Effect of liquid water path

In this section, the effect of the liquid water path (lwp) on the up-welling brightness temperature (T_b) will be discussed. Similar toiwp, lwpis the integrated liquid water content expressed in units of kg m⁻².

5 Effect of clouds on microwave radiances

Table 5.3: The different liquid cloud cases considered in this study. As in Table 5.1 for ice clouds, each row represents one set of simulations with corresponding values of cloud parameters, surface emissivity and line of sight given in each column.

lwc [ g m⁻³] reff [µm] cloud ht. [km] emissivity los

0 – 0.8 15 4 – 5 0.7 180^◦

0.4 6 – 20 4 – 5 0.7 180^◦

0.4 15 1 – 2 to 8 – 9 0.7 180^◦

0.4 15 4 – 5 0.4 – 1.0 180^◦

0.4 15 4 – 5 0.7 130^◦– 180^◦

The case discussed here corresponds to the first row of Table 5.3. The liquid water content (lwc) is varied from 0 to 0.8 g m⁻³. This range of lwcis reasonable as can be seen from Chapter 2 on the discussion on liquid clouds. The cloud is assumed to be 1 km thick and is placed at an altitude of 4 to 5 km. The cloud is assumed to be homogeneous in the vertical direction. The liquid particles are considered to be spher-ical in shape following a gamma size distribution with an effective radius of 15µm. The upper and lower radius limits of the particles in the distribution are taken to be 5 and 60µm respectively. The surface emissivity is assumed to be 0.7 and the simulations are done for nadir viewing geometry. Figure 5.18 shows the effect on brightness temper-ature due to an increasing lwpfor the tropical and the midlatitude winter atmospheric scenarios, and Figure 5.19 shows the difference be-tween the clear sky simulation and a simulation including only liquid clouds. The difference ∆T_b(∆T_b=T_b^cloud−T_b^clear) is plotted at each lwp.

It can be seen that there is a marked difference compared to the behavior of ice clouds. The reason is as mentioned earlier, in this frequency range, liquid clouds mostly absorb whereas ice clouds mostly scatter. This can also be seen from the single scattering properties of ice and liquid particles in Figures 3.9 and 3.5. In the case of ice clouds, 89 GHz was almost inert to any changes in iwp, whereas for liquid clouds 89 GHz is the frequency that is most sensitive to changes in the lwp. As thelwpincreases, the brightness temperature corresponding to 89 GHz increases. This increase inT_bin the presence of liquid clouds

0.0 0.2 0.4 0.6 0.8 Liquid water path [kg/m²] 250

260 270 280

Brightness Temperature [K]

0.0 0.2 0.4 0.6 0.8

Liquid water path [kg/m²] 220

230 240 250 260

Brightness Temperature [K]

Figure 5.18: Effect of the liquid water path on T_b for the nadir viewing geometry. The left plot represents the tropical gas absorption scenario and the right plot represents the midlatitude winter gas absorption scenario.

The solid line stands for 89 GHz, the dotted line for 150 GHz, the dashed line for 184 GHz, the dash-dotted line for 186 GHz, and the dash-dot-dotted line for 190 GHz.

is because of the absorption of liquid clouds positioned at a warmer layer against the radiometrically cold surface background which leads to cloud emission. The emission increases as the optical depth through the cloud increases for higher lwp. For both the tropical and the midlatitude winter cases, there is initially a rapid increase ofT_b with lwp. Beyond a certain value oflwp, the increase is rather slow and the T_bs tend to saturate as the cloud optical thickness is very high. The maximum enhancement is about 15 K for the tropical scenario and about 40 K for the midlatitude winter scenario. For the midlatitude winter case, the increase in T_b is larger than for the tropical case.

This is because for the midlatitude winter scenario the atmosphere is comparatively drier than the tropical scenario. Also, the surface is radiometrically much colder compared to the tropical scenario which increases the radiometric contrast.

Among the frequencies close to the center of the water vapor line, 184 GHz is inert to any changes in thelwp. This is because, the con-tribution to radiation at this frequency comes from altitudes that are above the position of the liquid cloud. Figure 5.19 shows that the

dif-5 Effect of clouds on microwave radiances

0.2 0.4 0.6 0.8

Liquid water path [kg/m²] -10

-5 0 5 10 15

TBcloud - TBclear [K]

0.2 0.4 0.6 0.8

Liquid water path [kg/m²] -10

0 10 20 30

TBcloud - TBclear [K]

Figure 5.19: ∆T_b (T_b^cloud−T_b^clear) for the nadir viewing geometry. The left plot represents the tropical scenario and the right plot represents the midlatitude winter scenario. The solid line stands for 89 GHz, the dotted line for 150 GHz, the dashed line for 184 GHz, the dash-dotted line for 186 GHz, and the dash-dot-dotted line for 190 GHz.

ference to the clear sky case is close to 0 K for the tropical case. The cloud altitude of 4 to 5 km is well below the sounding altitude of this channel (see Figure 5.2). For the midlatitude winter case, since the weighting function peaks at a slightly lower altitude, the presence of clouds has a small effect of about 2 K.

For other frequencies, namely 186 and 190 GHz, as the lwp in-creases, T_b decreases. At these frequencies the presence of the highly absorbing liquid cloud makes theT_bs to saturate at colder higher al-titudes. For 186 GHz also the weighting function peaks well above the cloud altitude for the tropical scenario. Therefore the effect of increas-inglwpwithin the cloud has only a negligible effect on the up-welling T_b, about 1 K at the maximum lwp. For the midlatitude-winter sce-nario, the maximum T_b depression is about 6 K. For 190 GHz, the maximum depression is about 7 K for the tropical case, and about 10 K for the midlatitude winter case. The decrease in T_b is due to the extinction by liquid clouds which mainly comes from absorption rather than scattering.

As we have seen for ice clouds, 150 GHz behaves more like a water

vapor channel for the tropical case and like a surface channel for the midlatitude winter case. This means that theT_b decreases with lwp for the tropical case and increases withlwpfor the midlatitude win-ter case. For the tropical scenario, the T_b depression for 150 GHz is comparatively higher than that for the water vapor frequencies. This is because the cloud is more visible to this frequency than the wa-ter vapour frequencies. The maximum depression is about 10 K. For the midlatitude winter scenario, theT_benhancement follows the same curve as 89 GHz up to anlwpof 0.2 kg m⁻² after which it saturates to a constantT_b.

For both scenarios it can be seen from Figure 5.18 that except for 184 GHz, 89 GHz has the lowestT_bfor the clear sky case corresponding to lwp = 0. For clear sky cases, this is true only when the surface emissivity is not very high, like over the oceans. Over land surfaces, the surface emissivity is usually very high, close to 0.95 and 89 GHz T_b can be higher than those for the frequencies close to the water vapour line at 183 GHz. Over regions of low surface emissivity, as the lwpis increased, theT_bfor 89 GHz increases and after a certainlwp has higher T_b compared to the T_b at the other frequencies. In this example it can be seen that for the tropical scenario, beyond lwp value of 0.6 kg m⁻², 89 GHz has the highestT_b.

In the simulation by Muller et al. (1994) at anlwpof 0.5 kg m⁻², the T_b enhancement for a 2–3 km liquid cloud is about 7 K. Usingarts, at the same lwp, the enhancement is about 15 K. The main reason for this discrepancy is that the surface emissivity used in Muller et al.

(1994) is 0.9, whereas here it is 0.7. It will be shown in 5.3.4, that as the surface emissivity increases, T_b enhancement decreases. For 176 GHz the 2–3 km cloud in Muller et al. (1994) shows a very low depression of about 1 K. This can be compared to a depression on about 5 K usingarts. The difference here is because the cloud is at an altitude of 4–5 km and this can contribute to a much colder emission.

5 Effect of clouds on microwave radiances