Download Analyses of Dry Intrusion and Instability during a Heavy Rainfall

ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2009, VOL. 2, NO. 2, 108−112
Analyses of Dry Intrusion and Instability during a Heavy Rainfall Event
that Occurred in Northern China
YANG Shuai, CUI Xiaopeng, and RAN Lingkun
Laboratory of Cloud-Precipitation Physics and Severe Storms (LACS), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
Received 15 December 2008; revised 1 March 2009; accepted 1 March 2009; published 16 March 2009
Abstract The 1°×1° National Center for Environmental
Prediction/National Center for Atmospheric Research
(NCEP/NCAR) data and mesoscale numerical simulation
data are analyzed to reveal a mechanism for the formation
of heavy rainfall in Northern China; this mechanism is the
non-uniformly saturated instability induced by a dry intrusion. The dry intrusion and the accompanying downward transport of air with a high value of potential vorticity (PV) are maintained during the precipitation event. As
the dry air intrudes down into the warm and moist sector
in the lower troposphere, the cold, dry air and the warm,
moist air mix with each other, and, as a result, the atmosphere becomes non-uniformly saturated. On the basis of
this non-uniform saturation, a new Brunt-Väisälä frequency (BVF) formula is derived and applied to the precipitation event. It is shown that, compared to the conditions of either a dry or a saturated atmosphere, the BVF in
a non-uniformly saturated, moist atmosphere (BVF*) may
be more appropriate for depicting the atmospheric instability in rainy regions.
Keywords: dry intrusion, instability, non-uniformly saturated atmosphere
Citation: Yang, S., X. Cui, and L. Ran, 2009: Analyses
of dry intrusion and instability during a heavy rainfall
event that occurred in Northern China, Atmos. Oceanic
Sci. Lett., 2, 108−112.
1
Introduction
Dry intrusion means that cold, dry air with a high value
of potential vorticity (PV) intrudes from the lower stratosphere and/or upper troposphere down into the lower troposphere. They have important roles in the explosive development of cyclones, the evolution of cold fronts, etc.
From synoptic charts, it is easily found that dry intrusions
descend above warm and moist sectors, and this may induce potential instability. When strong convections occur,
precipitation is triggered. Therefore, the studies on dry
intrusions are very important for weather forecasting.
Danielsen (1964) demonstrated that the three-dimensional structure of dry intrusion airflows fanning out
down from the tropopause fold along the isentropic surface. In recent years, the satellite infrared, visible and
water-vapor imageries have been widely used to study
these dynamics. Dry intrusions have been seen clearly in
satellite imageries. They are referred to as “dry slots” in
cloud imageries and “dark areas” in water-vapor imageCorresponding author: YANG shuai, [email protected]
ries (Browning and Monk, 1982; Browning et al., 1995).
Browning and Monk (1982) provided a split-cold front
model, with an over-running upper front ahead of a surface front, which provides a useful representation of the
principal characteristics of many katafronts. Browning
and Roberts (1994) utilized numerical weather-prediction
model products and satellite radar imagery to propose a
conceptual model of a mid-latitude developing frontal
cyclone or wave (Fig. 8 in their paper), which integrated a
number of well-known features such as warm conveyor
belts (Harrold, 1973), cold conveyor belts (Carlson, 1980),
split fronts (Browning and Monk, 1982), frontal fractures
(Shapiro and Keyser, 1990), dry slots (Weldon and
Holmes, 1991), line convections (Browning and Harrold,
1970), cloud head (Bottger et al., 1975), and the dry intrusion airflow (Reed and Danielsen, 1959). Browning
and Roberts (1994) elucidated the influence of dry intrusions on rapidly-deepening cyclones during its different
development stages. Browning and Golding (1995) analyzed the occurrence of a rapidly-deepening cyclone that
crossed the British Isles. Browning et al. (1995) derived
the effect of dry intrusions on the mesoscale structure and
evolution of parts of a frontal cyclone developing over the
eastern North Atlantic Ocean. Browning and Roberts
(1996) illuminated the role of dry intrusions on cold frontal precipitation. Browning (1997) analyzed the development of dry intrusions and extra-tropic cyclones.
The research mentioned above considers many aspects
of dry intrusions, but the effects of dry intrusions on
heavy rainfall events in Northern China have not been
studied as thoroughly. Northern China lies in middle latitudes. Dry and cold air from the North and the upper troposphere often reach the region. Warm and moist air from
the South also enters frequently. Thus, these airflows may
encounter each other and lead to precipitation by the convergence-lift effect. According to the survey results of
meteorological observatories, dry intrusions frequently
occur in Northern China. Therefore, it is necessary to
study dry intrusions in order to analyze the mechanisms
for the formation of heavy rainfall event in Northern
China and to improve weather forecast. In section 2, a
case study of the features of dry intrusions is performed in
detail. In section 3, the instability induced by dry intrusions is studied. A summary is given in the last section.
2 The analysis of the characteristics of dry
intrusions
One case is from 0000 UTC 12 to 0000 UTC 13 Au-
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YANG ET AL.: ANALYSES OF DRY INTRUSION
gust 2004. The circulation background and precipitation
process are described in the literature (Yang et al., 2007a).
The 6-h 1° × 1° National Center for Environmental Prediction/National Center for Atmospheric Research
(NCEP/NCAR) data are used to analyze the characteristics of the dry intrusion for the above case.
The meridional-vertical cross-section along 116°E of
the 6-h time-averaged equivalent potential temperature,
stream field, tropopause, horizontal velocity, specific humidity, and the observed 6-h cumulated precipitation and
PV are shown in Figs. 1a and 1b. From Figs. 1a and 1b,
the dry intrusion airflow, and accompanying high values
of PV from the near-tropopause level at 45°N, intrudes
down towards the warm and moist sector. The PV=1 contour extends down almost to 850 hPa, between 40°N and
45°N. The distributions of relative humidity and the temperature perturbation (Figs. 1c and 1d) show that the dry
intrusion is not only dry but also cold. Also from Fig. 1a,
the dense θe isolines are stretched vertically from the surface near 40°N upwards towards higher elevations. A
large humidity ridge exists ahead of the surface of the
cold front, where very warm and moist ascending airflows
are present and precipitation occurs. Across the front,
there is a positive secondary circulation that ascends in
the warm sector and descends in the cold one. In the
lower troposphere, the convergence of strong southerlies
and northerlies drive θe isolines towards 35−40°N, and
the front strengthens.
From 0600 UTC to 1200 UTC 12 August 2004, the dry
intrusion, and accompanying high PV (Figs. 1e and 1f),
moves southwards slightly and intrudes downwards to
850 hPa. Correspondingly, the front and its precipitation
move to the south of 40°N. In contrast to the circulation
in Fig. 1a, the closed circulation no longer exists in Fig.
1e. Dry intrusions dominate the whole upper level. The
front and precipitation move southwards, which may be
associated with the upper-level dry intrusion.
From the zonal-vertical cross-section along 38°N (Figs.
2a and 2b), strong cold, dry westerlies from the upper
level intrude downwards into the warm and moist sector
and move eastwards. Correspondingly, surface precipitation moves eastwards.
By the above analysis of this precipitation event in
Northern China, the southward dry intrusion in the upper
level can either reach the boundary of the warm and moist
sector or intrude down into the warm and moist sector in
the lower level, while the eastward dry intrusions from the
upper level can either reach above the warm and moist
sector or mix with the lower-level moist air. When the dry
intrusion airflow in the mid-upper troposphere overruns
parts of the lower-level warm and moist sector, and lies
slanted above the rainy region, the differences in temperature and moisture between the lower and upper levels
are increased; therefore, a potential instability is realized.
3 The dynamical analysis of the non-uniformly
saturated, moist instability induced by dry
intrusion
From the above analysis, the higher-level dry intrusion
109
descends and reaches to above the moist sector or mixes
with the lower-level moist air; therefore, instability is
readily built up. It is easy to understand that the approach
of an eastward and downward dry intrusion that originates
in the upper level and mixes with the lower-level moist air
moving upwards can easily lead to a non-uniform saturation. Therefore, the generalized potential temperature,
which is more applicable to a real moist atmosphere and
the idea of non-uniform saturation (Gao et al., 2004; Gao
and Cao, 2007; Yang et al., 2007b), is used here to analyze the instability in the non-uniformly saturated, moist
atmosphere.
3.1 The modification of the Brunt-Väisälä frequency
(BVF) for a moist flow
In dry and unsaturated air, BVF can be calculated from
the expression
d ln θ
,
(1)
N2 = g
dz
where θ = T ( p0 / p ) p (T is the temperature, p is the
pressure, p0=1000 hPa, R is the universal gas constant,
and cp is the specific heat of dry air at a constant pressure
p, θ is the potential temperature, and g is the gravitational
acceleration).
In moist saturated air, by using the equivalent potential
temperature θe (expressed as θe = θ exp (Lqs/cpT), where L
is the latent heat of vaporization and qs is saturated specific humidity) instead of θ, saturated BVF becomes
d ln θ e
N m2 = g
.
(2)
dz
When considering the non-uniformly saturated characteristics after the mixing between the dry intrusion and the
moist airflows, the generalized potential temperature θ*
that is appropriate to non-uniformly saturated flow is used.
Then the BVF is modified to BVF* in a non-uniformly
saturated, moist flow. It is expressed as
d ln θ *
N *2 = g
,
(3)
dz
where θ* is expressed as θ* = θ exp[(Lqs/cpT)(q/qs)k] (q is
specific humidity and k=9) (Gao et al., 2004; Gao and
Cao, 2007; Yang et al., 2007b).
R/c
3.2 The diagnostic analysis and application of the
modified BVF*
Also, for the above case, a simulation was carried out
by using the three-dimensional non-hydrostatic Weather
Research and Forecasting (WRF) Model. The details of
the parameter scheme and a comparison between the observations and model results can be found in the work of
Yang et al. (2007a). By using the above-mentioned credible WRF simulated data, the diagnostic analysis and the
application of the modified BVF were performed for the
rainy regions of the precipitation event. Figure 3 is the
meridional-vertical cross-section of different BVFs along
114.5°E at 0000 UTC 12 August 2004. The shaded regions have BVF values less than 0. Before the occurrence
of precipitation, there is no instability in Fig. 3a (shown
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Figure 1 The meridional-vertical cross-sections along 116°E for 6-h (from 0000 UTC to 0600 UTC 12 August 2004) time-averaged data for the: (a)
equivalent potential temperature (thick solid line, units: K), stream line (thin solid line with vector arrows), tropopause (thick dashed line), specific
humidity (shaded in the lower level, units: g kg−1), and observed 6-h cumulated (0000 UTC−0600 UTC 12 August 2004) precipitation (histogram,
units: mm); (b) potential vorticity (units: PVU); (c) temperature perturbation (negative values are shaded, units: K); (d) relative humidity (the relative
humidity less than 70% is shaded) and temperature (isoline, units: K); and (e) and (f) are the same as (a) and (b), but for the 6-h time-averaged from
0600 UTC to 1200 UTC 12 August 2004. Shaded arrows represent the dry-intrusion airflow in (a) and (e).
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YANG ET AL.: ANALYSES OF DRY INTRUSION
111
Figure 2 The zonal-vertical cross-sections along 38°N of the 6-h
time-averaged equivalent potential temperature (thick solid line, units:
K), stream line (thin solid line with vector arrows), specific humidity
(shaded, units: g kg−1), and the observed 6-h cumulated precipitation
(histogram, units: mm): (a) from 0000 UTC to 0600 UTC 12 August
2004; and (b) from 0600 UTC to 1200 UTC 12 August 2004. Shaded
arrows are the same as in Fig. 1.
by BVF values > 0). Only a weak instability was present
at 38°N in the saturated case (Fig. 3b). And the region of
instability calculated by using a modified BVF in the
non-uniformly saturated, moist atmosphere stretched vertically upwards between 36°N and 38°N (Fig. 3c). Then,
precipitation started. Till 1200 UTC, a stable region
dominated nearly the whole troposphere and instability
only occurred below 2 km (Fig. 4a). In Fig. 4b, the coverage of the instability is larger than that in dry air. While
the deep, instable region stretched vertically over the
rainy region for about 13 km in a non-uniformly saturated,
moist atmosphere (Fig. 4c). Compared with Figs. 4a and
4b, the BVF formulas used for dry and saturated atmospheres depict the instability of rainy regions far worse
than those using BVF* in a non-uniformly saturated,
moist flow (Fig. 4c). In theory, the BVF* formula is more
reasonable because it considers the non-uniformly saturated property of a real atmosphere and, therefore, a
greater release of latent heat. Furthermore, the diagnostic
analysis of BVF* shows its better applicability in the future for portraying real atmospheric instability.
Figure 3
The meridional-vertical cross-sections along 114.5°E of (a)
N2; (b) N m2 ; and (c) N*2 at 0000 UTC 12 August 2004 (negative values
are shaded, units: 104 s−2).
4
Summary
From the analysis of a rainfall event in Northern China,
the characteristics of a dry intrusion and the atmospheric
instability were revealed. As the dry air intruded downwards into the warm and moist sector in the lower troposphere, cold, dry air and warm, moist air mixed with each
other, and the atmosphere became non-uniformly saturated. On the basis of this non-uniform saturation, a new
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rated airflows. It can more accurately predict and diagnose the location of heavy rainfall compared to those for
dry and saturated atmospheres.
Acknowledgements. This study was supported by the National
Natural Science Foundation of China (under Grant No. 40805001),
and the Knowledge Innovation Program of the Chinese Academy of
Sciences (under Grant Nos. KCL14014, IAP07201, and IAP07214).
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Figure 4 The meridional-vertical cross-sections along 114.5°E of (a)
N2; (b) N m2 ; and (c) N*2 at 1200 UTC 12 August 2004 (negative values
are shaded, units: 104 s−2), from and the 6-h precipitation (histogram,
units: mm) from 0600 UTC to 1200 UTC 12 August 2004.
BVF formula was derived and applied to the precipitation
event by using the meso-scale numerically simulated results from WRF. It is shown that the use of BVF* may be
more appropriate to depict the instability in rainy regions
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