The unique hydro-climatological cycle in CL montane cloud forest can be summarized: (1) the early peak of LH flux in CL montane cloud-fog forest makes the near-surface temperature increase slowly in the morning. (2) During the daytime, valley wind brings water vapor from lowland combined with evapotranspiration from local forest, resulting in water vapor accumulation until 3 p.m. (3) Because of the small diurnal temperature range, water vapor can easily reach saturation in the afternoon, thus favoring fog formation. Fog further serves as a source of canopy water in addition to the dew and precipitation. (4) Plentiful canopy water can sustain throughout the night because of the high relative humidity. The drying trend of leaf wetness after sunrise implies the critical role of canopy water on the early peak of LH flux. This unique hydro-climatological cycle in the montane cloud-fog forest reflects the inseparable relationship between the canopy and near-surface meteorology at the diurnal cycle, and such unique cycle can be seen in all seasons (Fig. 5.1). The offline simulations also suggest the asymmetric LH flux is principally attributed to high canopy evaporation in the early morning.
Fog, precipitation, and dew comprise the plentiful canopy water. From the sensitivity tests, precipitation forcing may be the controlling factor to the plentiful canopy water in the early morning and significantly affect the peak of canopy evaporation. Such abundant canopy water in CL may be attributed to the frequent drizzle phenomenon. Besides, downward longwave radiation and temperature forcing are the minor contributors to the asymmetric LH flux. The downward radiation forcing leads to an increase in nighttime dew, having less contribution compared with precipitation. The change in temperature forcing mainly results in the change in total transpiration, which may affect the asymmetry of LH flux when total transpiration outweighs total canopy evaporation. In
summary, the source of canopy water is highly associated with the atmospheric forcing.
Under future projections, a decrease in canopy water may happen due to a reduction of fog formation, a larger variation in precipitation, and a change in local circulations, which could lead to the disappearance of the asymmetric LH flux and further influence the eco-hydroclimatology in the montane cloud forests.
In this study, where the water vapor comes from and how the asymmetric LH flux will be influenced by different atmospheric forcing under climate change remain uncertain. Future works may require isotopic measurement to distinguish local and advection water vapor supply. In addition, idealized model simulation may be needed to discuss how the mean and variance of different atmospheric forcing may respectively affect the hydro-climatological cycle in montane cloud-fog forests.
FIGURES
Figure 1.1 The comparison of the diurnal cycle of net radiation (Rn: dashed lines) and latent heat flux (LH flux: solid lines) between CL (Chi-Lan: blue lines) and LHC (LienHuaChih: red lines). The shading color represents the variation of the energy fluxes between the first quartile and the third quartile from four years of data from 2008 to 2011 in CL and two years of data from 2012 to 2013 in LHC.
Figure 2.1 The location of CL flux tower site (green triangle). (The figure is taken from Klemm et al. (2006).)
Figure 2.2 The location of LHC flux tower site (red dot). (The figure is taken from Chen et al. (2012).)
Figure 2.3 The land type comparison between CL and LHC and the measurements in CL.
(The middle map in which green area represents the distribution of montane cloud-fog forests in Taiwan is taken from Schulz et al. (2017).)
Figure 3.1 Five meteorological variables obtained from the flux towers in CL (blue lines) and LHC (red lines): (a) temperature, (b) specific humidity (solid lines) and saturated specific humidity (dashed lines), (c) wind speed, and (d) relative humidity. The shading color represents the variation of each meteorological variable between the first quartile and the third quartile from four years of data in CL and five years of data in LHC.
Figure 3.2 (a) Simulations conducted by the Community Land Model V4: with (CTR:
blue lines) and without (EXP: orange lines) canopy water representation. (b) The comparison of the diurnal cycle of net radiation (dashed lines) and LH flux (solid lines) between CTR and EXP. (c) (d) The partition of LH flux (including ground evaporation (brown lines), transpiration (red lines), and canopy evaporation (blue lines)) for (c)CTR and (d)EXP. The shading color represents the variation of the energy fluxes between the first quartile and the third quartile from the last eight years of the simulations.
Figure 3.3 (a) The contribution of different source of water on the canopy, including fog (blue bars), rain (green bars) and dew (red bars). (b) The comparison of the diurnal cycle of LH flux among CTR (blue line), Rain+Dew (green line) and DEWonly (red line). The shading color represents the variation of the LH fluxes between the first quartile and the third quartile from the last eight years of each simulation. (c) The partition of LH flux among CTR (blue lines), Rain+Dew (green lines) and DEWonly (red lines). The solid lines, dashed lines and dotted lines represent canopy evaporation, transpiration, and ground evaporation, respectively.
Figure 3.4 (a) The comparison of the diurnal cycle of LH flux among CTR_3yr (blue line), LHCatm_CLsurf (red line) and CLatm_LHCsurf (green line). The shading color represents the variation of the LH fluxes between the first quartile and the third quartile from the last nine years of each simulation. (b) The partition of LH flux among CTR_3yr (blue lines), LHCatm_CLsurf (red lines) and CLatm_LHCsurf (green lines). The solid lines, dashed lines and dotted lines represent canopy evaporation, transpiration, and ground evaporation, respectively. (c) The comparison of the diurnal cycle of canopy water among CTR_3yr, LHCatm_CLsurf and CLatm_LHCsurf. The shading color represents the variation of the canopy water between the first quartile and the third quartile from the last nine years of each simulation.
Figure 3.5 Atmospheric forcing sensitivity test: comparing the proportion of the partition of LH flux including canopy evaporation (blue bars), transpiration (red bars) and ground evaporation (brown bars).
Figure 3.6 Atmospheric forcing sensitivity test: comparing (a) canopy evaporation and (b) canopy water in each sensitivity run with CTR_3yr and LHCatm_CLsurf.
Figure 4.1 The comparison of the occurrence probability of the daily maximum LH flux between CL (blue line) and LHC (red line).
Figure 4.2 (a) The comparison of the diurnal cycle of canopy water among CTR (blue line), max_cw_0.2 (purple line) and max_cw_0.1 (dark magenta line) and max_cw_0.05 (light magenta line). The shading color represents the variation of the canopy water between the first quartile and the third quartile from the last eight years of each simulation.
(b) The comparison of the diurnal cycle of LH fluxes among CTR, max_cw_0.2 and max_cw_0.1 and max_cw_0.05. The shading color represents the variation of the canopy water between the first quartile and the third quartile from the last eight years of each simulation. (c) The partition of LH flux among CTR, max_cw_0.2 and max_cw_0.1 and max_cw_0.05. The solid lines, dashed lines and dotted lines represent canopy evaporation, transpiration, and ground evaporation, respectively.
Figure 4.3 The comparison of probability density function (P.D.F.) in precipitation between CL (blue line) and LHC (red line).
Figure 4.4 (a) The comparison of probability density function in precipitation between LHCatm_CLsurf (red line) and LHCatm_Clim_precip (orange line). (b) The comparison of the diurnal cycle of canopy water among LHCatm_CLsurf and LHCatm_Clim_precip The shading color represents the variation of the canopy water between the first quartile and the third quartile from the last nine years of each simulation. (c) The partition of LH flux among LHCatm_CLsurf and LHCatm_Clim_precip The solid lines, dashed lines and dotted lines represent canopy evaporation, transpiration, and ground evaporation, respectively. The shading color represents the variation of the canopy water between the first quartile and the third quartile from the last nine years of each simulation. (d) The comparison of the diurnal cycle of LH flux among LHCatm_CLsurf and LHCatm_Clim_precip The shading color represents the variation of the LH fluxes between the first quartile and the third quartile from the last nine years of each simulation.
Figure 4.5 The effects of fog on (a) LH flux and (b) CO2 flux in CL montane cloud-fog forest. The solid dots represent the mean values of fluxes in foggy conditions in each time step, while the hollow dots represent those in fogless conditions. The solid line shows the averaged fluxes in foggy conditions from 6 a.m. to 6:30 p.m., while the dashed line shows those in fogless conditions from 6 a.m. to 6:30 p.m.
Figure 5.1 Schematic plot of the hydro-climatological cycle in CL montane cloud forest.
TABLES
Table 2.1 Experiment design: the contribution of canopy water on the LH flux Name of per 30 mins when visibility <
1km.
Water will drip on the ground as soon as it form or intercept on the canopy.
Table 2.2 Experiment design: canopy water sensitivity test per 30 mins when visibility <
1km. include fog as a source of canopy water.
Table 2.3 Experiment design: the controlling factors to the asymmetric LH flux in CL
LHCatm_CLsurf CL LHC precip.
dew
Table 2.4 Experiment design: the controlling factors to the asymmetric LH flux in CL
LHCatm_CLsurf CL LHC precip.
dew
using CL downward solar radiation
LHCatm_CL_LWdw CL
LHC atm. but CL
downward longwave radiation
precip.
dew
Land:
the same as CTR Atm.:
the same as
LHCatm_CLsurf but using CL downward londwave radiation
Table 3.1 The difference of leaf wetness between 6 a.m. and 9 a.m. in 3 different canopy layers. The positive mean value represents the canopy being wetter at 6 a.m. than at 9 a.m.
Height(m)
Difference of leaf wetness between 6 a.m. and 9 a.m.
(mean(mV) ± std)
5.3 32.39 ± 62.31*
8.3 70.25 ± 102.44*
11.2 1.87 ± 26.9
3 layer averaged 36.95 ± 56.81*
*The value shows significant difference at the 1% significance level, according to one-tailed t test
Table 4.1 Experiment design: the sensitivity test of the maximum allowed canopy water
Table 4.2 Experiment design: the impact of drizzle on the asymmetry of LH flux
LHCatm_CLsurf CL LHC precip.
dew
LHCatm_Clim_precip CL LHC precip.
dew
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