Abstract. This study examines the diurnal variation in precipitation
over Hainan Island in the South China Sea using
gauge observations from 1951 to 2012 and Climate Prediction
Center MORPHing technique (CMORPH) satellite estimates
from 2006 to 2015, as well as numerical simulations.
The simulations are the first to use climatological mean initial
and lateral boundary conditions to study the dynamic
and thermodynamic processes (and the impacts of land–sea
breeze circulations) that control the rainfall distribution and
climatology. Precipitation is most significant from April to
October and exhibits a strong diurnal cycle resulting from
land–sea breeze circulations. More than 60% of the total annual
precipitation over the island is attributable to the diurnal
cycle with a significant monthly variability. The CMORPH
and gauge datasets agree well, except that the CMORPH
data underestimate precipitation and have a 1 h peak delay.
The diurnal cycle of the rainfall and the related land–
sea breeze circulations during May and June were well captured
by convection-permitting numerical simulations with
the Weather Research and Forecasting (WRF) model, which
were initiated from a 10-year average ERA-Interim reanalysis.
The simulations have a slight overestimation of rainfall
amounts and a 1 h delay in peak rainfall time. The diurnal
cycle of precipitation is driven by the occurrence of moist
convection around noontime owing to low-level convergence
associated with the sea-breeze circulations. The precipitation
intensifies rapidly thereafter and peaks in the afternoon with
the collisions of sea-breeze fronts from different sides of the
island. Cold pools of the convective storms contribute to the
inland propagation of the sea breeze. Generally, precipitation
dissipates quickly in the evening due to the cooling and stabilization
of the lower troposphere and decrease of boundary
layer moisture. Interestingly, the rather high island orography
is not a dominant factor in the diurnal variation in precipitation
over the island.
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Boundary Layer Quasi-Equilibrium Limits Convective Intensity Enhancement from the Diurnal Cycle in Surface Heating
A combination of cloud-permitting model (CPM) simulations, satellite, and reanalysis data are used to test whether the diurnal cycle in surface temperature has a significant impact on the intensity of deep convection as measured by high-percentile updraft velocities, lightning, and CAPE. The land–ocean contrast in lightning activity shows that convective intensity varies between land and ocean independently from convective quantity. Thus, a mechanism that explains the land–ocean contrast must be able to do so even after controlling for precipitation variations. Motivated by the land–ocean contrast, we use idealized CPM simulations to test the impact of the diurnal cycle on high-percentile updrafts. In simulations, updrafts are somewhat enhanced due to large-scale precipitation enhancement by the diurnal cycle. To control for large-scale precipitation, we use statistical sampling techniques. After controlling for precipitation enhancement, the diurnal cycle does not affect convective intensities. To explain why sampled updrafts are not enhanced, we note that CAPE is also not increased, likely due to boundary layer quasi equilibrium (BLQE) occurring over our land area. Analysis of BLQE in terms of net positive and negative mass flux finds that boundary layer entrainment, and even more importantly downdrafts, account for most of the moist static energy (MSE) sink that is balancing surface fluxes. Using ERA-Interim data, we also find qualitative evidence for BLQE over land in the real world, as high percentiles of CAPE are not greater over land than over ocean.
more » « less- NSF-PAR ID:
- 10127534
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 77
- Issue:
- 1
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- p. 217-237
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. This study examines the diurnal variation in precipitation over Hainan Island in the South China Sea using gauge observations from 1951 to 2012 and Climate Prediction Center MORPHing technique (CMORPH) satellite estimates from 2006 to 2015, as well as numerical simulations. The simulations are the first to use climatological mean initial and lateral boundary conditions to study the dynamic and thermodynamic processes (and the impacts of land–sea breeze circulations) that control the rainfall distribution and climatology. Precipitation is most significant from April to October and exhibits a strong diurnal cycle resulting from land–sea breeze circulations. More than 60% of the total annual precipitation over the island is attributable to the diurnal cycle with a significant monthly variability. The CMORPH and gauge datasets agree well, except that the CMORPH data underestimate precipitation and have a 1h peak delay. The diurnal cycle of the rainfall and the related land–sea breeze circulations during May and June were well captured by convection-permitting numerical simulations with the Weather Research and Forecasting (WRF) model, which were initiated from a 10-year average ERA-Interim reanalysis. The simulations have a slight overestimation of rainfall amounts and a 1h delay in peak rainfall time. The diurnal cycle of precipitation is driven by the occurrence of moist convection around noontime owing to low-level convergence associated with the sea-breeze circulations. The precipitation intensifies rapidly thereafter and peaks in the afternoon with the collisions of sea-breeze fronts from different sides of the island. Cold pools of the convective storms contribute to the inland propagation of the sea breeze. Generally, precipitation dissipates quickly in the evening due to the cooling and stabilization of the lower troposphere and decrease of boundary layer moisture. Interestingly, the rather high island orography is not a dominant factor in the diurnal variation in precipitation over the island.
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