Documentation of the Arctic Global Radiation (AGR) Data Set Mark C. Serreze, Jason E. Box and James A. Maslanik Cooperative Institute for Research in Environmental Sciences Campus Box 449 University of Colorado Boulder CO 80309-0449 Jeffrey R. Key Department of Geography Boston University 675 Commonwealth Avenue Boston, MA 02215 1. SUMMARY The often-cited Arctic radiation climatologies by Marshunova [1961], Marshunova and Chernigovskii [1966, 1971] Vowinckel and Orvig [1962,1963,1964] and Gavrilova [1963], are based on data from coastal sites and sparse Arctic Ocean data then available from the Russian "North Pole" (NP) series of drifting ice stations. Fletcher [1966] noted that differences between Marshunova's and Vowinckel and Orvig's climatologies are much greater than the estimated accuracies. On this basis he questioned Gavrilova's [1963] arguments that global radiation in the Arctic is known to an accuracy of 2.5% for annual fluxes, 5-10% for monthly fluxes, 10-15% for absorbed solar radiation, 15-20% for outgoing longwave radiation and 20-30% for the radiation balance. Nevertheless, he concluded that the work of Marshunova [1961] probably represented the most accurate description of the Arctic radiation climate. Ohmura [1981] arrived at the same conclusion 15 years later. Much more recently, Marshunova and Mishin [1994] have summarized results from the Russian "North Pole" (NP) stations 2-31, covering the period 1950-1991. The need to validate climate model output and satellite- derived radiation fluxes warrants the development of new climatologies of surface radiative fluxes [cf. Marshunova and Mishin, 1994]. To this end, NSIDC has assembled the two-part Arctic Global Radiation (AGR)data set. AGR-1 represents times series of monthly fluxes from land stations, ocean drifting stations and empirically-derived long-term climatological estimates from earlier Russian studies. While many of the station time series are quite short, some span over 40 years. Data are provided for the region north of 60oN. AGR-2 is a long- term monthly mean gridded climatology for the region north of 65oN, based on the AGR-1 data set. Work is underway to update the AGR monthly data set and to develop companion data sets of daily and hourly global and net radiation fluxes for release on CD-ROM. The AGR data set documentation which follows below largely follows a recent paper by Serreze et al. [submitted]. 2. AGR-1 2.1 Data Sources Starting in 1950 the Soviets one to three "North Pole" drifting ice stations in the Arctic Ocean. Thirty ice stations were manned between 1950 and 1991 (NP-2 through NP-31; NP-1, the first ice station, operated during 1937-1938). Radiation measurements were made on a regular basis. The average duration of each station was 2.4 years. Russian scientists at the Arctic and Antarctic Research Institute (AARI) have assembled these records to provide monthly averages at the mean positions of the stations, climatological monthly fluxes on a coarse grid and daily values from NP-17 through NP-31 [Marshunova and Mishin, 1994]. The monthly data used here were obtained from a CD-ROM compiled by University of Washington's Polar Science Center (PSC)and NSIDC. The monthly means for NP 17-31 were processed from the daily values. We required than at least 20 observations be available for each month, otherwise the flux was coded as missing. Radiation measurements from the U.S. ice island "T-3" (also known as "Fletcher's Ice Island" and "Ice Station B") for primarily spring-autumn months of 1953, 1957-1959 and 1971-1973 were digitized from tables in Marshunova and Chernigovskii [1971] and Weller and Holmgren [1974], with mean monthly positions obtained from the PSC and the latter report. From 1953-1959, T-3 drifted from north of Ellesmere Island along the Canadian coast and into the Beaufort Sea. It then continued to drift clockwise around the Canada Basin again to north of Ellesmere Island, remaining nearly motionless at approximately 85oN, 85oW during 1971-1973. Monthly mean fluxes and positions from the U.S. "Arlis II" drift station for January 1964 through May 1965 were digitized from tables published by Roulet [1969]. During this interval, Arlis II drifted from near the Pole to just north of Greenland. Efforts were also made to recover radiation fluxes in the Beaufort Sea collected at four drifting camps during the Arctic Ice Dynamics Joint Experiment (AIDJEX) [Pautzke and Hornof, 1978]. Careful inspection of these data revealed numerous large errors. Following recommendations from a scientist involved in the measurement program (G. Weller, pers. comm.), these data were discarded. The resulting number of available station months from combining the Russian NP and U.S. drifting records is over 70 from June through September and over 40 for March through May as well as October. Few data points are available from November through February, ranging from one in December to 15 in February. Data from land stations were obtained from several sources. The Global Energy Balance Archive (GEBA) [Ohmura and Gilgen, 1991] contains monthly means from 57 land stations north of 60oN. Records range in length from 1 to 49 years, with data for stations in continual operation available through 1987. We also use data from 11 Alaskan stations north of 60oN from the National Oceanic and Atmospheric Administration (NOAA) Solar and Meteorological Surface Observations (SAMSON) archive for the time period 1961-1990. Records of typically 20 years duration (most extending through part of 1993) for 14 sites in the Canadian Arctic were obtained from the Atmospheric Environment Service (AES), Ottawa. There were a number of duplicate stations in the GEBA, SAMSON and AES archives with overlapping periods of record. Efforts were made to assemble the most compete record possible from duplicate stations. Both the AES and SAMSON archives represent hourly observations, from which we computed monthly means. Several years of monthly data (1971-1973) were obtained for Broughton Island, off the east coast of Baffin Island, collected as part of a field experiment [Jabobs et al., 1974] and for the Carey Islands (1973-1974), Coburg Island (1972-1974) and Cape Herschel (1973-1974), collected during the Baffin Bay "North Water Experiment" [Muller et al., 1976]. Monthly data were also obtained for six sites over the Greenland Ice Sheet from the Program for Arctic Regional Climate Assessment (PARCA) (headed by K. Steffen, University of Colorado and sponsored by the National Aeronautics and Space Administration and the National Science Foundation). Four sites over western Greenland provide a single year of data between 1995 and 1996, with the summit station providing only summer data for 1996. A longer record, complete for 1994-1995 with partial records for 1990-1993 and 1996 is provided from the PARCA University of Colorado Greenland Camp. Climatological monthly means for two Eurasian sites from Gavrilova [1963] are also included in our data set. It was found that the above archives provide poor coverage of the North Atlantic between about 60oN-65oN and for coastal Greenland. To obtain coverage over these areas, it was necessary to resort to estimated rather than measured values. For coastal Greenland stations and Jan Mayen we made use of climatological values from Marshunova and Chernigovskii [1971]. For the North Atlantic, a series of points was chosen from contour maps provided by Smetannikovoi [1983]. Marshunova and Chernigovskii employ the method of Vowinkel and Orvig [1964] to estimate station fluxes from consideration of clear-sky radiation and attenuation by aerosols, cloud amount and type. These relationships are then assumed to hold for other stations for which no fluxes are measured. The Smetannikovoi [1983] values are calculated similarly. Marshunova and Chernigovskii find that this technique is generally accurate within 10%, but their Table 13 indicates that it may be in error by as much as 25% in autumn. 2.2 File Structure AGR-1 is an ASCII file with the following format: (A20,1x,F7.2,F8.2,I5,12F6.1,I3,I6,1x,A1) The variables (1-19) in each record are as follows: 1) STATION NAME (A20): Name of station. Abbreviations "Is." correspond to "Island" and "Obs." to "Observatory". Climatological values from Smetannikovoi [1983] are listed as "North Atlantic". One unknown station is listed as "Unknown". 2) LATITUDE (F7.2): Latitude in degrees and decimals. 3) LONGITUDE (F8.2): Longitude in degrees and decimals. 4) YEAR(I5): Year. For empirical estimates or long-term climatological measured values, YEAR has a value of zero. 5-16) FLUX (12F6.1): Mean radiation flux in W m-2 for January through December, respectively. Missing values are -99. With few exceptions, the positions of the drifting stations change every month. To preserve a file structure consistent with the fixed- location land data, each individual drifting station measurement (referenced to the average position of the station) appears as an individual record, with all months except that represented by the monthly position padded with -99. For example, if the measurement is for June 1982, January-May and July-December will be padded with -99. As the positions corresponding to the data points pulled from the maps of Smetannikovoi [1983] are not entirely consistent between months, these estimates are reported in the same fashion. 17) SOURCE CODE (I3): Source of the data (codes are listed below). 18) STATION IDENTIFICATION (I6): Station code used in the original data source. It takes a value of zero if a code was not provided with the original data source or if no code is applicable. 19) TYPE FLAG (A1): Flag indicating whether the flux is measured ("M") or estimated ("E"). Source Codes (Variable 17) 1) AES Archive (Canadian Stations) 2) SAMSON Archive (Alaska Stations) 3) GEBA Archive (Land Stations North of 65oN) 4) PARCA Archive (Greenland Stations) 5) Smetannikovoi [1983] (North Atlantic Estimates) 6) Gavrilova [1963] (Long-Term Climatologies) 7) Marshunova and Chernigovskii [1971](Long-Term Climatologies) 8) Muller et al. [1976] (Northwater Region) 9) Jabobs et al. [1974](Broughton Island) 10) PSC/NSIDC, Marshunova and Mishin [1994] (NP Stations) 11) Marshunova and Chernigovskii [1971], Weller and Holmgren [1974] (T-3) 12)Roulet [1969](ARLIS-II) 3. AGR-2 3.1 Interpolation The objective is to use AGR-1 to provide long-term monthly means at a regular grid of points. A Cressman interpolation [Cressman, 1959] was used to interpolate the irregularly spaced data to an 89x89 grid point (7921 grids) subsection of a 100x100 km version of the EASE-Grid centered over the pole [Armstrong and Brodzik, 1995]. This is a lower-resolution form of the same equal-area grid being used at the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado, for producing SSM/I global brightness temperature grids and geophysical products from the NOAA/NASA SSM/I Pathfinder program. The Cressman interpolation has the form Fease= äFiWi/äWi Wi = (N2-d2)/N2+d2) where Fease is the flux at a desired EASE-Grid point, Fi is the flux at an observed location i, Wi is the weight given to that observation, d is the distance between the observation and the EASE-Grid point, and N is the search radius, beyond which the weight is zero. An initial search radius is set, with the summations performed for all observations falling within that radius. If no observation falls within that radius, the search is extended over larger values of N. The use of the raw values in the interpolation procedure may introduce biases related to latitudinal dependencies of the flux. Therefore, the interpolation is instead based on values expressed as the difference between the observed radiation (Gobs) and an estimate of the clear-sky downwelling shortwave flux (Gclr). The adjustment by Gclr essentially normalizes the data with respect to latitudinal variations in solar zenith angle/daylength and associated path-length dependencies of non-cloud atmospheric absorption and scattering. It also acts to normalize the data with respect to the minor effect of varying surface albedo (Gclr tends to increase with respect to albedo due to the effects of multiple scattering). After the interpolation, Gclr is added back to the grid-point values. Gclr is computed using a neural network implementation [Key et al., 1996] of the two-stream radiative transfer model used by Schweiger and Key [1994] that was employed in the computation of radiative fluxes from the International Satellite Cloud Climatology Program (ISCCP) monthly (C2) cloud product [Rossow and Schiffer, 1991]. The clear sky flux calculations were done using ISCCP-C2 all-sky surface albedos [Schweiger et al., 1993], an aerosol optical depth of 0.05 representing background tropospheric conditions, month- specific total precipitable water ranging from 1000 g m-2 (January) to 10,000 g m-2 (August), and total column ozone of 7 g m-2. The effect of uncertainties in these quantities is small; e.g., for a 20% error in an albedo of 0.85, the error in a calculated flux of 400 W m-2 is only 5 W m-2. The AGR-1 land data sources with monthly resolution vary widely in terms of record length, with some of the land and ocean data records expressed instead as climatological monthly means. With few exceptions, each individual monthly drifting station record represents a sample from a unique location. Simply passing all available data values into the interpolation without addressing these inconsistencies would result in serious spatial biases. Similar to the procedure used by Serreze et al. [1995] to provide a gridded Arctic climatology of precipitable water and vertically-integrated water vapor fluxes, we adopted a two-step interpolation approach whereby the drifting station and fixed- location records are equally represented. First, we passed all individual monthly values from the drifting stations into the interpolation (each adjusted by the climatological clear-sky flux), providing climatological monthly means at ocean grid points only (determined from a land/ocean mask). We required that for each grid point the interpolated value be based on data from at least five different years; otherwise, it was coded as missing. We initially used a 500 km search radius. If fewer than five values were represented in the interpolated flux, the search radius was enlarged to 750 km. The monthly land station measurements were then processed to provide climatological monthly means, also expressed as departures from clear sky values. Climatological means based on fewer than three years were discarded. The exception is for the Greenland ice sheet stations. Means for all of these irregularly-distributed fixed locations were then appended to the estimated values described in Section 2.1 and ocean grid point means derived from the initial interpolation, with the combined data set then passed through a second interpolation, using search radii of 500 km, 750 km, 1000 km, and 1250 km and a minimum of two values for each radius. The larger search radius was necessary due to the comparatively sparse distribution of the land stations. Initial results indicated a regional problem related to sharp flux gradients between the PARCA Greenland ice sheet stations and surrounding coastal sites. Consequently, we modified the interpolation so that fluxes over central Greenland are interpolated from the PARCA values only. As a result of the second interpolation, ocean grid points experience some additional adjustment by surrounding ocean points, or if near the coast, from land stations. In turn, coastal land grid points will be adjusted by the ocean grid values. Few observations are available for the drifting stations from November through February, resulting in missing data values over the central Arctic Ocean. Corresponding values of Gclr, are, of course, very small, generally less than 15 W m-2 during this time of year. For the sake of completeness we simply filled missing values for which Gclr is less than 15 W m-2 by multiplying Gclr by an assumed effective cloud transmittance(Gobs/Gclr) of 0.60. The two step interpolation and the latter adjustment provided nearly complete coverage for all EASE-Grid points north of 65oN. 3.2 File Structure AGR-2 is an ASCII file with the following format: (I4,F5.1,F7.1,12F7.1,I4) The variables (1-16) in each record are as follows: 1) GRID (I4): Grid number from 1-7921 (defines an 89x89 array) 2) LATITUDE (F5.1): Latitude of the grid number 3) LONGITUDE (F7.1): Longitude of the grid number 4-15) FLUX (12f7.1): Mean radiation flux in W m-2 for January through December, respectively. Missing values are 9999 16)FLAG (I4): Indicator of ocean grid (O) or land grid (255) 4.0 LIMITATIONS The absolute accuracy of the radiometers used for the measurements is generally ñ 5-10 W m-2. Sensor drift is assumed to be negligible under the assumption that the sensors underwent regular calibration, maintenance, and replacement when necessary. The AGR-1 data sources have been quality controlled by the investigators who developed them. As such, for compiling AGR-2, we use these data largely as provided. Obvious outliers were flagged through manual inspection and limits checks. However, when the solar zenith angle is extreme (e.g., greater than 85o), measurement errors can be large due to limitations in the cosine response of the instruments and multiple reflections within the radiometer domes. Furthermore, the neural network used to estimate Gclr has only been trained up to solar zenith angles of 85o. These zenith angle problems are primarily limited to the months October-March when fluxes are small, especially at high latitudes. Regarding the interpolations, although on a year-to-year basis the drifting stations provide only one or two points per month, taken over the entire period of record they provide reasonably good coverage. However, for different ocean regions we are sampling from different years so biases may be introduced. The coverage is fairly sparse over the Canadian and Eurasian sectors, but apart from the short records for the Northwater region and Broughton Island, the station means are based on many years (a 23-year modal station duration for the GEBA records). Clearly, we would like to have data for more years over Greenland, but we are limited by the record presently available. Results for the northern North Atlantic and coastal Greenland, which are based largely on empirically-derived values, should also be viewed cautiously. Because the interpolations are based on a large search radius, inhomogeneities in the gridded fields due to data sampling problems will tend to be minimized. However, by the same token, real local variations are smoothed. 5. ACKNOWLEDGEMENTS Development of the AGR data set was provided by NSF Grants OPP-9321547, OPP-9504201, OPP-9614297 and NASA grant NAGW-2407. Documentation and final data set assembly was supported by the National Snow and Ice Data Center DAAC under NASA contract NASA- 32392. K. Steffen is thanked for the PARCA data. 6. REFERENCES Armstrong, R.L. and M.J. Brodzik, 1995: An earth-gridded SSM/I data set for cryospheric studies and global change monitoring, Adv. Space Res., 16(10), 155-163. Cressman G.P., 1959: An operational objective analysis system, Mon. Wea. Rev., 87, 367-374. Fletcher, J.O., 1966: The Arctic heat budget and atmospheric circulation, Proc. Symp. Arctic Heat Budget and Atmospheric Circulation (J.O. Fletcher, ed.), Rand Corp. Mem. RM-5233-NSF, 23-43. Gavrilova, M.K., 1963: Radiatsionnyi klimat Arktiki (Radiation Climate of the Arctic), Gidrometeorologicheskoe Izdatel'stvo, Leningrad (Translated from Russian, Israel Program for Scientific Translations, Jerusalem, 1966), 178 pp. Jacobs, J.J., R.G. Barry, R.S. Bradley and R.L. 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