Effect of cloud altitude

5 Effect of clouds on microwave radiances

0 50 100 150 200

0 0.2 0.4 0.6 0.8

1 x 10

−3

Radius of particle [µm]

Scattering coefficient [m−1]

Scattering Coefficent of Ice 89 Ghz

150 GHz 184 GHz 186 GHz 190 GHz

Figure 5.8: The scattering coefficient of the ice particle distribution as a function of effective radius for each frequency. Theiwcis fixed at 0.4 g m⁻³.

150µm where

T_b^cloud₁₉₀ < T_b^cloud₁₈₆ < T_b^cloud₁₈₄ (5.6) The ice water path considered here is 800 g m⁻² which is lower than the cross-over point in Figure 5.4. Therefore it can be inferred that even for a low ice water path this effect can be seen if the cloud particle effective radius is large.

Clear1 2 3 4 5 6 7 8 9 10 11 12 Cloud base height [km]

230 240 250 260 270 280

Brightness Temperature [K]

Clear 1 2 3 4 5 6 7 8

Cloud base height [km]

220 230 240 250 260

Brightness Temperature [K]

Figure 5.9: Effect of the cloud altitude on T_b for nadir viewing geometry andimc= 0.4 g m⁻³. 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.

and from theecmwfdata (c.f. Section 2.5) show the presence of ice clouds at rather low altitudes. Therefore these heights are included in the results to cover all possible ranges of cloud altitudes. Figure 5.9 shows the effect of different cloud altitudes onT_bfor the tropical and the midlatitude scenarios. Figure 5.10 shows the T_b depression,∆T_b for both scenarios.

The results shown here are in a way indicative of the altitude each frequency is sensitive to. For the tropical case, theT_b for 184 GHz re-mains a constant, equal to the clear sky case for cloud base heights up to 6 km. The reason for this is that the lower clouds are not visible at this frequency because of water vapor emission above the cloud. Above 6 km, aT_bdepression is observed. From Figure 5.10 it can be seen that the maximum depression is about 27 K when the cloud is placed at the highest altitude considered here, 12–14 km. For 186 GHz, up to a cloud height of 3–5 km, there is no change in theT_b with changes in cloud altitude as this frequency is sensitive only above about 5 km for the tropical case. Beyond this, there is a steeper depression up to a cloud altitude of 9 to 11 km. Thereafter, theT_bs saturate to a constant

5 Effect of clouds on microwave radiances

Clear1 2 3 4 5 6 7 8 9 10 11 12 Cloud base height [km]

-30 -20 -10 0

TBcloud - TBclear [K]

Clear 1 2 3 4 5 6 7 8

Cloud base height [km]

-30 -25 -20 -15 -10 -5 0

TBcloud - TBclear [K]

Figure 5.10: Effect of the cloud altitude on T_b depression for the nadir viewing geometry forimc= 0.4 g m⁻³. 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.

value because the clouds are above the sensing altitude and they have more or less the same background. Therefore for the cloud altitudes above the Jacobian peak, theT_bs remain constant irrespective of the position of the cloud. From Figures 5.9 and 5.10 it can be seen that the maximum depression is about 30 K at the highest cloud altitude. For 190 GHz, the depression starts already at the first cloud altitude 1 to 3 km. The steep depression behavior extends up to the cloud altitude of 7 to 9 km and then tends to remain constant. The maximum de-pression is about 34 K at the highest cloud altitude. Also for 150 GHz, theT_bdepression starts with the first cloud altitude (1 to 3 km). The steep depression behavior extends up to cloud altitude 3 to 5 km and thereafter theT_bs remain constant. The maximum depression is about 16 K. For 89 GHz, the effect of changing the cloud altitude is negligible on the up-welling T_b as this frequency is not sensitive to ice clouds.

For the midlatitude winter case, the tropopause is at about 10.1 km.

Therefore cloud heights above 10 km are not considered. Here 184 GHz starts getting sensitive to different cloud positions at a cloud top height of 5 km compared to 8 km for the tropical case. All the other

higher frequencies, 186, 190 and 150 GHz start showing depression at the lower most cloud altitude, 1 to 3 km. This is obvious because the midlatitude winter atmosphere is dry compared to the tropical case and these frequencies are sensitive to lower altitudes. This can be seen from their corresponding clear sky water vapor Jacobians in Figures 5.2 and 5.3. As in the tropical case, 89 GHz is not affected by cloud positions. For this frequency the scattering effect is very small.

Another interesting feature is the intersection of the different T_b curves corresponding to the frequencies 184, 186, and 190 GHz. As can be seen from Figure 5.9, for the midlatitude winter scenario, the clear skyT_b relationship

T_b^clear₁₉₀ > T_b^clear₁₈₆ > T_b^clear₁₈₄ (5.7) is maintained for the cloud case up to a cloud altitude of 1 to 3 km. For a cloud height of 2 to 4 km, it can be seen that while 184 GHz still has the minimumT_bamong these three frequencies, 186 GHzT_b is higher than 190 GHzT_b. As the cloud heights are increased, the T_b changes in such a way to establish a reverse of the clear sky relationship. This happens at a cloud base altitude of 5 km where,

T_b^5-7₁₈₄> T_b^5-7₁₈₆> T_b^5-7₁₉₀ (5.8) Beyond this, for cloud heights of 6 to 8 km and 8 to 10 km, the initial clear sky relation ship is re-established. For the tropical case, the clear sky relationship holds except for cloud base heights of 4, 5, and 6 km, where 186 GHzT_b was slightly higher than 190 GHzT_b. But 184 GHz had always the lowestT_b similar to the clear sky case. This confirms the fact that the cross-over effect discussed in Sections 5.2.1 and 5.2.2 is valid independently of the cloud altitude.

In Muller et al. (1994), when the cloud base height increases from 4 to 8 km, theT_bdepression increased by 20, 33, and 15 K respectively at 182, 180 and 176 GHz. The iwp was 0.85 kg m⁻² and the cloud thickness was 4 km. The correspondingT_b increase as the cloud base altitude increases from 4 km to 8 km using arts is 18, 20 and 13 K respectively. Theiwpfor the simulation usingartsis 0.8 kg m⁻²and the cloud thickness is 2 km.

5 Effect of clouds on microwave radiances