Intraseasonal Variability of Cloud Cover in Midlatitudes during Boreal Winter

We statistically investigated the zonal phase relation between intraseasonal variation (15-30 day) in cloud cover and that in geopotential height in the mid-latitude of the Northern Hemisphere during boreal winter. The intraseasonal variation of the geopotential height is well examined and mostly explained by Rossby waves having almost equivalent barotropic vertical structure. However, cloud cover during boreal winter has not been studied as much in mid-latitude (25–40°N), where Rossby waves exist in large meridional temperature gradient in basic state. Does the longitude of the maximum of cloud cover coincide with that of the maximum of southern wind rising along the isentropic surface?

We investigated the zonal phase relation between cloud cover, wind, and geopotential height. The total cloud cover, which we calculated using the D1 data created by the International Satellite Cloud Climatology Project (ISCCP) was used. For other physical quantities including high/medium/low-cloud covers, ERA-Interim reanalysis data were used. The periods are January and February from 1984 to 2008 for D1 data, and January and February from 1980 to 2016 for ERA-Interim.

First, we analyze the zonal phase relation between the total cloud cover and the geopotential height at 300 hPa (Z300). The phase relation between these two parameters is statistically significant only above the continents, where the position of the local maximum of total cloud cover is closer to the trough than the center between the trough and the ridge, where the maximum of the southern wind occurs. To elucidate the reason for the mismatch between the positions of the maximum total cloud cover and the maximum of the southern wind as well as the reason for the zonal dependence in their phase relation, we investigated the zonal phase relation between the high/medium/low-cloud covers and Z300.

The maximum high-cloud cover is found between the position of the southern wind maximum and the center of the ridge. In contrast, the position of the maximum upward motion in the upper layer almost coincides with the position of the maximum of the southern wind. These relations mean that high-cloud cover occurs east of the maximum of upward motion in most of the region. The distance between the two maxima is larger in the region where the westerly jet is stronger. This suggests that cloud advection by the westerly jet may be responsible. It is noted that this zonal phase relation does not hold over the eastern part of the Eurasian continent; the high-cloud cover fluctuates simultaneously in zonally elongated region there. We infer that the variation in high-cloud cover is related to unique Rossby waves with a long wavelength and fixed phase in almost all winters.

The maximum medium-cloud cover is located around or slightly east of the center of the trough. In contrast, the position of the maximum upward motion in the medium layer is almost coincident with the position of the maximum of the southern wind. These relations mean that the maximum medium-cloud cover is located west of the maximum upward motion in almost the entire region. However, extracting only the diabatic component of the vertical motion and examining the zonal phase relationship with the medium-cloud cover, their positions of the maximum are almost coincident. Furthermore, the position of the maximum of precipitation is located at almost same position. These results suggest that the variation of the medium-cloud cover is strongly influenced by diabatic heating in low-pressure systems with synoptic or smaller scales.

There is strong regional dependence in the zonal phase relation between low-cloud cover and Z300. Over the continents (oceans), maximum low-cloud cover is located east of the trough (ridge). It should be noted that in the reanalysis data, low-cloud cover is not well represented due to the lack of satellite observation near the ground.

These results explain the significant zonal phase relation between the ISCCP total cloud cover and Z300 mainly over the continents. The position of the maximum cloud cover can be explained by the fact that the maxima of high-, medium-, and low-cloud cover are close to each other.