Effect of cloud altitude

Here, the effect of varying the altitude of a liquid cloud on the up-wellingT_bwill be examined. The set up for the calculation corresponds to the third row of Table 5.3. This means all the cloud and the atmo-spheric parameters remain the same as in Section 5.3.1, except that thelwcis assumed to be a constant 0.4 g m⁻³, and that the cloud al-titude is varied within a reasonable range. The clouds are all assumed to be one kilometer thick. The cloud base altitude ranges from 1 to 4 km for the midlatitude winter scenario and 1 to 8 km for the tropical scenario. Figure 5.22 shows theT_bat all the frequencies as a function of cloud base height.

It can be seen from Figure 5.23 that theT_bdifference in the presence

5 Effect of clouds on microwave radiances

20 40 60 80 100

Particle radius (µm) -5

0 5 10

TBcloud - TBclear [K]

20 40 60 80 100

Particle radius (µm) -10

0 10 20

TBcloud - TBclear [K]

Figure 5.21: Effect of particle size on ∆T_b for a nadir viewing geometry when the liquid water content is kept constant. 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 dotted line for 186 GHz, and the dash-dot-dotted line for 190 GHz.

of liquid clouds is highly sensitive to the cloud altitude. Especially for 89 and 150 GHz, the T_b difference can be seen at all cloud base al-titudes whereas for the higher frequencies, the effect is present only if the cloud is above the sounding altitude of the channels which can be seen in the clear sky Jacobians in Figure 5.3. For the tropical sce-nario, the clear sky T_b at 89 GHz is about 258 K. If a liquid cloud is present at a height of 1–2 km, the T_b increases to about 282 K.

As mentioned in the previous sections, this increase in T_b at 89 GHz is due to the emission from liquid clouds against the radiometrically cold surface background. For the midlatitude winter scenario the T_b increase is about 40 K due to liquid clouds. The larger difference for the midlatitude winter scenario compared to the tropical scenario re-sults from the fact that the atmosphere is drier and that the surface is colder. As the cloud height is increased to 2–3 km, the cloud emis-sion is coming from colder temperatures. This is responsible for the brightness temperature decrease with cloud height. Therefore the T_b enhancement due to the clouds is reduced. For 150 GHz, the result for the midlatitude winter scenario is similar to the result for 89 GHz.

Clear 1 2 3 4 5 6 7 8 Cloud base height [km]

250 260 270 280

Brightness Temperature [K]

Clear 1 2 3 4

Cloud base height [km]

220 230 240 250 260

Brightness Temperature [K]

Figure 5.22: Effect of cloud height onT_bfor a 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.

The T_b increase compared to the clear sky case is about 30 K for a cloud altitude of 1–2 km. As the cloud height changes to higher alti-tudes, theT_b enhancement decreases due to the colder emission from the cloud. For the tropical case, theT_bdoes increase compared to the clear skyT_bfor a cloud height of 1–2 km. But when the cloud is placed at higher and higher altitudes, the tropical scenario being wetter, the surface emission factor becomes negligible. The presence of clouds fur-ther decreases the contribution from the lower altitudes. This means that theT_bdecreases with higher cloud base heights. When the cloud is at 8–9 km, theT_b depression is about 25 K.

The water vapor frequencies behave in a similar way as in the pres-ence of ice clouds. At these frequencies, T_b decreases with respect to the clear sky case when the cloud is at a height above the peak of their clear sky weighting function. For the tropical scenario, 184 GHz is inert to all clouds having a cloud base height below 6 km. Beyond this cloud base height, the presence of the cloud decreases theT_band the maximumT_bdepression of about 6 K is seen for the highest cloud at 8–9 km. The midlatitude winter scenario being drier than the

trop-5 Effect of clouds on microwave radiances

Clear 1 2 3 4 5 6 7 8

Cloud base height [km]

-20 -10 0 10 20

TBcloud - TBclear [K]

Clear 1 2 3 4

Cloud base height [km]

0 10 20 30 40

TBcloud - TBclear [K]

Figure 5.23: ∆T_b for different cloud heights for a 1 km thick cloud for a nadir viewing geometry. The left plot is for 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.

ical, 184 GHz is sensitive to clouds at lower altitudes compared to the tropical. The behavior is similar for the other frequencies, namely, 186 and 190 GHz except that they are sensitive to much lower cloud altitudes.

For 190 GHz, there is a slight increase in T_b when the cloud is at 1–2 km, because the drier midlatitude winter scenario can have some surface contribution. But as the cloud is at higher and higher altitudes, the atmosphere below the cloud decreases the surface contribution and theT_bdecreases with respect to the clear sky case. The maximumT_b depression is shown by 190 GHz.