sites and for first order sites.

iii) To identify meteorological conditions associated with larger temperature effects of the rooftop locations.

The ASOS hourly observations will be used to define weather conditions. The expectation is that large differences are due to weather conditions or local characteristics of the roof. Primary weather conditions are wind speed and solar radiation. High wind speed and overcast skies will lead to small temperature differences which are due to instrument differences. Low wind speed and clear skies will lead to larger temperature differences. These will be investigated individually on days of occurrence to understand the causes.

iv) To assess the impact of rooftop locations on defining the climate (normals) of a location, and assess the potential impact of rooftop observations on long-term climate research.

The results of (iii) above are the important source for this assessment. Stable biases will just produce a bias in the normal and long term climate records. Variable relationships and highly skewed relationships will have a very different effect on climate normals.

v) To present study results at national meetings to inform a wide audience.

Results will be presented to several meetings but two are known. The American Meteorological Society has an annual meeting in January of each year and the American Association of State Climatologists has an annual meeting each August.

FY 1999 Work plan

Milestones:

#1/Dec 98: To identify rooftop stations.
#2/Feb 99: Identify ground level station suitable for comparison and obtain comparable data.
#3/Jun99: Conduct field study with RMY instruments.

Deliverables:

#1/Sep 99: A list of known NWS and non-NOAA rooftop weather stations.
#2/Sep 99: Data set collected.

FY 2000 Work plan:

Milestones:

#1/Dec 99: Analyze co-located data from roof and surface.
#2/Jun 00: Analyze rooftop vs. Non-rooftop data.
#3/Jun 00: Present results to scientific community.


NOAA, National Weather Service
Office of Climate, Water, and Weather Services
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Last Updated: September 24, 2013

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