Exercise 1   Deriving Essential Climatic Variables for Driving 3-PG Model

Problem: 3-PG requires meteorological data not available at standard weather stations.

Solution: Use techniques from meteorology to derive variables other than precipitation and temperature extremes.

Procedures for estimating vapor pressure deficit, frost frequency, and solar radiation:

A. Open sheet labeled’ Exercise 1’ and copy monthly max and min temperatures from “Climate data” block.

B.Open sheet labeled ‘Tools’ and go to bottom of page and find module for estimating daytime average vapor pressure deficits (green box). Paste Tmax and Tmin along side the green VPD box—then paste both columns separately over the existing information in the VPD box (Past special, transpose).

C. Copy monthly mean VPD values generated, to Exercise 1 in the blue Climate data block (Past special, values, transpose)

D. Return to ‘Tools’ and copy minimum temperatures for Exercise 1 into the green frost days box at bottom of page. Copy frost days/month into Climatic Data box in Exercise 1.  Then delete the pasted material that you placed alongside the green box.

E. Open sheet’ Radiation Calculation’ (solar) and paste in mean monthly (daily averages of) max and min temperatures on the columns with red headings (TMAX24H, TMIN24H). Using the following formula, the sheet will generate estimates of mean solar radiation when latitude  (-41 degrees) and elevation (750 m) are inserted where labeled. Do calculations for each of the 12 months and list in a column at the left side of the page.

F. Copy column of modeled estimates of solar radiation and return to Exercise 1 sheet to paste values (special paste as values (M Joules m2) in column labeled ‘predicted’. Transfer values from Climatic data of solar radiation into the column labeled ‘observed’.  Compare predicted with observed by making a graph and determining the equation and r2.

G. Return to’Tool’ page and note that the equation for predicting solar radiation at SETRES site in North Carolina has a slope far from 1:1.  This is probably related to a higher fraction of diffuse to direct radiation associated with either more cloudy days than recorded in Bariloche, Argentina or more haze recorded on clear days in N.C. than in Bariloche.

H. Although the day-time maximum vapor pressure deficit is relatively easy to estimate, based on the assumption that the water vapor concentration is the air is a function of the mean minimum temperature, the day-time average appears to be somewhat variable, usually falling between 0.6 and 0.7 of the mean maximum vapor pressure deficit.