Abstract. Studies have shown that changes in cloud cover are responsible
for the rapid climate warming over the Tibetan Plateau (TP) in the past 3
decades. To simulate the total cloud cover, atmospheric models have to
reasonably represent the characteristics of vertical overlap between cloud
layers. Until now, however, this subject has received little attention due to
the limited availability of observations, especially over the TP. Based on
the above information, the main aim of this study is to examine the
properties of cloud overlaps over the TP region and to build an empirical
relationship between cloud overlap properties and large-scale atmospheric
dynamics using 4 years (2007–2010) of data from the CloudSat cloud product
and collocated ERA-Interim reanalysis data. To do this, the cloud overlap
parameter α, which is an inverse exponential function of the cloud
layer separation D and decorrelation length scale L, is calculated using
CloudSat and is discussed. The parameters α and L are both widely
used to characterize the transition from the maximum to random overlap
assumption with increasing layer separations. For those non-adjacent layers
without clear sky between them (that is, contiguous cloud layers), it is
found that the overlap parameter α is sensitive to the unique
thermodynamic and dynamic environment over the TP, i.e., the unstable
atmospheric stratification and corresponding weak wind shear, which leads to
maximum overlap (that is, greater α values). This finding agrees well
with the previous studies. Finally, we parameterize the decorrelation length
scale L as a function of the wind shear and atmospheric stability based on
a multiple linear regression. Compared with previous parameterizations, this
new scheme can improve the simulation of total cloud cover over the TP when
the separations between cloud layers are greater than 1 km. This study thus
suggests that the effects of both wind shear and atmospheric stability on
cloud overlap should be taken into account in the parameterization of
decorrelation length scale L in order to further improve the calculation of
the radiative budget and the prediction of climate change over the TP in the
atmospheric models.
Comparison of Length Scale Parameterization Methodologies
