Supplementary Materials for
Impacts of Vertical Structure of Large-Scale Vertical Motion in Tropical Climate:
Moist Static Energy Framework
Hien Xuan Bui 1 , Jia-Yuh Yu 2 and Chia Chou 3
1
Taiwan International Graduate Program - Earth System Science Program, Research Center for
Environmental Changes, Academia Sinica and Department of Atmospheric Sciences, National
Central University, Taiwan
2
3
Department of Atmospheric Sciences, National Central University, Taoyuan City, Taiwan
Research Center for Environmental Changes, Academia Sinica and Department of Atmospheric
Sciences, National Taiwan University, Taiwan
Correspondence to: [email protected] or [email protected]
1
1. Datasets
In this study, we tested our hypothesis on two different datasets. The first one is the
reanalysis ERAITM (Dee et al. 2011), containing the daily outputs with a grid size of 1.5o  1.5o
spatial resolution at 37 pressure levels. Since most of the analysis corroborates Back and
Bretherton (2006), we want to stress that by using the ERAITM our results are improved
compared to previous studies [i.e., Back and Bretherton (2006); Peters et al. (2008)]. The
ERAITM reanalysis data is a product from a much improved atmospheric model and
assimilation system. It is more skillful than the ERA40, which was used by Back and Bretherton
(2006). As mentioned in many papers, the ERA40 does not include rainfall observations in its
assimilation process, nor it does include constraints to maintain a balanced hydrological budget,
and thus had a too strong precipitation over tropical oceans. Figure S1 shows a comparison of the
daily time-series of precipitation over the eastern and the western Pacific ITCZs, from the
TRMM and ERAITM datasets. The rainfall characteristics are mostly identical, but ERAITM’s
precipitation appears to have smaller fluctuations due to its lower horizontal resolution compared
to TRMM data. The domains are taken over the tropical oceans, so there is no particular bias
from
topographical
effect.
The
data
are
available
from
http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/
We also used a model simulation from the CESM, containing 30 years of daily data with 1o
 1o spatial resolution. Testing the CESM is the first step along the line of our current works to
set up a series of hierarchical model sensitivity tests to examine the role of shallow convection in
tropical convection and climate.
2
2. Domains selection
The top- and bottom-heavy convection in our study are identified based on the
climatological structure of convection averaged over the two heavy precipitation domains in the
Pacific ocean. To justify the domains selection, we show some evidences from the ISCCP
satellite retrieved products and the ERAITM reanalysis data.
2.1. ISCCP dataset
Figure S2 shows the spatial distribution of climatological temperature and pressure at the
cloud-top level over the tropical Pacific. Both the cloud-top temperature and pressure differences
between the western and the eastern Pacific are obvious. As the ISCCP cloud-top data
statistically average the cloud top of various kinds of cloud at each pixel once a cloud is detected,
the higher cloud-top temperature and greater cloud-top pressure in the eastern Pacific domain
suggest that the eastern Pacific is dominated by more bottom-heavy convection. On the other
hand, the lower cloud-top temperature and smaller cloud-top pressure show that deep convection
clouds are much more frequently to occur in the western Pacific.
2.2. Reanalysis ERAITM
Before presenting the detailed EOF modes of omega field, we first show the spatial
distribution of the pressure velocity at 500 hPa (Figure S3). This quantity ( 500 ) is often used to
define the domain of mean ascending regions (Chen et al. 2016). To find out whether the vertical
motion profile changes in different parts of the tropics, in additional to the entire tropical
domain, the EOF analysis of omega field was conducted respectively over three sub-domains
within the ITCZs, namely the western Pacific convergence zone (WPCZ, 0-10oN, 130-170oE),
the south Pacific convergence zone (SPCZ, 5-15oS, 150oE-140oW) and the eastern Pacific
convergence zone (EPCZ, 5-10oN, 180-110oW).
3
The first three EOF modes of pressure velocity field are plotted over the entire tropical
convergence zone (Figure S4), the western Pacific convergence zone (Figure S5), the eastern
Pacific convergence zone (Figure S6) and the south Pacific convergence zone (Figure S7). A
clear bimodal distribution of convection occurs in the climatologically ascending regions (see
Fig. S4), suggesting the coexistence of shallow and deep convection. We note that the first EOF
mode in the WPCZ and SPCZ are dominated by a top-heavy structure of convection (see Figs.
S5 and S7) while the eastern Pacific is dominated by a bottom-heavy structure of convection (see
Fig. S6), consistent with the results shown in the paper. The second and third EOFs only account
for a small fraction of the total variance and we neglect their discussions here for brevity.
A latitude–height section of the pressure velocity and stream function at longitudes 150oE
and 120oW is plotted in Figure S8. Over the western Pacific (at 150oE), active deep convection is
observed between 15oS and 15oN with a peak ascending motion at around 400-500 hPa,
indicating a typical top-heavy structure of convection (Fig. S8a). Over the eastern Pacific (at
120oW), the north-south extent of ascending motion appears much narrow (5~15oN) compared to
the western Pacific (Fig. S8b). The peak ascending motion in the eastern Pacific ITCZ occurs at
around 850hPa, showing a typical bottom-heavy structure of convection. The strong surface
convergence and middle level divergence appear over the eastern Pacific ITCZ, suggesting the
predominance of strong SST gradients in driving the boundary layer convergence in this region.
The above arguments demonstrate that the two domains we have selected in this study are
typical for analyzing the impacts of deep and shallow convection on tropical climate.
4
References:
Back, L. E., and C. S. Bretherton, 2006: Geographic variability in the export of moist static
energy and vertical motion profiles in the tropical pacific. Geophysical Research Letters, 33 (17),
doi: 10.1029/2006GL026672.
Chen, C.-A., J.-Y. Yu, and C. Chou, 2016: Impacts of vertical structure of convection in global
warming: The role of shallow convection. J. Climate, 29, doi: http://dx.doi.org/10.1175/JCLI-D15-0563.1.
Dee, D. P., and Coauthors, 2011: The era-interim reanalysis: configuration and performance of
the data assimilation system. Quarterly Journal of the Royal Meteorological Society, 137 (656),
23553–597, doi:10.1002/qj.828.
Peters, M. E., Z. Kuang, and C. C. Walker, 2008: Analysis of atmospheric energy transport in
ERA-40 and implications for simple models of the mean tropical circulation. J. Climate, 21 (20),
5229–5241, doi:10.1175/2008JCLI2073.1.
5
Figure S1: Daily time-series of precipitation of year 2002 (unit is mm day-1) from the TRMM
(red) and ERAITM (blue) datasets in the (a) western and (b) eastern Pacific regions.
6
Figure S2: Similar to Fig. 1 but for the (a) cloud-top temperature (unit is K) and (b) cloud-top
pressure (unit is hPa) from the ISCCP monthly dataset during the period from 1983
to 2008.
7
Figure S3: Spatial pattern of pressure velocity at 500 hPa (unit is Pa s-1), a quantify typically
used to define the domain of climatologically ascending regions (i.e., ITCZ), derived
from the 0.5o resolution ERAITM daily reanalysis product during the period of 20012010. The solid curves represent the zero contours and the three rectangular boxes
denotes the regions, including the western Pacific convergence zone (WPCZ, 010oN, 130-170oE), the eastern Pacific convergence zone (EPCZ, 5-10oN, 180110oW) and the south Pacific convergence zone (SPCZ, 5-15oS, 150oE-140oW) for
conducting EOF analysis of the omega (vertical velocity) field.
8
Figure S4: The first three EOF modes of omega field in the entire tropical convergence zone
(i.e., the area enclosed by the zero contours of pressure velocity at 500 hPa).
9
Figure S5: Similar to Fig. S4 but for the western Pacific convergence zone (WPCZ, 0-10oN, 130170oE).
10
Figure S6: Similar to Fig. S4 but for the eastern Pacific convergence zone (EPCZ, 5-10oN, 180110oW).
11
Figure S7: Similar to Fig. S4 but for the south Pacific convergence zone (SPCZ, 5-15oS, 150oE140oW).
12
Figure S8: Latitude – height section of the mean vertical velocity (color shades, in Pa s-1) and
streamlines (in vectors) at (a) 150oE and (b) 120oW. Data are taken from the ERAinterim reanalysis averaged over the period from 2001 to 2010.
13