Direct Solar Radiation

Geologische Abhandlungen von Hessen 2004, 114, 24-35 (Engl. translation 2010) DIETER HOPPMANN* Direct Solar Radiation 1. Introduction Even today, it is very difficult to produce a tion reaching the soil. Part of the incipient so- detailed, area-wide presentation of the basic cli- lar radiation absorbed by the soil or vegetation matic factors that influence grape quality. Any cover is emitted back to the surrounding air or endeavor to survey one of the most important of the adjacent plants. Therefore, those sites that these, the relationship between daily and annual are optimally inclined to receive solar radiation, temperature variation and site topography invol- will heat up more rapidly than less optimally in- ves a vast effort in terms of scientific instrumen- clined locations. tation and data processing. The first climatic site However, direct solar radiation is not only de- evaluation surveys were therefore restricted to termined by astronomical factors. Hourly sunshi- determining the physiologically important he- ne duration, atmospheric turbidity and terrestrial at balance along the northern margin of wine shielding also affect the amount of energy availa- growth using easily obtainable measurements. ble from solar radiation. The so-called Offenbach The first Viticultural Survey of Hesse (ZAKOSEK Evaluation Scheme (BRANDTNER 1973) for calcu- et al. 1967) already included a map of the ma- lating radiation takes these effects into account. ximum potential radiation over the complete Long-term measurements of atmospheric tur- vegetation period between April and October bidity and sunshine duration (1951 – 1980) in in relation to aspect and slope. The justification various regions of Germany have improved solar for this procedure is based on the fact that site radiation calculations. temperature is related to the amount of radia- 2. The model for calculating direct solar radiation 2.1 Theory A good indicator of the thermal conditions of lar radiation correlates to the slope and aspect of a location is the net solar radiation Q (equation the site and is not affected by ground conditions. 1). Q is the sum of all short and long wave radia- Pre-requisite to this procedure is a near balan- tion effects at ground level. Positive values indi- ce at our latitude between energy uptake by dif- cate radiation energy surplus, whereas negative fuse sky radiation (D) and atmospheric long wave values signify energy loss. However, Q can only radiation (G) on one side and loss by terrestrial be estimated for a few locations since net radi- long wave radiation (Rk) and albedo (A) on the ation also takes into account soil conditions and other over long periods of time. Consequently, plant cover. This is why direct solar radiation (I) direct solar radiation (I) is the only site-specific is used to indicate thermal conditions. Direct so- variable in the radiation energy balance equation. * Dr. D. Hoppmann (e-mail: [email protected]), National Meteorological Service (DWD), Agrometeorology, Kreuzweg 25, D-65366 Geisenheim. 24 Geologische Abhandlungen von Hessen 2004, 114, 24-35 (Engl. translation 2010) atmosphere and (Io) is the intensity of solar ra- Q = I + D + G – Rk – A (1) diation at the top of the atmosphere (solar constant). (am) is the transmission coefficient of the This procedure is justified for long-term ob- atmosphere (depending on the thickness of the servations over the complete vegetation period atmosphere m) (Tm) is the turbidity factor of the (HOPPMANN 1978). Thus, the available energy for atmosphere and () is the angle between the in- a specific location is estimated by measuring the coming rays and the reference plane. Angle () hourly direct solar radiation (Joule/cm2) for each is correlated to latitude (), solar declination (), day during the vegetation period (April 1 to Oc- hour angle (), slope aspect () and the angle of tober 31) in correlation with the mean percent hourly sunshine. This procedure explicitly em- sin = (sin × cos υ–cos × sin υ × cos)× (3) phasizes the diurnal and annual variability due sin + (cos × cos υ + sin × to site position. In addition to this, it also takes sin υ × cos ) × cos × cos + into account the distinct differences in daytime sin υ× sin × cos × sin cloud distribution between each location. Clou- diness and sunshine hours are directly correla- inclination (υ) as follows: ted. Direct solar radiation (I) is calculated with the STRA = I × (SS / SSmax) (4) following formula: The actual solar radiation (STRA) reaching the earth surface is reduced by clouds and is given I = I × exp (–a T ) × sin (2) o m m by: Where SS is the actual duration of sunshine Here (I) is defined as the maximum possible and SSma is the maximum possible duration of solar radiation that may pass through a cloudless sunshine. 2.2 The effect of turbidity, slope and aspect on direct solar radiation estimates The atmospheric turbidity factor (Tm) is set at The turbidity factor is a measure of the extinc- a constant 3.0 for the whole vegetation period in tion of direct solar radiation as it passes through the original model for calculating solar radiation the atmosphere and interacts with aerosols and (BRANDTNER 1973). This calculation is still used trace gases including water vapor (KASTEN 1984: for climate evaluations of vineyard locations ac- 28 f.). Revised calculations using the new turbi- cording to the Wine Economy Law. However, dity factors are compared with the previous ones long-term turbidity factor measurements by the in the next section. Meteorological Observatory Hamburg provided The calculations were performed for five in- the mean monthly results shown in Table 1. clinations 5, 10, 15, 20 and 25 degrees for each 25 Geologische Abhandlungen von Hessen 2004, 114, 24-35 (Engl. translation 2010) Tab. 1. Mean monthly turbidity factors Tm (Met. Observatorium Hamburg) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Tm 3.7 4.1 4.6 5.0 5.1 6.0 6.2 5.8 5.5 4.2 3.7 3.6 of the eight directions N, NE, E, SE, S, SW, W cularly important when considering the radiation and NW at the Geisenheim station located at 50° model according to BRANDTNER (1973) used for latitude using the higher and the lower turbidity vineyard evaluations. This model is based on a factors (Table 2). constant turbidity factor of 3.0. The mean percent hourly sunshine data used The results (Table 2) clearly show how the in these calculations was obtained from the 1951 revised turbidity factors affect the calculations. – 1980 measurements (Table 3). The aim of this The average available energy from direct solar comparison is to ascertain the effect of the diffe- radiation at high turbidity values (new values) is rent turbidity factors on the results. This is parti- 45.5 kJoule/(cm² ×Vp) less than at low turbidity Tab. 2. Comparison of the energy gain from direct solar radiation (kJoule/cm2/Vp) using old and new turbidity factors Tm Vp : Vegetation period (April 01 – Oct 31) Geisenheim (1951–1980) 2 kJoule/(cm ×Vp) slope aspect Tm (old) N NE E SE S SW W NW Slope 0° 45° 90° 135° 180° 225° 270° 315° 0° 207 207 207 207 207 207 207 207 5° 196 199 206 214 216 214 207 200 10° 183 189 204 218 224 219 204 190 15° 169 178 200 221 230 223 202 179 20° 154 166 195 223 234 225 197 167 25° 138 152 189 224 238 226 192 154 2 kJoule/(cm ×Vp) slope aspect Tm (new) N NE E SE S SW W NW Slope 0° 45° 90° 135° 180° 225° 270° 315° 0° 160 160 160 160 160 160 160 160 5° 151 154 160 166 168 166 160 155 10° 141 146 158 169 174 170 158 146 15° 130 137 155 172 179 173 156 138 20° 118 128 151 174 182 175 152 128 25° 105 116 146 175 185 176 148 116 26 Geologische Abhandlungen von Hessen 2004, 114, 24-35 (Engl. translation 2010) values (old values). The energy differences vary dered in this case. The distribution of direct so- between 36 and 52 kJoule/(cm² ×Vp) depending lar radiation emphasizes the profound effect of on exposition. For example, the difference is gre- aspect and slope on the total available energy ater between north and south exposed steep slo- during the vegetation period from April to Octo- pes. Since the revised turbidity values form the ber. A north exposed slope inclined at 10 degrees basis for these maps, these differences must be receives about 33 kJoule/(cm²× Vp) less energy taken into account when comparing the results than a comparable south exposed slope. On stee- with the energy values proscribed by the Wine per slopes inclined at 25 degrees, the energy Business Law. difference increases to about 80 kJoule/(cm² × Table 2 also provides an overview of radiation Vp). In these examples the deficits of the north balance differences in correlation to slope and exposed slopes are due to the unfavorable angle aspect. The effects of elevation are not consi- of incidence of solar radiation. Tab. 3: Mean percent hourly sunshine in per mille [‰] Geisenheim (1951–1980) MLT 3–4 4–5 5–6 6–7 7–8 8–9 9–10 10–1111–12 12–13 13–14 14–15 15–16 16–17 17–18 18–19 19–20 20–21 Apr 0 0 17 223 412 488 541 543 553 559 548 525 500 461 351 95 0 0 May 0 2 158 420 495 535 566 584 580 565 580 562 538 498 458 333 45 0 Jun 0 5 254 464 538 552 555 558 554 538 550 540 510 496 465 389 119 0 Jul 0 4 186 425 511 548 570 576 567 553 565 569 520 501 469 385 73 0 Aug 0 0 34 276 470 536 581 602 612 602 589 558 535 490 422 199 3 0 Sep 0 0 0 57 278 447 523 564 585 576 575 541 499 448 235 13 0 0 Oct 0 0 0 1 54 177 273 341 397 421 431 435 384 235 20 0 0 0 Wiesbaden (1951–1980) MLT 3–4 4–5 5–6 6–7 7–8 8–9 9–10 10–1111–12 12–13 13–14 14–15 15–16 16–17 17–18 18–19 19–20 20–21 Apr 0 0 26 215 403 474

Load more