Photosynthetically available radiation at high latitudes

Transcription

1 Limnol. Oceanogr., 34(8), 1989, Q 1989, by the American Society of Limnology and Oceanography, Inc. Photosynthetically available radiation at high latitudes Janet W. Campbell and Thorkild Aarup Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine Abstract Low solar elevations at high latitudes result in two phenomena that affect the quantity and quality of light entering the sea. High surface reflectances significantly reduce the direct solar irradiance and, to a lesser extent, the global irradiance. Furthermore, there is an apparent spectral shift such that proportionately more blue (diffuse) light is transmitted and more red (direct) light is reflected by the sea surface. A model of photosynthetically available radiation (PAR} has been used to quantify these effects. The model was developed to predict daily broadband ( nm) photon flux as a function of latitude and time of year for varying cloud-free atmospheric conditions. Seasonal and spectral variability of the surface albedo is described at latitudes between 40 and 7O N. Ranges arc established for surface albedo which encompass variability due to atmospheric turbidity and wind-induced surface waves. Light availability is an important limiting factor in oceanic primary production at high latitudes (Steemann Nielsen 1974). Polar and subpolar regions experience much greater seasonal variability in photosynthetically available radiation (PAR) than do temperate and tropical latitudes (Fig. 1). During the transition seasons (spring and fall), high-latitude regions experience rapid temporal variations in daily PAR, yet in the summer PAR reaches levels that are comparable to PAR at lower latitudes. Latitudinal differences in solar elevation (Fig. 2) can affect the quantity and quality of PAR entering the water column. Low solar elevations at high latitudes result in longer atmospheric paths (i.e. more scattering); consequently, light is more diffuse. Another consequence of lower solar elevations is increased surface reflection. Losses of incoming radiation due to reflection, which are on the order of 4-6% at lower latitudes, may be significantly greater at high latitudes. The quality or spectral distribution of PAR entering the water column at high latitudes may be affected by a dependence of reflectance on wavelength, which is known to exist at low sun angles (Sauberer and Ruttner 1941). The reason for this is that diffuse and direct light are subject to differ- Acknowledgments This work was supported under grants from the NASA Ocean Processes Branch (16 l-35-01) and the Danish Natural Science Research Council (1 l-6345). This is Bigelow Contribution ent reflectances (Jerlov 1976). Diffuse light, dominated by the shorter (blue) wavelengths, is subject to a reflectance of 57% at the water surface (Preisendorfer 1976). Reflectance of the direct beam, containing the unscattered red light, is dependent on solar elevation; at low solar elevations, a large portion is reflected at the surface. Thus, the transmittance of global (direct + diffuse) irradiance through the air-water interface tends to be higher in the blue than in the red regions of the spectrum. Jitts et al. (1976) observed that the rate of attenuation of irradiance with depth tended to be insensitive to solar elevation. They explained it to be a result of variations in the proportion of direct and diffuse radiation with solar elevation. Dominance of diffuse light at low solar elevations tends to reduce the sun-angle dependence of the global irradiance. Smith and Baker (1986) investigated the transmittance of the air-water interface to energy flux (W me2) in the range nm using a model similar to that developed here. Their results apply to instantaneous fluxes as a function of solar zenith angle and include the effects of variations in the ratio of diffuse to total irradiance. They concluded that transmittances are relatively insensitive to variations in this ratio except for low solar elevations. This case is specifically addressed here. Our purpose here is to quantify the effects of low solar elevations on daily PAR at high latitudes. We have used a model to address

2 PAR at high latitudes 1491 J FMAMJJASOND Fig. 1. Seasonal patterns in daily photon flux at the surface of the ocean under clear skies at different latitudes in the northern hemisphere. Values are derived from a PAR model assuming very clear atmospheric conditions and hence represent an upper limit. two questions: what is the effective reflectance of daily PAR at high latitudes, and what is the magnitude of spectral differences between PAR incident on the water surface and that entering the water? We answer these questions first for the limiting case of an exceptionally clear atmosphere (visibility of 100 km) and flat sea surface which maximizes effective reflectance at low solar elevations, and then consider how increased atmospheric turbidity and wind-induced capillary waves affect these answers. PAR model A model was developed for predicting the daily flux of PAR, specifically the total photon flux (mole photons m-2 d-l) integrated between 400 and 700 nm at sea level and under clear (cloud-free) atmospheric conditions. Solar radiation models found in the literature (Iqbal 1983; Justus and Paris 1985; Bird and Riordan 1986; Green and Chai 198 8) generally give spectral energy flux as W mm2 nm-, or broadband energy flux for the whole solar spectrum (Bird and Hul- Strom 198 1). Approximations exist for converting the broadband flux to that in the photosynthetic range (Baker and Frouin 1987) and for converting energy to quanta in that range (Morel and Smith 1974). The PAR model used in this investigation is based on a model of spectral energy flux (W m-2 nm-l) d eveloped by Bird and Riordan (1986). An earlier version of this model Fig. 2. Seasonal patterns in solar elevation at midday at latitudes in the northern hemisphere. (Bird 1984) was used by Sathyendranath and Platt (1988) to describe the direct and diffuse components of spectral irradiance at sea level. This model was chosen because of its ability to reproduce measured spectra at solar elevations as low as 10 (Bird and Riordan 1986). The model has been tested extensively against measured spectra (Solar Energy Res. Inst., TR , 1986) and against simulated spectra from rigorous radiative transfer codes (BRITE; Bird 1983). Extraterrestrial solar spectral irradiance, adjusted for seasonal variation due to solar declination and earth-sun distances, is differentially absorbed or scattered along its atmospheric path. Atmospheric transmittance is subject to Rayleigh and aerosol scattering, and absorption by aerosols, water vapor, and ozone. Losses from the direct solar beam due to Rayleigh and aerosol scattering contribute to the diffuse radiation, which also includes radiation scattered from the surface to the atmosphere and back. Details of the model are summarized in Table 1. Instantaneous spectral irradiance (W rnb2 nm- ) incident on the sea surface at time t and wavelength X is W, 0 = J%,@, 0 + EdiXx9 t) (1) where Edir(X, t) is the direct (transmitted) irradiance and EdiXX, t) is the diffuse (sky) irradiance (units given in list of symbols). The latter is the sum of three components: Ed& t) = E,(k t> + EA& t) + Ed& t) (2)

3 1492 Campbell and Aarup Table 1. Details of the solar energy model used in this paper. Further details and formulae are given elsewhere (Bird and Riordan 1986). Variable Extraterrestrial solar energy spectrum Air mass Rayleigh scattering Aerosol extinction coefficients Ozone absorption Water vapor absorption Absorption by uniformly mixed gases Forward scatterance Aerosol single scattering albcdo Multiply reflected diffuse irradiance Remarks/references Neckel and Labs (198 1) revised spectrum as given in table 1 of Bird and Riordan (1986) for 122 wavelengths between 0.3 and 4.0 pm Relative air mass of Kasten (1966), which is accurate to better than 0.1% for zenith angles up to 86 (Iqbal 1983). Pressure-corrected air mass is used in formulae for Rayleigh scattering and absorption by uniformly mixed gases Equation adapted from LOWTRANS code (Air Force Geophys. Lab. TR , 1980) Based on turbidity formula of Angstrom (196 1); optical depth at wavelength X is given by 7h = To*(x/xo)**( -a) where TP = 0.10 at X0 =500 nm; (Y = at X <500 nm and a! = at h >500 nm Leckner s (1978) transmittance equation; spectral extinction coefficients from table 1 of Bird and Riordan (1986). Results based on assumed total ozone = cm Leckner s (1978) transmittance equation; spectral extinction coefficients from table 1 of Bird and Riordan (1986). Results based on assumed water vapor = 0 Leckner s (1978) transmittance equation; spectral extinction coefficients from table 1 of Bird and Riordan (1986) Assumed = 0.5 for Rayleigh scattering. Formula for aerosol scattering given by Bird and Riordan (1986); dependent on solar zenith and aerosol asymmetry factor (cos 0) = 0.65 Formula given by Bird and Riordan (1986); dependent on wavelength and assumed single scattering albedo = at 400 nm Assumed sky reflectivity based on air mass = 1.8 where E,(X, t) is the irradiance due to air molecule (Rayleigh) scattering, E,(X, t) the irradiance due to aerosol scattering, and E,&, t) the irradiance due to multiple reflection from the ground to the atmosphere and back. The photon flux associated with each spectral irradiance term E,(X, t) in Eq. 1 and 2 is given by where h (= x 1O-34 J s-l) is Planck s constant and c (= x 1017 nm s-l) the speed of light in a vacuum (Morel and Smith 1974). To obtain the photon flux entering the water column, we calculate a Fresnel reflectance from the solar elevation and apply it to the direct component air@, t) and apply a reflectance of (Preisendorfer 1976) to the diffuse component QdiXX, t). Spectra above and below the air-sea interface are integrated from sunrise to sunset to obtain the spectral distribution of daily PAR, and then these, respectively, are integrated over wavelengths from 400 to 700 nm to obtain PAR(0-t) and PAR(O-). The effective surface reflectance for PAR is defined as P= PAR(O+) - PAR(O-) PAR(O+). (4) In a similar manner, the effective direct reflectance &jr and diffuse reflectance p&f are defined from the direct and diffuse components of PAR. The relationship among the three is P = (1 - r)pdir + rpdif (5) where r is the ratio of diffuse to global PAR (Jerlov 1976; Jitts et al. 1976). To evaluate the effects of wind-induced surface waves on reflectance, we made use of results published by Preisendorfer and Mobley (1986) which are based on Monte Carlo simulations. According to their results, winds decrease direct reflectance at

4 PAR at high latitudes 1493 E.A 0 h x PAR PAR(O+) PAR(0 --) QCh 0 r P Pdif, Pdir Significant symbols Speed of light, nm s- Global instantaneous spectral irradiance, W m-2 nm-l Diffuse (sky) and direct (sun) components of the global irradiance, W m-2 nm- Components of diffuse irradiance due to aerosol (x = A), Rayleigh (R), and multiple (M) scattering, W m-2 nm- Planck s constant, J s-l Wavelength, nm Photosynthetically available radiation defined as Q(X, t) integrated over day and over wavelengths nm, mole photons m-2 d-l (=Einst rnd2 d-l) PAR incident on (0+) or transmitted through (0-) the water surface, mole photons m-2 d- Global instantaneous spectral photon flux, photons m-2 s I nm- Diffuse and direct components of global photon flux, photons m-2 s-l nm-* Instantaneous spectral photon flux, where subscripts x are same as for E,(X, t) above; photons m-2 s-l nm-l Ratio of diffuse to global radiation Effective surface reflectance of PAR Effective surface reflectance of diffuse and direct PAR Time, s low solar elevations, but increase direct reflectance at higher sun angles. They also found that the diffuse reflectance decreases with increased wind speeds, with values ranging from at 0 m s-l to at 20 m s-l. The results of Preisendorfer and Mobley (1986) for direct and diffuse reflectance were digitized from their figures 16 and 21 and incorporated into the model to calculate the dependence of the global reflectance on wind speed, We chose the more extreme case of a crosswind (light source at right angles to wind direction) to test whether the upper limit of reflectance (flat surface) is exceeded in the case of a wind-roughened surface, and we interpolated to obtain reflectances between the solar elevation angles shown in their figures. The effects of increased atmospheric turbidity on reflectance were examined by increasing the aerosol optical depth. Aerosol optical depth was varied to correspond to visibilities of 3, 6, 13, 25, 50, and 100 km, according to a formula relating optical depth to visibility (Iqbal 1983). It should be noted that the Bird and Riordan (1986) model is based on rural aerosols which differ from maritime aerosols. This difference is not important for the extremely clear atmosphere case where all atmospheric attenuation is minimal, but becomes more important with increased aerosols. Unless stated otherwise, references to model results in the following section will refer to the case of an extremely clear atmosphere (visibility = 100 km) and flat sea surface. Results Instantaneous solar fluxes (photons mm2 s-l) predicted by the model at varying sun elevation angles were compared with measured fluxes from a variety of geographic locations including high-latitude regions. Model predictions describe an upper limit to measurements made just beneath the water surface (Hojerslev 1982) and prove to be accurate estimates of solar fluxes on clear days for a range of solar elevations (Fig. 3A). In addition, model estimates of daily PAR were compared with measurements made during the MARMAP cruises (O Reilly and Busch 1984), and again model results represent an upper limit for measured values (Fig. 3B). The ratio of diffuse to global radiation exhibits strong seasonal and latitudinal variation north of 4O N (Fig. 4). Increases in this ratio tend to reduce global reflectance, compensating for losses due to increased direct reflectance (Fig. 5). The solid curves in Fig. 5 are the effective global reflectance p defined by Eq. 4. The dashed lines are the effective direct p&r and diffuse reflectance p&f (see Eq. 5). At 60 and 70 N, maximal values of global reflectance as high as 23% can occur during winter. Dates of maximal reflectance do not coincide with the winter solstice, however, because of the dominance of the diffuse component at that time of year. Seasonal patterns are less pronounced at 40 and 5O N but still present, with global reflectance ranging from 4% in summer to 11 or 17% (respectively) at the winter solstice.

5 1494 Campbell and Aarup m ; m 100 $ Et. North Atlantic 3 ii o Mediterranean 0 l Off California gj 10 * Indian Ocean Solar Elevation, Degs. Latitudes J FMAMJJASOND Fig. 3. Comparison of model-derived photon flux (curves) with data (symbols). A. Instantaneous flux ( nm) just beneath the surface of the ocean. Line is model prediction. Symbols are measurements from various geographic locations (Hojerslev 1982). B. Daily PAR measurements made as part of MARMAP cruises to the northwest Atlantic continental shelf between 35 and 45 N, (C) Reilly and Busch 1984). The spectral character of global reflec- in reflectance is made clearer by examining tance on the summer solstice (A), vernal the spectral distribution of the daily photon equinox (B), and two winter dates (C, D) is flux in absolute terms (Fig. 7). In Fig. 7, shown in Fig. 6 for four latitudes. As might each pair of spectra separated by a hatched be expected, reflectance increases with low- area represents the spectral distribution of er solar elevations, and the spectral depen- PAR(0 +) and PAR(0 -). The dates of greatdence is more pronounced to the north. est spectral variability in reflectance corre- The importance of the spectral variability spond to dates of very low solar energy flux

6 PAR at high latitudes 1495 I I I,,,,,, I I I JFMAMJJASOND Fig. 4. Ditise (sky) component as percentage of global daily PAR as a function of time of year and latitude. and, hence, are relatively insignificant to the energy budgets driving photosynthesis on an annual scale. Global reflectance curves for varying wind speeds are shown in Fig. 8. A very light wind (2 m s-l) increased global reflectance slightly on some dates, but <0.4% in all cases. Higher winds (S-20 m s-l) decreased global reflectance significantly. The global reflec- tance defined for a flat sea surface remains as an upper limit, for all practical purposes, for the case of an exceptionally clear atmosphere. The effect of turbidity (as increased aerosols) is shown in Fig. 9 for the flat surface (no wind) situation. Increasing aerosols cause the reflectance to approach that of the diffuse component, as expected since increasing aerosol optical depth increases the diffuse component of surface irradiance at the expense of the direct component. The trends shown in Fig. 9 would not change with alternative aerosol types (e.g. maritime, urban, etc.), but the absolute values may change. Discussion Results presented in Figs. l-7 are for a very clear, cloud-free atmosphere and a flat sea surface (no wind). In calculating the effects of surface waves (Fig. 8) and increased aerosols (Fig. 9) on the global reflectance, we see that the very clear, zero-wind situation (Fig. 5) defines the outer edge of an 1 40 N 50' N 0;,,,,,,,,,,, 1 01 I I I I 1 I , 1 JFYAMJJASOND JFYAMJJASOND 60' N 70 N 3o \ 25- \ N \. ti j$ ii!:; d lo- IG _ 5- l 5-0 II I I II 1 I I,, 1 JFYAYJJASOND JFYAMJJASOND Fig. 5. Seasonal patterns in the effective daily reflectance of direct, diffuse, and global PAR. The global reflectance is for extremely clear atmospheric conditions (visibility = 100 km) and a flat sea surface.

7 1496 Campbell and Aarup ' N N N 1 D 20 0 ( I I I 0: I I oonm oollm Fig. 6. Spectral reflectance of global PAR on four dates: 2 1 June (A), 21 March (B), 21 February (C), and 10 February (D). envelope. During winter months, this rep- the most extreme effect. The model does not resents an upper limit for reflectance, and include the effects of clouds, so we are rein summer it generally defines a lower limit stricted to a more qualitative discussion of (though not always). their influence. The other edge of the envelope is defined If cloud cover were included, the daily by the diffuse reflectance Pdif. During winter, PAR would range from < 10% to 100% of this is a lower limit, which may be as low the maximum predicted by the model for as or less. The value corre- each date and latitude. In the MARMAP sponds to a totally overcast sky and a wind data of Fig. 33, for example, measured valspeed of 20 m s-l (Preisendorfer and Mob- ues of PAR were 5 1% of their maximum ley 1986). Since overcast skies and high values on average, and monthly averages winds are common at high latitudes, the ranged from 40 to 60%, depending on. seaupper limit defined by clear skies and no son. With increases in cloud cover, the difwinds may seldom be realized. fuse component of global PAR (i.e. the ratio Our model allows us to consider the ef- r in Eq. 5) becomes proportionately larger, fects of increasing aerosols, absorbing gases, and hence global reflectance approaches the and water vapor on the magnitude and spec- diffuse reflectance value Pdifi tral distribution of PAR, and on surface re- The second question we addressed in this flectances. Of all the variables that affect study concerned the magnitude of spectral surface irradiance, however, cloud cover has shifts in PAR that occur with surface re-

8 PAR at high latitudes N ' N.3.2 A.a 1 70 N.1 B C D oonm.l oonm Fig. 7. Spectral photon flux above and below the surface on four dates: 2 1 June (A), 21 March (B), 2 1 February (C), and 10 February (D). The pair of curves separated by hatching correspond to the above and below surface values. flection. We observed that reflectance was generally spectrally flat except on dates (and locations) when incident PAR was very low. These results were for the very clear sky, flat surface situation. Since the maximal spcctral variations are associated with maximal reflectance, effects of winds and atmospheric turbidity should also diminish the spectral variability. This is certainly true for increases in turbidity since the incident radiation becomes more diffuse. We consider the magnitude of spectral shifts in PAR to be insignificant. Much greater spectral variation exists in surface irradiance due to changes in cloud cover and other atmospheric constituents. The latter occur over short time scales and are highly stochastic in nature. Conclusions Reflectance of daily PAR at the air-water interface falls within an envelope defined by the reflectance of diffuse radiation and that of global radiation as predicted for clear skies with no wind. The range of reflectance values widens in winter at all latitudes and is relatively narrow during summer months. During summer, surface reflectance is generally between 4 and 7% at all latitudes (40-70 ) and under all atmospheric and wind conditions. During winter months, the maximum reflectance ranges from 11% at 40 N to 23% at 70 N, but the dates of highest reflectance receive very low solar radiation. Wind-induced surface waves generally increase the amount of light entering the sea

9 be 25 g c N JFYAYJJASOND N 60 N 70 N : 1 1 I I 1 I I I I I I I JFYAYJJASOND 0; I I I I I I I I I I JFYAYJJASON~ ws me ms - Fig. 8. Effective reflectance of global PAR at the surface for variable wind speeds (WS) ranging from 0 to 20 m s-l. The effects of wind are based on albedo effects of a crosswind (derived by Preisendorfer and Mobley 1986) N N o! I I I I I I I l 11 JFMAYJJASOm 1 60' N lo N 0; I I I I 1 l I I I l I JFMAMJJASOND VIS km

10 PAR at high latitudes 1499 and thus decrease reflectance. Similarly, the presence of clouds and/or haze affects reflectance by increasing the diffuse component of total PAR. With increases in turbidity, the global reflectance approaches the diffuse reflectance. The effect may be either an increase or decrease in reflectance, depending on the season. Spectral shifts associated with surface transmittance at low sun angles are monotonically increasing with wavelength. We consider their magnitude to be insignificant as compared with spectral variability in the incident radiation due to clouds and other atmospheric constituents. References ANGSTROM, A Technique of determining the turbidity of the atmosphere. Tellus 13: BAKER, K. S., AND R. FROUIN Relation between photosynthetically available radiation and total insolation at the ocean surface under clear skies. Limnol. Oceanogr. 32: 1370-l 377. BIRD, R. E Terrestrial solar spectral modeling. Sol. Cells 7: A simple solar spectral model for direct-normal and diffuse horizontal irradiance. Sol. Energy 32: 46 l , AND R. L. HULSTROM Direct insolation models. Trans. ASME (Am. Sot. Mech. Eng.) J. Sol. Energy Eng. 103: AND C. RIORDAN Simple solar spectral model for direct and diffuse irradiance on horizontal and tilted planes at the earth s surface for cloudless atmospheres. J. Climate Appl. Meteorol. 25: GREEN, A. E. S., AND S.-T. CHAI Solar spectral irradiance in the visible and infrared regions. Photochem. Photobiol. 48: HBJERSLEV, N. K Bio-optical properties of the Fladen Ground* Meteor -FLEX-75 and FLEX- 76. J. Cons. Cons. Int. Explor. Mer 40: IQBAL, M An introduction to solar radiation. Academic. JERLOV, N. G Marine optics, 2nd ed. Elsevier. JITTS, H. R., A. MOREL, AND Y. SAIJO The relation of oceanic primary production to available photosynthetic irradiance. Aust. J. Mar. Freshwater Res. 27: JUSTUS, C. G., AND M. V. PARIS A model for solar spectral irradiance and radiance at the bottom and top of a cloudless atmosphere. J. Climate Appl. Meteorol. 24: KASTEN, F A new table and approximate formula for relative optical air mass. Arch. Meteorol. Geophys. Bioklimatol. Ser. B 14: LECKNER, B The spectral distribution of solar radiation at the earth s surface-elements of a model. Sol. Energy 20: MOREL, A., AND R. C. SMITH Relation between total quanta and total energy for aquatic photosynthesis. Limnol. Oceanogr. 19: 59 l-600. NECKEL, H., AND D. LABS Improved data of solar spectral irradiance from 0.33 to 1.25 pm. Sol. Phys. 74: O REILLY, J. E., AND D. A. BUSCH Phytoplankton primary production on the northwestern Atlantic shelf. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 183, p PREISENJXIRFER, R. L Hydrologic optics. 6 V. Natl. Tech. Inform. Serv., Springfield, Va. -, AND C. D. MOBLEY Albedos and glitter patterns of the wind-roughened sea surface. J. Phys. Oceanogr. 16: 1293-l 316. SATHYENDFCANATH, S., AND T. PLATT The spectral irradiance field at the surface and in the interior of the ocean: A model for applications in oceanography and remote sensing. J. Geophys. Res. 93: SAUBERER, F., AND F. RUTTNER Die Strahlungsverhaltnisse der Binnengewasser. Akad.-Verlag. SMITH, R. C., AND K. S. BAKER Analysis of ocean optical data 2, p In Ocean Optics 8, Proc. SPIE 637. STEEMANN NIELSEN, E Light and primary production, p. 36 l-387. In N. Jerlovand E. Steemann Nielsen [eds.], Optical aspects of oceanography. Academic. c Fig. 9. Effective reflectance of global PAR at the surface for variable aerosol optical depths. Optical depths correspond to variations in visibility (VIS) ranging from 3 to 100 km.