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  • View in gallery

    Nominal spectral response of the 940-nm CIMEL sun radiometer (dotted line) superimposed on the water vapor transmittance simulated with MODTRAN in midlatitude summer conditions (continuous line). The 940-nm narrow-band filter has a spectral bandwidth [full width at half maximum (FWHM)] of 10 nm.

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    Variation of the 940-nm atmospheric transmittance calculated using the MODTRAN radiative transfer code as a function of observed slant water vapor (air mass times IWV). The IWV values used to carry out this simulation were retrieved from GPS meteorology measurements at the Sherbrooke site (45°22′N, 71°55′W). The spectral response employed to generate these points was the nominal (passband) curve of Fig. 1.

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    Influence of the 940-nm filter spectral shift on the a and b parameters. These values were derived using the MODTRAN (version 3.7) radiative transfer code to generate transmissions for an ensemble of IWV and m values.

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    Time series of IWV records from both GPS meteorology and (AERONET V1) sun radiometry for three Canadian AEROCAN sites that undergo different climatic regimes: (a) Churchill (58°45′N, 94°5′W), (b) Saturna Island (48°46′N, 123°07′W), and (c) Sherbrooke (45°22′N, 71°55′W) (see Table 1 for the measurement period).

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    Example of the (A, a, b) retrieval procedure [Eq. (5)] in the case of 2002 Saturna Island observations. The points represent the aerosol and Rayleigh corrected transmission for the sun radiometer 940-nm channel [the Y term from Eq. (6)] as a function of the slant water vapor content.

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    (a) Uncertainty in the IWV retrieval as a function of the relative difference between the predicted and observed IWV values in the 940-nm band for the Saturna Island site (48°46′N, 123°07′W). The same plot profile is observed for the others considered sites. A 10% IWV uncertainty (accuracy of water vapor determination from radiosonde) corresponds to a 1% relative deviation in the CIMEL sun radiometer logarithm signal prediction. (b) Variation of the Y term [from Eq. (6)] with the slant water vapor content shown in Fig. 5, and corrected for outlier radiometric points (points greater than 10% IWV or 1% deviation threshold have been excluded).

  • View in gallery

    Comparison between IWV-GPS and IWV-SUN (AERONET V1 calibration; rhombus symbols), IWV-GPS and IWV-SUN (AERONET V2 calibration; plus symbol), and IWV-GPS and IWV-SUN (GPS calibration; gray star symbols) for the (a) Saturna Island and (b) Sherbrooke sites. The statistical results of the regressions are reported in Table 7.

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    Day-to-day variation of calibration constants (a) a, (b) b, and (c) and the associated daily number of observations. (d) For the Saturna Island (48°46′N, 123°07′W) site during the year 2002.

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    Plot of Y as a function of m × IWV corresponding to the single set of a, b, and A values extracted from the overall observations of the sun radiometer 940-nm channel for the Saturna Island (48°46′N, 123°07′W) site during the year 2002.

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Calibration of Sun Radiometer–Based Atmospheric Water Vapor Retrievals Using GPS Meteorology

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  • 1 Centre d’Applications et de Recherches en Télédétection (CARTEL), Université de Sherbrooke, Québec, Canada
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Abstract

A study of the validation and calibration process for integrated water vapor (IWV) measurements derived from sun radiometry at the 940-nm solar absorption channel employed in the Aerosol Robotic Network (AERONET) Aerosol Canada (AEROCAN) is presented. The sun radiometer data are compared with GPS meteorology records used as a reference. Three Canadian sites from different climatic regimes covering the period 2000–04 are considered. The observations from five different sun radiometers (IWV-SUN) were processed using the initial AERONET IWV retrieval procedure (V1) whereas GPS-derived IWV (IWV-GPS) was retrieved using “GPSpace” software developed by the Geodetic Survey division of Natural Resources Canada.

A sensitivity study is carried out to highlight the influence of both central wavelength and signal amplitude on the 940-nm filter characteristics, which are instrument dependent and can drift due to aging. The comparison between IWV-SUN (V1) and IWV-GPS shows an average rmse of 0.23 ± 0.11 g cm−2 (22%) and a mean bias of −0.09 ± 0.16 g cm−2 (9%). Furthermore, it is shown that the use of GPS for determining the 940-nm channel calibration constants for the solar radiometers improves IWV retrievals (rmse reduced by about 35% and bias by a factor of 3–10) without any knowledge of the 940-nm filter characteristics. These results are discussed within the context of the new AERONET IWV processing procedure (V2), which accounts for solar 940-nm region filter characteristics. The GPS receiver technique appears to be a powerful calibration tool because of its continuous observation capability, its robustness, and its operational simplicity.

* Current affiliation: Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada, Montreal, Québec, Canada

Corresponding author address: Amadou Idrissa Bokoye, Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada—Quebec Region, Place Bonaventure, 800, rue de la Gauchetière Ouest, Tour Nord-Est, bureau 7810, Montreal, QC H5A 1L9, Canada. Email: AmadouIdrissa.Bokoye@ec.gc.ca

Abstract

A study of the validation and calibration process for integrated water vapor (IWV) measurements derived from sun radiometry at the 940-nm solar absorption channel employed in the Aerosol Robotic Network (AERONET) Aerosol Canada (AEROCAN) is presented. The sun radiometer data are compared with GPS meteorology records used as a reference. Three Canadian sites from different climatic regimes covering the period 2000–04 are considered. The observations from five different sun radiometers (IWV-SUN) were processed using the initial AERONET IWV retrieval procedure (V1) whereas GPS-derived IWV (IWV-GPS) was retrieved using “GPSpace” software developed by the Geodetic Survey division of Natural Resources Canada.

A sensitivity study is carried out to highlight the influence of both central wavelength and signal amplitude on the 940-nm filter characteristics, which are instrument dependent and can drift due to aging. The comparison between IWV-SUN (V1) and IWV-GPS shows an average rmse of 0.23 ± 0.11 g cm−2 (22%) and a mean bias of −0.09 ± 0.16 g cm−2 (9%). Furthermore, it is shown that the use of GPS for determining the 940-nm channel calibration constants for the solar radiometers improves IWV retrievals (rmse reduced by about 35% and bias by a factor of 3–10) without any knowledge of the 940-nm filter characteristics. These results are discussed within the context of the new AERONET IWV processing procedure (V2), which accounts for solar 940-nm region filter characteristics. The GPS receiver technique appears to be a powerful calibration tool because of its continuous observation capability, its robustness, and its operational simplicity.

* Current affiliation: Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada, Montreal, Québec, Canada

Corresponding author address: Amadou Idrissa Bokoye, Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada—Quebec Region, Place Bonaventure, 800, rue de la Gauchetière Ouest, Tour Nord-Est, bureau 7810, Montreal, QC H5A 1L9, Canada. Email: AmadouIdrissa.Bokoye@ec.gc.ca

1. Introduction

Water vapor in the atmosphere varies considerably in time and space. These variations have considerable influence on weather and climate systems at local and global scales (American Geophysical Union 1995; Gradinarsky et al. 2002; Minschwaner and Dessler 2004). As the primary greenhouse gas, it plays an important role in the control of temperature in the troposphere as well as in cloud formation (Held and Soden 2000). The interactions of water vapor with the other atmospheric and climate components are complex and global (Inamdar and Ramanathan 1998; Soden et al. 2002). The spatial and temporal characterization of water vapor variation remains a global challenge in spite of the motivational impetus provided by the obvious need to better understand this key atmospheric constituent. Significant progress was made over the last few decades concerning water vapor observations and the modeling of these observations using general circulation models (GCMs.) The current satellite-based monitoring systems (Johnsen and Kidder 2002; Dubuisson et al. 2004) can be used to track weather systems ranging in size from a few kilometers to several hundred kilometers in dimension. Ground-based observations of water vapor are nonetheless still needed to monitor local phenomena and climate trends at the local scale. There are several ground-based measurement systems such as radiosondes, passive microwaves, GPS meteorology, and sun radiometry for water vapor monitoring (Bevis et al. 1994; Bokoye et al. 2003).

The research presented in this paper is performed in the context of worldwide sun radiometer networks such as the Aerosol Robotic Network (AERONET; Holben et al. 1998) and associated networks such as AEROCAN (Bokoye et al. 2001). To enhance the monitoring capabilities of these primarily aerosol monitoring networks (more than 150 instruments across the world) a channel for water vapor retrieval is included as part of the standard instrumental protocol. Calibration and validation requirements for data quality assurance of both aerosol and water vapor channels are demanding at the scale of a large network such as AERONET. In this paper we propose an independent and instrument-specific water vapor calibration, a method based on intercomparisons between sun radiometer and GPS meteorology. For that purpose, the high temporal resolution of GPS observations is more appropriate than that of radiosonde limited to two observations per day. Furthermore, the considered methodology appeared to be a simple means of dynamically adapting calibration constants retrieved by traditional radiometric techniques. The data required to develop and test this new calibration method are presented in section 2. The methodologies related to integrated water vapor (IWV) retrievals from the 940-nm water vapor channel calibration process are outlined in section 3. The results are then presented in section 4 and discussed in section 5.

2. Database description

Table 1 shows the operating sites and observation periods (between 2000 and 2004) for the sun radiometric and GPS databases employed in this study. Meteorological data, acquired alongside GPS recordings, were needed for reducing GPS raw data into IWV output. Sun radiometric IWV data (IWV-SUN) were retrieved from the AERONET/AEROCAN network (Bokoye et al. 2001). The extinction and sky-scanning instrument employed in this network is the CIMEL Electronique multiwavelength and automatic sun radiometer. This instrument permits IWV retrievals from direct solar measurements in the 940-nm water vapor channel (Halthore et al. 1997; Holben et al. 1998). The data corresponding to IWV-SUN were, until recently, processed and archived according to an earlier version of the AERONET IWV retrieval procedure (referred to as V1; Holben et al. 1998). A new retrieval procedure, V2, was introduced in 2004 (Smirnov et al. 2004). Three product levels were defined with AERONET: raw level: 1.0, cloud-screened level: 1.5 (Smirnov et al. 2000), and quality-assured level: 2.0 (Holben et al. 2001). The difference between V1 and V2 retrieval procedures is discussed in sections 4 and 5.

GPS observations from Churchill, Manitoba, Canada (58°43′30″N, 94°07′00″W), and Victoria, British Columbia, Canada (48°23′23.2″N, 123°29′14″W), were derived using dual-frequency GPS receivers from the active network run by the Geodetic Survey Division of Natural Resources Canada. GPS data from Sherbrooke, Quebec, Canada (45°22′N, 71°55′W), was collected from our own dual-frequency GPS receiver system (Bokoye et al. 2003). Note that the Sherbrooke site is referred to as the Centre d’Applications et de Recherches en Télédétection (CARTEL) within the AERONET network. Observational GPS data consist of calibrated satellite code and carrier phase observations from continuous tracking GPS satellites at 30-s intervals. Data are archived daily in Receiver Independent Exchange Format (RINEX), version 2. Precise satellite ephemerides (GPS orbits) and precise satellite clock corrections necessary for GPS-based IWV retrieval were also downloaded from the International GPS Service (IGS) Web site. A meteorological station is associated with each GPS receiver station. Atmospheric pressure and air temperature were recorded at 15-min intervals and archived in a daily file. The processing methods applied to the overall database are briefly outlined in the following section.

3. Methodology

This section describes IWV retrieval procedures using sun radiometry and GPS meteorology. The calibration principle employed for the sun radiometer water vapor absorption channel is first reviewed.

a. Water vapor from sun radiometry

Since the original work of Volz (1974), numerous authors have investigated water vapor estimates derived from solar transmittance measurements (see, e.g., Bruegge et al. 1992; Schmid et al. 1996, 2001; Halthore et al. 1997; Bokoye et al. 2003). The present work is closely linked to the contribution of Ingold et al. (2000) related to IWV retrieval from solar transmittance measurements. The IWV retrieval procedure is based on the well-known modified Langley technique (Reagan et al. 1987) governed by the following relationship:
i1520-0426-24-6-964-e1
where V(λ) is the sun radiometer output at wavelength λ, and Vo(λ) is the sun radiometric calibration constant (the instrument response at the top of the atmosphere). Here, R is the earth–sun distance in astronomical units; m is the air mass [assumed constituent-independent for solar zenith angles <75° and computed using the Kasten relationship (Kasten 1966)]; and τi is the optical depth of scattering and absorbing aerosols, scattering molecules, and molecular absorbers other than water vapor. The index i represents the ith extinction constituent at wavelength λ. Here, Tw is the spectral band weighted transmission of the solar irradiance in the 940-nm band. It can be expressed by a two-constant (a, b) model (Schmid et al. 1996):
i1520-0426-24-6-964-e2
Once a, b, and Vo(λ) are determined, the sun radiometric IWV can be computed by combining Eqs. (1) and (2):
i1520-0426-24-6-964-e3
The subscripts r and a refer, respectively, to Rayleigh scattering and aerosol scattering and absorption. The pressure-corrected air mass mr = m P/Po, where Po = 1013 mb and P is the measured surface pressure in millibars. In the AERONET V1 retrieval procedure, the ratio P/Po is approximated by (1 – Z/H), where Z is the altitude of the site and H is the molecular scale height, that is, ∼8 km. For the water vapor observation sites considered in this study, the correction is less than 3% of mr. The new V2 procedure uses National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis and air pressure inputs for Rayleigh corrections (Kalnay et al. 1996; 6-h data and a spatial resolution of 2.5° × 2.5°).
The aerosol optical depth (AOD) at 940 nm is computed from the aerosol optical depth at a nearby wavelength (λ0) and from the Angstrom exponent (Angstrom 1929) α as follows:
i1520-0426-24-6-964-e4
In this study, we used the channel at λ0 = 1020 nm corrected for temperature dependence and relatively free of water vapor absorption. The exponent α was computed from the aerosol optical depths at 440- and 870-nm wavelength. For historical reasons, a part of the database, that is, Sherbrooke (2000, 2001) was processed with a constant α value of 1.3. The AERONET V1 procedure uses λ0 = 870 nm as the pivot wavelength with a value of α = 1. The new AERONET V2 procedure uses a pivot wavelength of λ0 = 870 and a value of α computed from the 670- and 870-nm wavelengths. A quadratic function giving τa(λ) in function of water vapor absorption band was considered in Schmid et al. (2001). It allows us to take into account short and long water vapor absorption bands. However, for the relatively clear-sky sun radiometric data considered in this study, the aerosol transmittance contribution is very weak across the 940-nm channel [the more so because τa(λ) decreases rapidly as per Angstrom’s exponential relation from visible to near-infrared wavelengths].

b. Water vapor from GPS meteorology

The principle of IWV determination from the GPS constellation is based on the delay introduced by the atmospheric water vapor in the propagation of the GPS signal. The absolute value of IWV can be computed from raw GPS observation data associated with the meteorological data (the GPS observations include RINEX data and orbital files from the International GPS Service for Geodynamics). In the present study, a geodetic processing software program “GPSpace” (Bokoye et al. 2003) based on the precise point positioning (PPP) technique was used to reduce dual-frequency GPS receiver and IGS orbital data to zenith tropospheric delay (ZTD). The latter can be partitioned into wet delay (ZWD) and hydrostatic delay (ZHD). ZHD is inferred from ground pressure measurements. Knowledge of ZTD and ZHD allows one to retrieve ZWD by a simple difference. ZWD is then calibrated to IWV. The IWV/ZWD calibration constant depends on meteorological conditions. A full description of IWV retrieval from GPS meteorology is given in Bokoye et al. (2003). Note that GPS observations were made every 30 s.

The accuracy of the GPS meteorology method is of the same order as that of radiosonde IWV retrievals, that is, about 0.1 g cm−2 (Bokoye et al. 2003; Mätzler et al. 2002; Niell et al. 2001) on average. Note that in certain cases the application of a correction algorithm to radiosonde raw data is necessary to improve the accuracy associated with this observation technique (Miloshevich et al. 2006). Errors associated with water vapor profiles are observed in the upper atmosphere using independent observation systems like lidar (Soden et al. 2004), but the impact of upper-atmospheric water vapor is limited as compared to the total columnar IWV. In fact, the lower atmosphere represents about 90% of total water vapor.

c. Sun radiometric calibration

In the context of the present study, sun radiometric calibration is assumed to be the process of determining values for the constants Vo(λ), a, and b [Eq. (3)].

The combination of Eqs. (1) and (2) yields a three-parameter model:
i1520-0426-24-6-964-e5
where [from Eqs. (1) and (3)]
i1520-0426-24-6-964-e6
i1520-0426-24-6-964-e7
The AEROCAN/AERONET water vapor channels are calibrated on a periodic basis by intercomparison with a master instrument calibrated at the Mauna Loa, Hawaii, observatory (19°32′N, 155°34′W) at 3397 m of altitude [using Eq. (1) in clear-sky and stable atmosphere conditions over a half-day]. For the AERONET V1 procedure, the coefficients a and b are maintained fixed and identical for all instruments based on low spectral resolution atmospheric transmittance (LOWTRAN; Kneizys et al. 1988) simulations. Halthore et al. (1997) reported that a = 0.616 and b = 0.594 for average water vapor conditions and a narrow-band filter of 10-nm spectral bandwidth. Note that the above values derived from moderate spectral resolution atmospheric transmittance (MODTRAN) water vapor transmittance simulations were not adopted by AERONET. For the new V2 procedure, the required a and b values are determined using the line-by-line radiative transfer model (LBLRTM) radiative transfer code (Smirnov et al. 2004), whose output is convolved with recent measured (if not available nominal) filter response functions. The alternative calibration method that is proposed here is based on the use of independent reference measurements of IWV provided by GPS meteorology. Such a procedure allows for the simultaneous retrieval of parameters A, a, and b. This is accomplished by solving Eq. (5) using a nonlinear least squares optimization method (Coleman and Li 1994, 1996) that requires initial values for A, a, and b. In this context, the initial values of A = 9.5, a = 0.616, and b = 0.593 were found, from a comprehensive sensitivity study, to be optimal (lowest fitting error). These initial a and b values correspond to the Halthore et al. (1997) values for midlatitude summer. Comparison between A, a, and b retrievals for this optimization procedure and that from Marquart’s algorithm (Press et al. 1986, section 17.1) led to a relative difference of 0.5% (A), 2% (a), and 5% (b), respectively. The constants A, a, and b depend on various factors such as central wavelength position and the width and shape of the sun radiometer filter function (see section 3d). The constants a and b also depend slightly on atmospheric conditions (see the last section), the vertical distribution of the water vapor (Halthore et al. 1997), and observation instrument altitude (Ingold et al. 2000).

d. Sensitivity of a and b to 940-nm filter characteristics

Sun radiometer filter characteristics degrade due to aging caused by the effects of long time solar exposure, temperature cycles, humidity, and lens surface contamination. The extent of the deterioration affects the spectral passband of the filter (Holben et al. 2001).

Figure 1 shows a typical 940-nm narrowband (10-nm bandwidth) filter spectral response. Atmospheric water vapor transmittance (continuous line) derived from the MODTRAN (version 3.7) radiative transfer code for midlatitude summer conditions was convolved with a 940-nm nominal filter spectral response. To highlight the sensitivity of a and b [Eq. (2)] to spectral shift and random noise in the filter spectral response for the sun radiometer filter characteristics in the 940-nm region, we computed the water vapor transmittance using simulated filter profiles. The 940-nm band-weighted water vapor transmittance Tw was calculated as follows:
i1520-0426-24-6-964-e8
where f (λ) represents the nominal filter spectral response and Tw(λ) is the output spectral transmittance from MODTRAN. Independent IWV time series derived from GPS meteorology were used to derive a and b from Eq. (2) for each record corresponding to a given solar zenith angle (i.e., an air mass m). Figure 2 shows the expected power-law Tw variation calculated using MOTRAN as a function of the measured slant water vapor amount [i.e., m × IWV; see Eq. (2)] for a single position of the filter function in Fig. 1 (the nominal curve labeled passband). Note that departures (outliers) from the power-law curve are observed. An analysis of these particular points indicated that they corresponded specifically to slant water vapor values greater than 3 g cm−2. These values were associated with large IWV-GPS values (IWV > 3.25 g cm−2) corresponding to low pressure conditions (P < 990 mb). Such meteorological conditions likely correspond to unstable, often cloudy or severe weather systems (Liou and Huang 2000). It was ascertained that the inclusion or removal of these outlying points did not significantly affect the fit results described below.

Note that a and b are retrieved from Eq. (5). Let us consider first the influence of a spectral shift on the evaluation of a and b. Figure 3 shows the derived values of a and b for the nominal 940-nm case and those derived by adding or subtracting a spectral shift of 2, 4, 6, and 8 nm to this nominal central wavelength. There is no direct experimental validation of this sensitivity analysis performed using synthetic transmittance data generated with the MODTRAN radiative transfer code for the above spectral shift values (with a maximum extent of ±8 nm) and the water vapor column content. However, several studies in spatial remote sensing (Vermote and Kaufman 1995; Bréon and Jackson 1999) reported spectral drifts of several nanometers of considered water vapor channel bandpass due to atmospheric contamination on the measurement or a degradation due to solar radiation effects.

The derived (a, b) values for the nominal 940-nm filter spectral response remain in the range of those reported by Halthore et al. (1997) and are comparable to the fixed AERONET V1 values as well as to the AERONET V2 values (see Table 7 where the V2 values of both a and b range between 0.6 and 0.7). When the central wavelength of the filter shifts (Fig. 3), a significant systematic decrease in the constant a is observed between 0-nm spectral shift and the extreme limits of the ±8-nm shift. The corresponding maximum relative variation in a is, respectively, −68% and −18% for the forward and backward shifts (Fig. 3). The constant b decreases moderately between the 0-nm and +4-nm spectral shifts by −7% and increases by about +6% for both the forward and backward 8-nm shifts. The combined variation of (a, b) introduces a strong variation in IWV retrievals. In fact, a typical error in IWV retrievals of more than 100% is obtained between a +4-nm spectral shift case and the nominal 940-nm case. These results indicate that the calibration process should take into account the possibility of spectral shifts as reported by Heath et al. (1998), who investigated environmental effects on filter central wavelength and bandwidth.

In the AERONET V2 procedure, a and b are computed using state-of-the-art spectroscopy (Rothman et al. 2003) and a measured (if available) filter function (which is different for each instrument). The LBLRTM radiative transfer code (Smirnov et al. 2004) is used to compute atmospheric transmittances in the 940-nm region and to subsequently extract a and b using the slope and intercept of the simulated function log(Tw) versus m × IWV (in similar fashion to the fitting procedure carried out in Fig. 2). For a given atmospheric state (a given value of IWV) computations are carried out over a range of airmass values (a range of m × IWV values). Note that each AERONET instrument has its own unique set of a and b values depending on the filter configuration. They are considered fixed until the filter is changed.

The effect of possible changes in filter response amplitude and the subsequent impact on integrated transmittance variation was also considered (without spectral shift). For example a reduction of filter output signal amplitude could lead to a decrease of water vapor channel signal measurement sensitivity, which would increase observation errors.

These errors could be reduced with a normalization of the signal. The influence of the amplitude can be linked also to some variations in filter spectral response shape, which is not studied in this paper.

For this purpose Tw was simulated using MODTRAN and the 940-nm nominal passband filter response (Fig. 1) while adding a certain percentage of random noisy signals (values between 0 and ± a certain percentage of random noise generated using the Matlab software random function). Then Tw was computed for each simulated IWV-GPS measurement. Our objective was to analyze the influence of random changes in filter passband amplitude on a and b retrievals. Table 2 shows the retrieved values of a and b for 0%, 10%, 30%, and 50% random noise in the nominal 940-nm filter passband response. These a and b values were always retrieved from log(Tw) versus m × IWV fits to the ensemble of input IWV-GPS values. The relative difference between the addition of 0% and 50% uncertainty in the nominal amplitude is −29.6% and −7.5%, respectively, for a and b (Table 2).

The above analysis shows that the coefficients a and b can undergo significant variations as a result of changes in the nominal 940-nm filter characteristics. These variations directly influence the IWV retrieval represented by Eq. (5). The constants a and b are typically computed using a nominal filter spectral response; while it would appear that it is necessary to account systematically for real variations in filter properties, the development of such a network-wide procedure by frequent and direct measurement of filter characteristics would be practically impossible on an operational scale (i.e., beyond the relatively infrequent updates provided by the AERONET V2 protocol). A dynamical combination of sun radiometry and GPS meteorology is the proposed alternative approach (see discussion in section 4b). This technique would allow for the determination of a and b without having to develop a protocol for the frequent measurement and updating of filter characteristics. Such a procedure appears to be very important in the framework of the development of an accurate worldwide sun radiometer–derived water vapor climatology. In ground-based sun radiometry the IWV retrieval is generally made in parallel to atmospheric aerosols observations through τa(λ) determination. Unfortunately, in existing sun radiometer networks including aerosols and water vapor channels, the logistical support is generally prioritized for τa(λ) determination, the main objective of such networks like AERONET. Our proposed procedure for A, a, and b determination in the water vapor absorption channel can be considered as a contribution to increase the water vapor sun radiometer network accuracy using a single sun radiometer site-to-sun radiometer site mobile GPS unit on a limited timeframe window.

4. Results

a. IWV-SUN versus IWV-GPS

Time series of IWV values derived from sun radiometry [V1 downloaded data] and GPS meteorology are shown in Fig. 4 for the three AERONET/AEROCAN sites. A general agreement in trend can be observed for the two independent measurement systems with a typical IWV increase during the summer. Table 3 summarizes the results of the comparison between IWV values derived from AERONET 940-nm sun radiometry [IWV-SUN (V1)] data and IWV values retrieved from simultaneous GPS observations (IWV-GPS) for each instrument. The comparison was performed for both AERONET levels 1.5 and 2.0 aerosol optical depth values [τa(940)] employed as the correction term of Eq. (3) [in Table 3, level 1.5 and level 2.0 refer only to the application of different interpolated τa(940) values]. As compared to the reference IWV-GPS data, the IWV root-mean-square error (rmse) for level 1.5 ranges between 0.14 g cm−2 (Churchill in 2002) and 0.48 g cm−2 (Sherbrooke in 2001) with a three-station average of 0.23 ± 0.11 g cm−2. The latter is about 22% of the mean observed IWV-GPS value of 1.00 ± 0.66 g cm−2 for all 2672 observations. The bias error ranges between −0.41g cm−2 (Sherbrooke in 2001) and 0.12 g cm−2 (Sherbrooke in 2003) with a three-station mean bias of −0.09 ± 0.16 g cm−2. No statistically significant improvement is obtained using the level 2.0 results (Table 3) relative to the level 1.5 results. Thus, the influence of non-cloud-screened versus cloud-screened atmospheric aerosol measurements on the 940-nm water absorption channel is weak for the data ensemble employed in our study. This suggests, as well, that the impact of AOD variations on IWV retrievals is minimal. AOD correction, such as the dynamic correction employed in the AERONET V2 protocol, does nonetheless contribute to an uncertainty reduction in water vapor retrieval (particularly in a hazy atmosphere). Except for Sherbrooke in 2001, the errors in Table 3 remain generally in the range of 0.15–0.2 g cm−2. This is similar to most IWV comparisons between various techniques such as GPS meteorology, radiosonde, numerical weather prediction models, or microwave radiometers (Liou et al. 2000; Mätzler et al. 2002; Niell et al. 2001; Pugnaghi et al. 2002). The large error obtained for Sherbrooke in 2001 may have been linked to a substantial calibration degradation, which may, in turn, have been induced by instrument misalignment or contamination.

Table 4 gives an overview of the calibration constants (A, a, and b) derived for each site and instrument, with the associated fitting error. An example of the retrieval approach for deriving these calibration constants using the IWV-GPS as reference is shown in Fig. 5 for the largest rmse of Table 4 [Eq. (6); Y versus slant water vapor]. The observed variations are reasonably well described by the regression fit (solid line in Fig. 5), despite some scatter possibly due to anomalous observations (cloud coverage, sun radiometer misalignment, etc.). Note that the fitting process itself [Eq. (5)] could also induce variations in (a, b) that are not strictly independent.

The AERONET A values are also reported in the same table for comparison. The AERONET V1 calibration constant (A) does not vary significantly (Table 4). This fact is linked to the constancy of a and b values in the V1 procedure. The mean difference between A from the AERONET V1 protocol and the derived GPS-based calibration values is about 5%. The derived (a, b) values of Table 4 are somewhat different from the nominal AERONET V2 values (see next section) and from those reported by Halthore et al. (1997) or Schmid et al. (1996). Filter degradation (i.e., changes in center wavelength and bandwidth characteristics) with time over the dataset period, or sun radiometer calibration errors (in A), may explain such differences in a and b. Note also that even though each CIMEL sun radiometer is equipped with the same type of ion-assisted deposition interference filters (Holben et al. 1991), the comparison with (a, b) values from literature are subject to the difference between filter responses.

The most important observation that can be drawn from the results of Table 4 is the fact that these constants change significantly in time and between instruments. This suggests different changes through time induced by differences in filter properties as suggested in the previous section. Such variations could not be entirely compensated for by adjusting the constant A (which is effectively what is done as part of the AERONET V1 protocol). The new AERONET V2 protocol (Smirnov et al. 2004) takes the filter characteristics in the a and b retrieval into account.

The use of these GPS-based constants for calibrating the sun radiometer calibration is analyzed in the next section and discussed in comparison with AERONET V2 protocol in section 5.

b. Analysis of the improvement obtained in the IWV-SUN retrieval using GPS-based calibration

This section describes the evaluation of the GPS-based IWV calibration compared with the V1 and V2 AERONET-based procedures. The evaluation is performed using the database corresponding to IWV-GPS data resampled to the sun radiometer times to create an ensemble of IWV-GPS points matched to the times of all sun radiometer signals. For a given instrument and period, this joint database was partitioned into two randomly selected subsets that were nearly equal in terms of number of observations and totally independent. The first subset is used to simultaneously compute the constants A, a, and b (GPS-based calibration) and the second is used for the comparison between the three calibration procedures, that is, IWV-GPS versus IWV-SUN (AERONET V1 calibration), IWV-GPS versus IWV-SUN (AERONET V2 calibration), and IWV-GPS versus IWV-SUN (GPS-based calibration). In this comparison, sun radiometric data were screened to retain only the data falling within the limit of uncertainty corresponding to typical radiosonde accuracies of 10% in IWV [as previously shown in Fig. 5, some points depart significantly from a fit to Eq. (5)]. These outliers might correspond to residual cloud cover or misalignment of the instrument in the solar direction. They can increase errors associated with IWV retrievals, and in turn, in the errors of the calibration constants derived from the linear regression-based procedure [Eq. (5)]. We thus chose to consider only values within 10% accuracy of IWV. This limit of uncertainty corresponds to a 1% departure from the model, that is, in the relative difference between predicted and observed logarithm of the sun radiometric signal as shown in Fig. 6a.

The above 1% threshold uncertainty criterion in Y was applied to the 940-nm signal for the first dataset yielding the constants A, a, and b (see the Saturna Island example in Fig. 6b, which is the filtered version of Fig. 5). One can note that in this case the application of the above screening procedure (and the subsequent reduction in the number of points) did not change the derived A, a, and b values. However, the 1% criterion in Y allowed the screening of outlier radiometric records that corresponds to bad observations (cloud coverage, instrument misalignment, sensor contamination, etc.).

The above approach was applied to the two databases having the greatest number of points, that is, Saturna Island (in 2002, instrument 81) and Sherbrooke (in 2003, instrument 82). It must be noted that the generation of a time-synchronous database between AERONET V1 and V2 databases and the GPS-retrieval database significantly reduced the number of GPS retrieval points. The retrieved A, a, and b values used for the sun radiometer calibration are reported in Table 5. The (IWV-SUN versus IWV-GPS) scatterplots for both AERONET V1 and GPS-based calibration are compared in Figs. 7a,b, respectively, for each site. In both cases, the GPS-based calibration significantly reduces the bias observed for IWV-SUN relative to the AERONET V1 and V2 calibration: the absolute value of the bias error in Table 6 indicates a reduction by a factor 3 and 11, respectively, for Saturna Island (in 2002) and Sherbrooke (in 2003). The bias reduction and the small but systematic rmse reduction of 30% and 40%, respectively, support the validation of the IWV calibration approach for the two test databases. One can note that results from AERONET V1 and V2 procedures are nearly the same for the two sites. However, Smirnov et al. (2004) reported more significant differences with a slope of 0.85, a y intercept of −0.05, and a correlation coefficient of 0.99 for a regression analysis between V2 and V1 carried out over a number of different sites. A V1 and V2 comparison using a large ensemble of data from the Sherbrooke site (1995–2004 data downloaded in March 2005, 15 615 observations) revealed a slope of 0.99, a y intercept of 0.00, and a correlation coefficient of 0.99. These results suggest that the difference between V1 and V2 depends on the site and may vary with the sun radiometer calibration history.

The results of this section indicate that improved calibration can be achieved using the GPS calibration methodology. This calibration approach can be easily implemented as part of a sun radiometer network calibration protocol by deploying one or more mobile bifrequency GPS receiver units to all stations during a regular calibration cycle.

5. Discussion

The new AERONET calibration procedure (Smirnov et al. 2004) referred to as V2 in this paper produces a and b values for each filter instrument. The calibration coefficient A is thereafter evaluated for a field instrument by intercomparison with a reference master sun radiometer. This new procedure resulted in a computed IWV reduction of about 13%–15% relative to the older AERONET V1 procedure (Smirnov et al. 2004) for the AERONET site of Greenbelt, Maryland. For the sites considered in this study the difference between V1 and V2 appeared to be statistically nonsignificant.

The IWV comparison between the AERONET V2 procedure and our GPS-based calibration approach for the Saturna Island site yielded an rmse of 0.24 g cm−2 and a bias error of 0.22 g cm−2 while the results for the Sherbrooke site were 0.15 g cm−2 rmse and a bias of −0.09 g cm−2. Table 6 highlights the difference between the AERONET V1 and V2 procedures and the GPS meteorology for various sites. Table 7 shows the a and b coefficients derived using V2. The results yield a and b values that differ from those of Table 5. This may be due to several factors including the fact that A, a, and b are not strictly independent in the iteration retrieval algorithm even if the set of (A, a, b) triplets that we employed yielded the lowest rmse. In other words, different triplet sets could give nearly the same rmse values.

Furthermore, the influence of the period over which regressions were performed on the A, a, and b retrieval procedure was investigated. Retrieval of the above parameters on a daily basis (computation of a and b for a daily observations) for the Saturna Island site (see Fig. 8) highlights the fact that the varying atmospheric conditions (as represented by different days of the year) can significantly affect the values of the derived constants. The coefficient of variation (standard deviation/mean) are, respectively, 1.28% for A, 9.07% for a, and 14.90% for b. The mean values of the daily-based averaged triplets over 3 months of measurements (Figs. 8 a–c, day 20 to day 180 in 2002) indicate a relative stability with a = 0.40 ± 0.07, b = 0.61 ± 0.11, and A = 9.15 ± 0.09. These values are reasonably close to the triplets derived from all 3-month data groupings (Fig. 9): a = 0.35, b = 0.78, and A = 9.36. The above sets of A, a, and b values remain different from the AERONET V2 mean values (see Table 7), respectively, by 4.32%, 43.8%, and 17.98% in terms of the coefficient of variation. The difference between the two approaches could result from the influence of possible filter aging effects in the AERONET V2 procedure. In this case, constants a and b remain constant for a given filter until its change; only the constant A is allowed to vary in the calibration fit. Our proposed GPS-based calibration procedure is advantageous in that it integrates all possible error sources such as detection response, bad filter characterization, or possible stray light (internal reflections).

6. Conclusions

The retrieval of IWV using the original AERONET V1 and AERONET V2 procedures can be validated and improved using GPS meteorology. Comparative results between IWV-SUN (V1) and IWV-GPS for seven instrument datasets from three sites in Canada over 3 yr highlight important temporal and site-to-site variations of the sun radiometer IWV accuracy with an rmse ranging between 0.14 and 0.48 g cm−2 and a bias between both databases (0.09 ± 0.16 g cm−2). These changes in accuracy are likely related to variations in the 940-nm filter response. It was demonstrated that the station-to-station and period-to-period variations could have a significant influence on the retrieved values of the three calibration constants (A, a, and b). The new AERONET V2 IWV retrieval procedure that employs instrument-dependent (a, b) constants is an improvement on the earlier V1 procedure.

The relevance of GPS meteorology for IWV calibration in sun radiometry was demonstrated in this work. The results show that the comparison between the retrieved values of IWV-GPS and IWV-SUN using GPS-based calibration reduced the rms and bias error (relative to more traditional nondynamic techniques) to about 0.1 (10%) and <0.06 g cm−2, respectively.

Given the advantage of simple, automatic, and continuous IWV measurements, GPS meteorology appears to be a powerful technique for assuring the quality and operationalization of water vapor retrievals.

Acknowledgments

The authors are grateful to Aichatou Ibrahim and Ovidu Pancrati for their help in data processing. We wish to thank Brent Holben and the AERONET staff at the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) for their commitment to the concept of providing timely public access to AERONET data. We are very grateful in particular to Alexander Smirnov for his contribution in terms of data and information on the AERONET IWV products. We wish to also thank Jim Freemantle and Bruce McArthur (MSC, Environment Canada) for their continuing contribution to AEROCAN, the Canadian sun radiometers network. The financial support of the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS), FQRNT, Québec, and NSERC Canada is gratefully acknowledged. We would also like to acknowledge the three anonymous reviewers, whose contributions have enriched the paper.

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Fig. 1.
Fig. 1.

Nominal spectral response of the 940-nm CIMEL sun radiometer (dotted line) superimposed on the water vapor transmittance simulated with MODTRAN in midlatitude summer conditions (continuous line). The 940-nm narrow-band filter has a spectral bandwidth [full width at half maximum (FWHM)] of 10 nm.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 2.
Fig. 2.

Variation of the 940-nm atmospheric transmittance calculated using the MODTRAN radiative transfer code as a function of observed slant water vapor (air mass times IWV). The IWV values used to carry out this simulation were retrieved from GPS meteorology measurements at the Sherbrooke site (45°22′N, 71°55′W). The spectral response employed to generate these points was the nominal (passband) curve of Fig. 1.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 3.
Fig. 3.

Influence of the 940-nm filter spectral shift on the a and b parameters. These values were derived using the MODTRAN (version 3.7) radiative transfer code to generate transmissions for an ensemble of IWV and m values.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 4.
Fig. 4.

Time series of IWV records from both GPS meteorology and (AERONET V1) sun radiometry for three Canadian AEROCAN sites that undergo different climatic regimes: (a) Churchill (58°45′N, 94°5′W), (b) Saturna Island (48°46′N, 123°07′W), and (c) Sherbrooke (45°22′N, 71°55′W) (see Table 1 for the measurement period).

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 5.
Fig. 5.

Example of the (A, a, b) retrieval procedure [Eq. (5)] in the case of 2002 Saturna Island observations. The points represent the aerosol and Rayleigh corrected transmission for the sun radiometer 940-nm channel [the Y term from Eq. (6)] as a function of the slant water vapor content.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 6.
Fig. 6.

(a) Uncertainty in the IWV retrieval as a function of the relative difference between the predicted and observed IWV values in the 940-nm band for the Saturna Island site (48°46′N, 123°07′W). The same plot profile is observed for the others considered sites. A 10% IWV uncertainty (accuracy of water vapor determination from radiosonde) corresponds to a 1% relative deviation in the CIMEL sun radiometer logarithm signal prediction. (b) Variation of the Y term [from Eq. (6)] with the slant water vapor content shown in Fig. 5, and corrected for outlier radiometric points (points greater than 10% IWV or 1% deviation threshold have been excluded).

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 7.
Fig. 7.

Comparison between IWV-GPS and IWV-SUN (AERONET V1 calibration; rhombus symbols), IWV-GPS and IWV-SUN (AERONET V2 calibration; plus symbol), and IWV-GPS and IWV-SUN (GPS calibration; gray star symbols) for the (a) Saturna Island and (b) Sherbrooke sites. The statistical results of the regressions are reported in Table 7.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 8.
Fig. 8.

Day-to-day variation of calibration constants (a) a, (b) b, and (c) and the associated daily number of observations. (d) For the Saturna Island (48°46′N, 123°07′W) site during the year 2002.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Fig. 9.
Fig. 9.

Plot of Y as a function of m × IWV corresponding to the single set of a, b, and A values extracted from the overall observations of the sun radiometer 940-nm channel for the Saturna Island (48°46′N, 123°07′W) site during the year 2002.

Citation: Journal of Atmospheric and Oceanic Technology 24, 6; 10.1175/JTECH2011.1

Table 1.

AEROCAN/AERONET sun radiometer sites, their corresponding GPS receiver locations, and the period of simultaneous GPS and sun radiometer observations.

Table 1.
Table 2.

Influence of variations in the filter spectral response (variable random noise) on the a and b coefficients. These values were generated using MODTRAN transmission outputs convolved with the nominal filter response of Fig. 1.

Table 2.
Table 3.

Comparison between GPS- and the sun radiometer–derived IWV (AERONET, V1) procedure (the level 1.5 and 2.0 labels refer only to the method of extrapolation of the AOD to 940 nm). The statistical parameters for the comparison are No. obs: number of observations, rmse: root-mean-square error between y and x, bias: bias error [mean (yx)], and r: correlation coefficient. The slope and yo values are defined from the linear regression: y = slope × x + Yo, where y and x stand, respectively, for dependent (IWV-SUN) and independent (IWV-GPS) variables.

Table 3.
Table 4.

CIMEL sun radiometer calibration parameters derived from the modified Langley technique compared with AERONET V1 results. For the AERONET V1 protocol, Note that A is variable while the values of a and b are fixed at 0.61 and 0.59, respectively [see Eq. (7)]. For the GPS meteorology technique, A, a, and b are variable [see Eq. (5)]. Rmse corresponds to the root-mean-square error in the Y term resulting from the curve fitting procedure.

Table 4.
Table 5.

Sun radiometric calibration constants A, a, and b retrieved using GPS measurements from half of the database considered. The second half of the database is used to validate the approach (see Table 7).

Table 5.
Table 6.

Comparison between the AERONET calibration approach and the GPS-based calibration approach (see Table 3 for the statistical parameter meanings).

Table 6.
Table 7.

Values of a and b derived from the AERONET V2 procedure for several CIMEL sun radiometers and their associated sites.

Table 7.
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