|Ph.D Student||Spond Hanan|
|Subject||Overcast versus Clear Sky Remote Sensing|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Maxim Shoshany|
|Full Thesis text|
The average diurnal cloud cover in Central and Northern Europe is above 60%, with a significant number of days of overcast conditions. Such a high cloud cover severely limits the use of satellite remote sensing in optical wavelength bands. With the unprecedented spread of near-surface platforms such as unmanned air vessels (UAVs) and drones, remote sensing below cloud cover allows for high resolution and frequent monitoring of the environment in a large number of applications. Clear-sky remote sensing is regarded by many as optimal for determining surface reflectance. One of its advantages over overcast remote sensing is its lower magnitude of sensing noise (instrument, atmospheric, and environmental) relative to the total irradiance (signal-to-noise ratio, or SNR). On the other hand, overcast conditions are advantageous owing to their lower directional irradiance heterogeneity and higher isotropic directional reflectance compared with clear skies. This research had three objectives: (1) to assess and model the effect of the irradiation field under overcast conditions and their affinity with cloud types; (2) to compare reflectance measurements from flat surfaces between overcast and clear skies conditions; and (3) to assess overcast versus clear sky remote sensing concerning the effect of multiple reflections within structured surfaces (i.e., urban, agricultural) on the radiance reflected from them. The methodologies developed included: field measurements of the spectral irradiance and reflectance from different flat and 3D structured surfaces; modelling the irradiance during overcast conditions using MODTRAN 4 simulations; and applying a radiosity model and a hardware model for measuring effects from multiple reflections. We conducted spectral measurements on three overcast days and two during clear sky conditions. The results of our irradiance measurements (the 1st objective) indicated that the total irradiance under clear sky conditions is approximately ten times higher than that under overcast conditions, while the stability of irradiance during overcast conditions was much higher than during clear sky. Spectral irradiance measured and modeled under overcast conditions indicated patterns characteristic of nimbostratus clouds on the first date, stratocumulus clouds on the second, and stratus clouds on the third. Surface reflectance was compared (the 2nd objective) for four targets (asphalt, pavement, grass, and a grey Styrene Butadiene Rubber sheet). Correlation coefficients (R2) between clear-sky and overcast reflectance estimates exceeded 0.9 for the majority of measurements. Higher correlation was obtained for the measurements during the overcast day with lower but more stable irradiance levels. Mutual assessment of irradiance and reflectance between clear sky and overcast conditions in a 3D box model using the spectral measurements and the radiosity model (objective #3) indicated that there is a low effect of multiple reflections effect during overcast conditions even with highly reflective surfaces. Overall, overcast remote sensing has an advantage in surface reflectance estimation over clear sky remote sensing where sensors have high SNRs and data acquisition is conducted under stable irradiance conditions with high homogeneity of incoming and upwelling directional reflectance fields. This advantage is of significant importance when utilizing low-altitude scanners and cameras with off-nadir viewing geometries.