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Earth’s radiative balance and clouds
The sun is the primary energy source for most processes in the earth’s system. Although the sun emits electromagnetic radiation at various wavelengths, most of the incoming solar radiations consist of visible and parts of ultraviolet and short infrared radiations. The electromagnetic (EM) radiations are characterized by their wavelength , or by its frequency . The two variables are related as × = , where is the speed of light ( = 3 × 108 −1 in vacuum). The wavelength of peak radiation emitted by an object is inversely related to its temperature (Wien’s law). Due to the high surface temperature of the sun (average 5500 ), the wavelength of peak radiation has high intensity and shorter wavelengths, hence called shortwave(SW). The earth’s surface and the atmosphere reflects as well as absorb these solar radiations (SW). A part of the radiation (around 30%) is reflected, and the fraction absorbed by the earth ( ∼ 300 ) is re-radiated at the longer wavelengths in the infrared region (about ∼ 10 ) called longwave (LW) with relatively less intensity. Figure 2.2 presents the radiation intensity and range of wavelengths of incoming solar radiations and the emitted radiations from the earth. Notice that the radiation intensity on the y-axis is relative.
Several factors influence the amount of solar radiation reaching the earth’s surface and the amount of radiation leaving the atmosphere. The interaction of radiation with atmospheric gases, water vapour, aerosols, and clouds includes absorption, emission, and scattering processes. These processes play a vital role in the thermodynamic conditions of the atmosphere.
Scattering, absorption and extinction processes
When radiation interacts with a particle, a part of the incident energy is absorbed, whereas the other is spatially redistributed in a non-isotropic direction. These processes are known as the absorption and scattering processes, respectively. The absorbed part 18 Figure 2.2: Incoming energy from the sun and outgoing energy from the earth relative to the wavelengths. Figure is extracted from Understanding Global Change(UCMP) webpage (see link) (https://ugc.berkeley.edu/background-content/re-radiation-of-heat/) of the radiation is converted into molecular kinetic and potential energies whereas, scattered radiation is simply redirected without any loss of energy. The extinction or attenuation of radiation by a particle represents the sum of absorption and scattering processes. An electromagnetic wave of intensity propagates along an optical path in an atmospheric layer gets attenuated by a factor which is given by: = − (2.1).
where is the extinction coefficients and has unit −1. The contributions of scattering and absorption to the extinction of the incident beam of radiation defines the scattering and absorption coefficient, such as: =+ (2.2).
Scattering is a process, which conserves the total amount of energy, but the direction in which the radiation propagates may be altered. The amount of scattering depends on several factors, including the wavelength of the radiation, the size of particles (or gas molecules), the amount of particles, and the incident and scattering angles. If we assume a spherical particle of radius , we define a dimensionless size parameter to be the ratio of the circumference of the particle to the wavelength of radiation: = 2 (2.3).
Figure 2.3: Scattering regimes and particle types based on size parameter ( ), wavelength and radius ( ) (replicated from Wallace and Hobbs, 2006[Wallace and Hobbs, 2006]) and radiation in different wavelength ranges. When the particle is small compared with the wavelength, the size parameter << 1, the scattering is weak and symmetrically distributed. In this so-called Rayleigh scattering regime, the scattering is divided evenly in the forward and backward direction, as shown in figure 2.4. The particle for which the size parameter is comparable to the wavelength ( ≈ 1) the scattering is referred to as Mie scattering regime [Mie, 1908]. When the particle becomes larger, the scattered energy is increasingly concentrated in the forward direction. For > 50, geometric optics methods have to be used to compute scattering properties.
Earth’s radiative equilibrium
Earth’s radiation Budget at the top of the atmosphere (TOA) describes the overall balance between the incoming energy from the sun and the outgoing thermal (longwave) and reflected (shortwave) energy from the earth. This flow of incoming and outgoing energy is earth’s energy budget. The earth system (atmosphere and surface) is heated by absorption of incoming solar radiations and cools by emitting longwave radiations to space. These thermal infrared radiation emitted from the earth surface are re-absorbed and re-radiated in the atmosphere many times by clouds and other greenhouse gases(water vapour, CO2 etc.), this process is known as the greenhouse effect. This is responsible for the temperate climate of earth, without it the average temperature of the planet would be about -19◦C instead of 15◦C.
Figure 2.6 illustrates the global energy balance, and the numbers represents annual and global averages of quantities that fluctuate substantially in space and time. The total instantaneous solar irradiance is 1360.8 −2, or 340 −2 averaged over the global sphere. Out of the 340 −2 received from the sun, about 100 −2 is reflected by clouds and atmospheric aerosols (e.g. sulfates, nitrates), leaving 240 −2 to be absorbed by atmosphere (71-82 −2) and surface (161-168 −2). Therefore, the planetary albedo (the fraction of SW radiation scattered back to space by the clouds, aerosols, and surface without being absorbed) is 0.29 [Stephens et al., 2015]. The radiative equilibrium at the TOA is balanced by emission of 237 −2 LW radiation from the earth system. A fraction of this thermal IR radiation escapes directly to space through the atmospheric window (the spectral band between about 8 and 12 ) when skies are clear. But, the presence of clouds reduces the amount of SW radiation reaching the surface and also contributes to additional IR radiation sent toward the surface. This effect of clouds on radiation is discussed in the next subsection.
Cloud radiative forcing
At TOA, the cloud radiative forcing (CRF) is defined as the difference between the downwelling (SW) and upwelling (LW) radiative fluxes in all-sky condition minus the difference in clear sky condition. =( ↓− ↑)−( ↓− ↑) (2.4).
The downwelling forcing ( ↓) is due to incoming SW radiations, and the upwelling ( ↑) is due to longwave cloud radiative effect, as all the wavelengths emitted by the earth do not reach into space. The atmosphere absorbs some of LW radiations while allowing other wavelengths to pass through.
The cloud radiative forcing due to SW and LW radiations are always in competition. In general, clouds with large optical thickness reflect most of the incoming shortwave radiation inducing a cooling effect, while clouds with a low cloud-top temperature trap outgoing longwave radiation inducing a warming effect [Hartmann et al., 1992]. Thus, net CRF depends both on the cloud optical thickness and the cloud top temperature.
Low-level clouds have high albedo, which means that these clouds reflect a part of incoming SW radiation back to the atmosphere. This cools the planet, and the size of the effect is determined primarily by cloud optical depth.
High level clouds have a weaker albedo. This means that solar radiation can penetrate deeper in the troposphere and heat the surface. These clouds also trap the IR radiation coming from the lower atmosphere. In the current climate, the global annual mean net CRF is about -17.1 −2 [Loeb et al., 2009]. Therefore, the net effect of clouds is slightly cooling. However, a change in radiative forcing can modify the occurrence and the radiative properties of clouds, which can further lead to an enhanced or weakened cooling effect of clouds, thus exerting a radiative feedback.
Cloud feedback on climate
Reflection of solar radiations by clouds serves as a key feedback mechanism for climate change. A reduction in reflection of SW radiations due to low cloud induce positive feedback, while increase in cloud water content with warming induce negative feedback on climate [Stephens et al., 2015]. Clouds and aerosols contribute to climate change in a variety of ways. As shown in figure 2.7, the global radiative balance is affected by anthropogenic forcing agents such as greenhouse gases and aerosols. When a forcing agent alters internal energy flows in the earth system, it affects cloud cover and other climate system components, which in turn affects the global energy budget. In contrast to changes in the global mean surface temperature, which are slowed by the huge heat capacity of the oceans, these adjustments often occur within a shorter time span (generally a few weeks). These rapid adjustments are associated with changes in climate variables that are mediated by a change in global mean surface temperature. These variables further contribute to the amplification or dampening in global temperatures through their effect on the radiative budget [Change, 2014].
Instruments used in this study
In this thesis, a method to estimate the microphysical characteristics of low-level clouds and fog is presented (in chapter 5) using a cloud radar and microwave radiometer synergy. Observations from BASTA cloud radar colocated with the HATPRO (Humidity And Temperature PROfiler) microwave radiometer at SIRTA observatory and SOFOG-3D field experiment are utilized. The fundamental concept of these remote sensing instru-ments is already introduced in this chapter, and this section describes these instruments and their capabilities in further depth. While the SIRTA observatory and SOFOG-3D experiment observation sites are detailed in section 3.5. The retrievals of the cloud mi-crophysical parameter using the mentioned synergy are compared with measurements from an in-situ sensor called CDP. To ensure that this section covers all instrumentation utilized in this research, the in-situ sensor is also described here.
Table of contents :
1 Introduction and Motivation
2 Clouds
2.1 Cloud formation and classification
2.2 Earth’s radiative balance and clouds
2.2.1 Scattering, absorption and extinction processes
2.2.2 Earth’s radiative equilibrium
2.2.3 Cloud radiative forcing
2.2.4 Cloud feedback on climate
3 Instruments for cloud observation
3.1 In-situ measurements
3.2 Remote Sensing
3.2.1 Passive sensors
3.2.2 Active sensors
3.3 Instruments used in this study
3.3.1 BASTA cloud radar
3.3.2 HATPRO microwave radiometer
3.3.3 Cloud Droplet Probe (CDP) on tethered balloon during SOFOG- 3D experiment
3.4 Observation platforms
3.5 Observation sites and field campaigns used in this study
3.5.1 SIRTA
3.5.2 SOFOG-3D
4 Prerequisites and overview of the literature for LWC retrieval
4.1 Microphysical parameters of liquid phase clouds
4.2 Classification of hydrometeors
4.3 Atmospheric Attenuation
4.4 Cloud radar based techniques for LWC retrieval
4.4.1 Empirical relation
4.4.2 Spectral Analysis
4.4.3 Multi-sensor retrieval techniques
5 LWC estimation using radar-microwave radiometer synergy
5.1 Introduction
5.2 Methodology of LWC retrieval
5.2.1 Optimal estimation formulation
5.2.2 Definition of the state and observation vectors
5.2.3 Description of the forward model and Jacobian matrix
5.2.4 Discussion of the retrieval uncertainty
5.2.5 Analysis of the method when microwave radiometer is available
5.3 Sensitivity analysis of retrieval algorithm using synthetic data
5.3.1 Description of synthetic data
5.3.2 Sensitivity analysis of impact of error in observation
5.3.3 Sensitivity analysis of impact of attenuation due to liquid droplets model
5.3.4 Sensitivity analysis of bias in Z and LWP
5.3.5 Sensitivity analysis of LWP assimilation
5.3.6 Sensitivity of parameter b
5.3.7 Analysis of the sensitivity exercise
5.4 Comparison of LWC retrieval with in-situ data
5.4.1 Presentation of the case study of 09 February 2020
5.4.2 Comparison between in-situ and radar measurements
5.5 Statistical analysis of retrievals to derive climatology
5.6 BASTA standalone LWC retrieval using climatology
5.6.1 BASTA standalone LWC retrieval approach
5.6.2 BASTA standalone LWC retrieval first assessment using LWP retrieved from MWR
6 Conclusion and outlook
6.1 Conclusion
6.2 Outlook
