Albedo, atmospheric solar absorption and heating rate measurements with stacked UAVs

Abstract

This paper reports unique measurements of albedo, atmospheric solar absorption, and heating rates in the visible (0.4 to 0.7 µm) and broadband (0.3 to 2.8 µm) spectral regions using vertically stacked multiple lightweight autonomous unmanned aerial vehicles (UAVs). The most significant finding of this study is that when absorbing aerosols and water vapour concentrations are measured accurately and accounted for in models, and when heating rates are measured directly with stacked aircraft, the simulated clear sky heating rates are consistent with the observed broadband heating rates within experimental errors (about 15%). We conclude that there is no need to invoke anomalous or excess absorption or unknown physics in clear skies.Aerosol-radiation-cloud measurements were made over the tropical Indian Ocean within the lowest 3 km of the atmosphere during the Maldives Autonomous UAV Campaign (MAC). The UAVs and ground-based remote sensing instruments determined most of the parameters required for calculating the albedo and vertical distribution of solar fluxes. The paper provides a refined analytical procedure to reduce errors and biases due to the offset errors arising from mounting of the radiometers on the aircraft and due to the aircraft attitude. Measured fluxes have been compared with those derived from a Monte-Carlo radiative transfer algorithm which can incorporate both gaseous and aerosol components. Under cloud-free conditions the calculated and measured incoming fluxes agree within 2-10 W m −2 (<1%) depending upon the altitudes. Similarly, the measured and calculated reflected fluxes agreed within 2-5 W m −2 (<5%). The analysis focuses on a cloud-free day when the air was polluted due to long-range transport from India, and the mean aerosol optical depth (AOD) was 0.31 and mean single scattering albedo was 0.92. The UAV-measured absorption AOD was 0.019 which agreed within 20% of the value of 0.024 reported by a ground-based instrument. The observed and simulated solar absorption agreed within 5% above 1.0 km and aerosol absorption accounted for 30% to 50% of the absorption depending upon the altitude and solar zenith angle. Thus there was no need to invoke spurious or anomalous absorption, provided we accounted for aerosol black carbon. The diurnal mean absorption values for altitudes between 0.5 and 3.0 km above mean sea level were observed to be 41 ± 3 W m −2 (1.5 K/day) in the broadband region and 8 ± 2 W m −2 (0.3 K/day) in the visible region. The contribution of absorbing aerosols to the heating rate was an order of magnitude larger than the contribution of CO 2 and one-third that of the water vapour. In the lowest 3 km of the tropical atmosphere, aerosols accounted for more than 80% of the atmospheric absorption in the visible region.

Simultaneous observations of aerosol–cloud–albedo interactions with three stacked unmanned aerial vehicles

“…Occasionally, the ''below-cloud'' AUAV flew above the clouds such that two AUAVs measure cloud albedo at different altitudes simultaneously. Radiometric data are corrected for pitch and roll and cloud albedos are normalized based on a clear sky model to remove solar zenith angle dependence (44). Only cases when the solar zenith angle is Ͻ30°are used for this study to minimize 3D effects of trade cu on albedo measurements.…”

Atmospheric pollutant outflow from southern Asia: a review

“…An effort to integrate some of the observations from the mainland and islands into a focused network is underway in a new program called ARFI (Aerosol Radiative Forcing over India; Satheesh et al, 2009). A particular highlight of the island-based measurements has been a small campaign called MAC (Maldives AUAV Campaign), which took place in March 2006, using AUAVs (automated unmanned aerial vehicles) flying in a stacked configuration to investigate the modification of the solar radiation budget by aerosols in the SAW-ABC (Ramanathan et al, 2007b;Ramana et al, 2007;Corrigan et al, 2008).…”

Capturing vertical profiles of aerosols and black carbon over the Indian Ocean using autonomous unmanned aerial vehicles

“…Specific to this work, the instrumentation included an aerosol inlet system; a meteorological package for measuring pressure, temperature, and relative humidity; a total particle counter; an optical particle counter (OPC); and an aerosol absorption photometer. Additional instruments modified for UAV deployment flown during the MAC missions include visible and broadband radiometers, a cloud droplet probe, and a liquid water content probe; results for these instruments are discussed elsewhere (Ramana et al, 2007;Ramanathan et al, 2007b;Roberts et al, 2008). Data from all instruments were collected at a rate of 1 Hz.…”