Kiran Sathaye Atmospheric Satellites and Aerosol Effects Spring 2010 1 Atmopsheric Observance Satellites and Cloud Aerosol Effects Kiran Sathaye ABSTRACT Atmospheric dynamics and indirect effects represent a large portion of the uncertainty in the understanding of Earth’s climate systems. One of the most useful tools in analyzing atmospheric processes is the use of satellite data. However, it is known that satellite data can be inaccurate, contributing uncertainty to climate models. To investigate climate change in the Arctic, one of the most vulnerable regions to global climate change, I evaluated the climate over Alaska using satellite-retrieved data. Cloud property changes over the Arctic are useful to understand how surface energy budgets and thus sea ice cover are impacted. To understand changes in clouds over the Arctic, I examined satellite retrievals of aerosol optical depth (AOD), used to indicate pollution levels, and other important climate variables such as liquid effective droplet radius (Reff) for the Alaskan region in the month of April 2008. I analyzed data from three satellite instruments: MODIS, CloudSat, and POLDER. I found that retrievals from MODIS and CloudSat showed greatly differing results for Reff and that MODIS and POLDER showed greatly differing results for cloud cover, with MODIS consistently showing higher values for both parameters. Additionally, no discernible relationship could be found between these two parameters and AOD. These results suggest that the further use of satellite retrievals to analyze the relationship between pollution and cloud properties in the Arctic may prove quite challenging. KEYWORDS Aerosol Indirect Effect, Arctic Circle, Climate Modeling, Cloud Properties, Energy FluxKiran Sathaye Atmospheric Satellites and Aerosol Effects Spring 2010 2 INTRODUCTION The Earth’s atmospheric systems are governed by the presence of many different variables such cloud cover, water content, aerosol content, and temperature, among others. Understanding the complex relationships between these processes is crucial to understanding Earth’s climate system and energy budget. The effect of aerosols is of great importance because of their ability to scatter and absorb sunlight, and because of their indirect effect on cloud formation and particle size. These processes all affect atmospheric energy flux, but the exact effects are unknown, contributing to uncertainty in global climate modeling (Cheng et al. 2010). The most effective method for gathering large amounts of atmospheric data on a global scale is satellite remote sensing. Satellites can continuously monitor the globe, making several orbits a day, and therefore quickly produce very large amounts of data. Region of interest The region and time period chosen for this study stretches from 170°W to 140°W and 55°N to 75°N for the month of April 2008. This covers the entire US state of Alaska, and some of the Arctic and Pacific Oceans as well, as shown in figure 1. This region was chosen because of marked warming in the Arctic in recent years, about double the global average (Solomon et al. 2007). Uncertainties in the Arctic climate system stem from complexities in cloud formation and dissipation processes and lack of reliable data (Liu, et al. 2010). Additionally, cloud observance is made increasingly difficult because of the presence of surface snow and ice. The small contrast between the cloud and surface color in the polar regions, combined with low solar flux in the Arctic, makes remote sensing very difficult in this region (Lubin and Morrow 1998). The time period of April 2008 was chosen to coincide with the Department of Energy Indirect and Semi-Direct Aerosol Campaign (ISDAC). The ISDAC study used aircraft-based instruments to measure atmosphericKiran Sathaye Atmospheric Satellites and Aerosol Effects Spring 2010 3 parameters similar to those measured by the satellite systems used in this study. By comparing satellite data from this time period, future studies can compare the aircraft measurements from the ISDAC study to the results of this study to determine satellite accuracy. Aerosol indirect effect Aerosols can have many different effects on clouds and solar energy flux. In addition to directly absorbing sunlight, they can act as Cloud Condensation Nuclei (CCN), increasing the number of particles in a cloud and in turn, increasing its reflectivity (IPCC Physical Science Basis, 2007). However, aerosols can also increase the number of ice particles in a cloud, decreasing reflectivity by increasing the amount of transparent ice in the cloud. Because of these varying effects of aerosols in the atmosphere, this study will focus on the effect of aerosols on cloud cover and droplet size, as well as discrepancies between for individual parameters, as measured by different satellite instruments. Satellite instruments The first of three satellite instruments used in this study was CloudSat, an atmospheric observance tool launched aboard the NASA satellite CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) in 2006. This satellite orbits at an altitude of 705 km above the surface in a sun-synchronous orbit, ensuring a constant solar illumination angle. CloudSat uses radar with wavelengths on the order of 1 millimeter, giving greater resolution and detection capability for cloud particle sizes than typical weather radars, which use wavelengths on the order of 1 centimeter. However, this resolution causes CloudSat to record data with a smaller field of view (NASA CloudSat Overview). Because of CloudSat’s high resolution and ability to estimate internal cloud conditions, liquid droplet effective radius (Reff) measurements from CloudSat were used in this study. The second satellite instrument used in this study was the Moderate Resolution Imaging Spectroradiometer (MODIS). This instrument was launched on the satellite Terra, the flagship of the NASA Earth Observing System (EOS) in 1999 and then on Aqua, a satellite launched specifically to monitor global water cycles, in 2002. MODIS uses a broadband radar spectrum to monitor both atmospheric and surface conditions (NASA MODIS Overview). MODIS measurements for cloud cover fraction, aerosol optical depth (AOD), and Reff were used in thisKiran Sathaye Atmospheric Satellites and Aerosol Effects Spring 2010 4 study. AOD is a unitless measure of
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