LE Accumulation, Net Radiation, and Drying with Tipped Sensors

Transcription

1 LE Accumulation, Net Radiation, and Drying with Tipped Sensors Three different situations were examined, where the influence that the deployment angle of the sensor has on the accumulation of latent heat during a period, and the amount of time required for a sensor to dry after a simulated rain event. The scenario first looks at the accumulation of latent heat during a dew period with clear skies day and night, and the influence that a simulated sensor angle would have on the onset and dry-off time of the sensor (Figure 2). The second and third situations examined drying of night rain on a clear morning and a cloudy morning from the simulated sensor surface depending on the degree to which the sensor was tipped. A modified Penman-Monteith model was used to calculate latent heat, with the inputs of solar radiation and net longwave radiation adjusted to reflect the angle of the sensor. Dew was allowed to accumulate to a maximum of.8mm, and rainfall to a maximum amount of.6mm. Clear Day/Night The accumulation and evaporation of LE was calculated for simulated sensors deployed at, 15, 3 and 45 for a chosen time period in, degrees 3 degrees 3 Net Radiation (Wm -2 ) Figure 2.2: The influence of tipped sensors on net radiation values when using clear sky radiation for 24. An examination of the net radiation calculated using solar radiation measured on a clear day showed that for, the morning net radiation became positive in the same hour for all sensor angles (Figure 1). The sensors at and 15 experienced negative evening net radiation one hour earlier than the sensors positioned at 3 and 45. It is important to note that the direct

2 and diffuse components of the solar radiation were determined from measured values of solar radiation at the Elora Research Station on a clear day degrees 3 degrees 2 Accumulated LE (Wm -2 ) Figure 1: Influence of sensor angle on the accumulation of LE during a dew period 24. Drying occurs when the accumulation of LE returns to zero in the morning. Table 1 shows the time of the onset of dew (the first hour in which the value of LE was positive), the time of drying (the first hour in which the LE value was negative) and the total wetness duration. The duration of wetness was determined by the number of hours in which the LE was positive. For the time period in (Figure 1), all the sensors showed dew onset and dry-off within the same hour. Table 1: The onset of wetness, the time of drying and the duration of wetness for a clear night and morning in with sensors positioned at, 15, 3 and 45. Rain Followed by Clear Morning ()

3 The maximum amount of rain (.6mm) was indicated to be on the sensor at 6am. The net radiation computed using solar radiation from a clear day (Figure 1) was again used as the input for the model to estimate the time required for the sensor to dry following the simulated rain event. The simulated event in (Figure 3) resulted in all sensor angles drying within the same hour..7.6 degrees 3 degrees Contents of Rain Reservoir (mm) Figure 3: Influence of sensor deployment angle on the time required to evaporate.6mm of water from the rain reservoir under clear sky solar radiation conditions for 24. Table 2.4: The onset of wetness, the time of drying and the duration of wetness for clear sky after night rain during with sensor positioned at, 15, 3 and 45. ()

4 2.2.3 Rain Followed by Overcast Sky A similar study was conducted to look at the net radiation that occurs when only the estimated diffuse component of the solar radiation is used to mimic an overcast sky. This situation simulated rain being followed by an overcast sky instead of a clear sky. The simulation of overcast sky began with the use of the same data as in Figure 1. However, during the period from sunrise to sunset, an overcast cloud condition was modeled. According to Haurwitz (in Smithsonian Meteorological Tables, 1949), the total amount of diffuse radiation under overcast conditions is 32% of the total incoming solar radiation for stratocumulus cloud coverage. For the month of, for all sensor angles, the net radiation became positive within the same hour in the morning. The sensor at indicated an evening net radiation that was negative an hour earlier than the other angles (Figure 4). However, the first negative net radiation experienced by the sensor at was -1 Wm -2, while the net radiation received at the sensor surface for deployment angles of 15, 3 and 45 in the same hour had positive values of 1, 3 and 6 Wm -2, respectively. This close range of values would have minimal impact on the time of dew onset on the sensor surface. 8 6 degrees 3 degrees 4 2 Net Radiation (Wm -2 ) Figure 4: Influence of the sensor deployment angle on the amount of net radiation received on the sensor surface for a cloudy day in 24. Once again, the maximum amount of rain (.6mm) was simulated to be on the sensor surface at 6am. The net radiation calculated using only the diffuse solar component (Figure 4) was used in the model to estimate the amount of time needed to dry a sensor positioned at, 15, 3 and 45. For, the sensor at 45 dried just at the end of the hour before the others (Figure 5).

5 .7.6 degrees 3 degrees Contents of Rain Reservoir (mm) Figure 2.6: Influence of sensor deployment angle on the amount of time required to evaporate.6mm from the rain reservoir under diffuse radiation conditions for 24. Table 2.5: The onset of wetness, the time of drying and the duration of wetness for cloudy sky after night rain in with sensors positioned at, 15, 3 and 45. () Many of these results correspond to studies that have been conducted by Sentelhas et al. (24b) and Lau et al. (2) on the exposure and deployment angles of sensors. The findings support the idea that the angle of deployment is not critical in the measurement of leaf wetness duration using a flat-plate sensor. However, the sensor should not be deployed at due to its ability to capture too much rain, as well as the fact that it does not mimic the natural arrangement of leaves in the environment (Lau et al., 2).