THE DIURNAL AND SEASONAL RADIATIVE EFFECTS OF CIRRUS CLOUDS UTILIZING LARGE AIRBORNE AND SPACE-BORNE LIDAR DATASETS

Abstract

Cirrus clouds are globally the most common cloud type, however, their

radiative impact on the Earth remains a large source of uncertainty in global climate

models. Cirrus are unique in that they are absorptive to terrestrial outgoing longwave

radiation, while also relatively transmissive to incoming solar radiation. The

interactions of this greenhouse and albedo effect determine the sign and magnitude of

cirrus radiative effects. Cirrus are microphysically complex, and can exhibit a variety

of different ice crystal shapes and sizes depending on the thermodynamic environment

in which they form, and their dynamic formation mechanism. Our ability to reliably

model cirrus radiative effects is dependent upon accurate observations and

parameterizations incorporated into radiative transfer simulations. Laser lidar

instruments provide valuable measurements of cirrus clouds unavailable by other radar

systems, passive remote sensors, or in-situ instruments alone.

In this dissertation I developed and tested an improved calibration technique for

the ACATS lidar instrument, and its impact on the direct retrieval of cirrus HSRL

optical properties. HSRL retrievals theoretically have reduced uncertainty over those

from a standard backscatter lidar. ACATS flew on two field campaigns in 2012 and

2015 where it was unable to consistently calibrate its etalon. It has been operating from

the lab in NASA GSFC collecting zenith pointing data of cirrus layers where the

improved calibration has resulted in consistent and reliable separation of the particulate

and Rayleigh signal components.

The diurnal trend of cirrus influence on the global scale has primarily been

limited to data provided by satellites in sun-synchronous orbit, which provide only a

snapshot of conditions at two times a day. Utilizing data from the CATS lidar aboard

the ISS I investigated cirrus at four periods throughout the day in morning, afternoon,

evening, and night across all seasons. Cirrus radiative effects were found to have a large

latitudinal dependence, and have a greater potential to cool than many studies suggest

with their primary warming contributions skewed towards the nighttime hours.

Constrained lidar retrievals reduce the assumptions made in retrieving cirrus

optical properties. Utilizing the expansive airborne CPL dataset from six flight

campaigns I model the radiative effects of over twenty thousand constrained cirrus

observations. Mid-latitude cirrus were found to have a mean positive daytime forcing

equivalent to that of the CO2 greenhouse effect. However, synoptic cirrus were found

to have a greater warming effect than convective cirrus, which were more likely to have

a cooling effect.