Search Results

You are looking at 1 - 8 of 8 items for :

  • Author or Editor: Changhai Liu x
  • Monthly Weather Review x
  • Refine by Access: All Content x
Clear All Modify Search
Changhai Liu
and
Mitchell W. Moncrieff

Abstract

A numerical model investigation is conducted of the effects of ambient flow and shear upon the propagation and morphology of density currents. The model is initialized with a horizontally homogeneous wind profile superimposed on a cold-air source that initiates and maintains the density currents. The base state is neutrally stratified and free-slip lower and upper boundary conditions are used.

A headwind (i.e., relative flow in the direction opposing the system movement) raises the density current head compared to calm surroundings, while a tailwind has the opposite effect. A weak or moderate shear elevates the head for the downshear-traveling system and a shallow multihead structure appears in strong shear. In contrast, the upshear-moving system is largely insensitive to the shear. In uniform flow, the propagation speed is linearly proportional to the ambient wind speed, reduced or enhanced by about three-quarters depending on the airflow direction. In uniform shear, a linear relationship approximates the relationship between the advance rate of density current and the value of the shear, particularly for the upshear-moving system.

An idealized dynamical model is developed for the moderate shear case in terms of a Froude number ℱ. The model has three branches, namely, a borelike region, an overturning updraft, and a stagnant region that moves bodily with the system. The Froude number calculated from the numerical model data is ℱ ≈ 0.7, which lies within die range of analytic solutions obtained.

With regard to the initiation of convection over an island or peninsula in an unsheared or weakly sheared ambient flow, a sea-breeze circulation will preferentially cause convection on the leeward side and a land breeze on the windward side. The opposite occurs when the ambient flow has moderate to strong low-level shear—that is, the sea breeze will cause convection on the windward side and a land breeze on the leeward side. The mean-flow momentum and mean-flow shear thus affect convection initiation in opposing ways. There is a dearth of observational data on density currents in shear flow with which to evaluate our dynamical model—in particular, the role of the overturning updraft, which is a new concept as regards density current dynamics.

Full access
Changhai Liu
and
Mitchell W. Moncrieff

Abstract

This paper investigates the effects of cloud microphysics parameterizations on simulations of warm-season precipitation at convection-permitting grid spacing. The objective is to assess the sensitivity of summertime convection predictions to the bulk microphysics parameterizations (BMPs) at fine-grid spacings applicable to the next generation of operational numerical weather prediction models. Four microphysical parameterization schemes are compared: simple ice (Dudhia), four-class mixed phase (Reisner et al.), Goddard five-class mixed phase (Tao and Simpson), and five-class mixed phase with graupel (Reisner et al.). The experimentation involves a 7-day episode (3–9 July 2003) of U.S. midsummer convection under moderate large-scale forcing. Overall, the precipitation coherency manifested as eastward-moving organized convection in the lee of the Rockies is insensitive to the choice of the microphysics schemes, and the latent heating profiles are also largely comparable among the BMPs. The upper-level condensate and cloudiness, upper-level radiative cooling/heating, and rainfall spectrum are the most sensitive, whereas the domain-mean rainfall rate and areal coverage display moderate sensitivity. Overall, the three mixed-phase schemes outperform the simple ice scheme, but a general conclusion about the degree of sophistication in the microphysics treatment and the performance is not achievable.

Full access
Mitchell W. Moncrieff
and
Changhai Liu

Abstract

Steady-state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection initiation. First, shear decreases the horizontal convergence, and therefore the mean ascent, associated with downshear propagating currents. Second, shear has a basic effect on the dynamical organization. If the downshear current travels at the speed of the ambient flow at a critical (steering) level, an overturning circulation provides deep lifting. Although mean ascent is increased by shear in the case of upshear propagating currents, the lifting is comparatively shallow because jumplike ascent occurs rather than deep overturning. The convection initiation mechanism involving the downshear current is therefore very different from the upshear case.

These basic principles are borne out in two-dimensional numerical simulations. Density currents generated by a stationary cold source imposed on an initially horizontally homogeneous, sheared, and neutrally stratified ambient flow are explored. Results show that (i) if the surface flow and low-level shear vectors are in the same direction, as in a low-level jet, the effects of shear and surface flow on the density current head height counteract one another; and (ii) if they oppose one another, as in a surface jet, both conspire to lower the density current head on the downwind side but raise it on the upwind side.

As regards convection initiation by sea breezes, point (i) above shows an approximately equal but weak preference for convection exists on the leeward and windward coasts. Point (ii) shows that initiation is strongly suppressed on the windward coast, but strongly enhanced on the leeward one. The hypothesis that sea breezes are more intense in offshore flow therefore holds only if shear and surface flow have opposite sign or if the flow is unsheared.

Concerning convection initiation by thunderstorm outflows, downshear propagating outflows provide the deepest lifting if they move at the speed of the ambient flow at a critical level, despite the fact that low-level convergence is decreased by shear. While shear strengthens the mean ascent in upshear propagating outflows there is no steering level to anchor the incipient convection to the organized ascent.

Full access
Changhai Liu
and
Mitchell W. Moncrieff

Abstract

The effects of three distinct stratifications on density current dynamics are investigated using a nonhydrostatic numerical model: (i) a stably stratified layer underneath a deep neutrally stratified flow, representing a nocturnal boundary layer over land; (ii) a neutrally stratified layer underlying a deep stably stratified flow, representing a daytime boundary layer; and (iii) a continuously stratified atmosphere.

In the first case, a weak or intermediate stratification decreases the height of density currents and increases the propagation speed. The same result holds in strongly stratified situations as long as the generated disturbances in the neighborhood of the head do not propagate away. Classical density currents occur in weak stratification, multiheaded density currents in intermediate stratification, and multiheaded density currents with solitary wave–like or borelike disturbances propagating ahead of the current in strong stratification.

In the second case, the upper-layer stratification consistently reduces the density-current height and its propagation speed. The simulated system resembles laboratory density currents and is not much affected by the overlying stratification.

Finally, in continuously stratified flow, the effect of stratification is similar to the second case. The density current becomes shallower and moves more slowly as the stratification is increased. The modeled system has the basic features of density currents if the stratification is weak or moderate, but it becomes progressively less elevated as stratification increases. In strong stratification the density current assumes a wedgelike structure.

The simulation results are compared with the authors’ previously obtained analytical results, and the physical mechanisms for the effect of stratification are discussed.

Full access
Changhai Liu
,
Mitchell W. Moncrieff
, and
Edward J. Zipser

Abstract

Based on environmental conditions of the 22 and 23 June squall lines during the Convection Profonde Tropicale in 1981 (COPT81) experiment in West Africa, a series of numerical simulations are performed with a two-dimensional nonhydrostatic cloud model to examine the dynamical effect of microphysics in tropical squall lines.

The role of ice-phase microphysics strongly depends on ambient conditions. For the environment of strong convective instability, the ice phase is important regarding the system-scale structure but is not important to the convective-scale dynamics. On the other hand, the ice influence is crucial to the squall-line convective system if the environment has a weak convective instability and is almost saturated at low levels.

Full access
Changhai Liu
,
Mitchell W. Moncrieff
, and
Wojciech W. Grabowski

Abstract

Convection and cloud processes are examined in a hierarchy of two-dimensional numerical realizations of cloud systems observed during the 19–26 December 1992 period of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The hierarchy consists of cloud-resolving simulations at a 2-km resolution, and two sets of 15-km resolution simulations; one attempts to treat convection explicitly and the other parameterizes convection using the Kain–Fritsch scheme.

The Kain–Fritsch parameterization shows reasonable results but shortcomings are found in comparison with the cloud-resolving model. (i) The entraining plumes in the parameterization excessively overshoot the tropopause, which produces a cold bias mostly through adiabatic cooling. The attendant moisture detrainment overproduces cirrus cloud. (ii) Because parameterized downdrafts detrain at the lowest level they generate a surface cold bias. (iii) The scheme fails to represent the trimodal convection (cumulonimbus reaching the tropopause, cumulus congestus around the melting level, and shallow convection regimes) realized by the cloud-resolving simulation and also seen in observations. The lack of shallow convection and cumulus congestus leads to an overprediction of the low-level moisture. (iv) The simulations are sensitive to the magnitude of moisture feedback from the convective parameterization to the grid scale but less sensitive to whether the moisture is in vapor or condensed phase.

These deficiencies are mostly a consequence of the single-plume model that represents updrafts and downdrafts in the parameterization scheme, along with the lack of a shallow convection scheme. A more realistic model of entrainment and detrainment that reduces overshoot and represents the cumulus congestus is required. Realistic downdraft detrainment and relative humidity are needed to improve the downdraft parameterization and alleviate the surface temperature bias.

Full access
Changhai Liu
,
Kyoko Ikeda
,
Gregory Thompson
,
Roy Rasmussen
, and
Jimy Dudhia

Abstract

An investigation was conducted on the effects of various physics parameterizations on wintertime precipitation predictions using a high-resolution regional climate model. The objective was to evaluate the sensitivity of cold-season mountainous snowfall to cloud microphysics schemes, planetary boundary layer (PBL) schemes, land surface schemes, and radiative transfer schemes at a 4-km grid spacing applicable to the next generation of regional climate models.

The results indicated that orographically enhanced precipitation was highly sensitive to cloud microphysics parameterizations. Of the tested 7 parameterizations, 2 schemes clearly outperformed the others that overpredicted the snowfall amount by as much as ~30%–60% on the basis of snow telemetry observations. Significant differences among these schemes were apparent in domain averages, spatial distributions of hydrometeors, latent heating profiles, and cloud fields. In comparison, model results showed relatively weak dependency on the land surface, PBL, and radiation schemes, roughly in the order of decreasing level of sensitivity.

Full access
Deepak Gopalakrishnan
,
Sourav Taraphdar
,
Olivier M. Pauluis
,
Lulin Xue
,
R. S. Ajayamohan
,
Noor Al Shamsi
,
Sisi Chen
,
Jared A. Lee
,
Wojciech W. Grabowski
,
Changhai Liu
,
Sarah A. Tessendorf
, and
Roy M. Rasmussen

Abstract

This study investigates the structure and evolution of a summertime convective event that occurred on 14 July 2015 over the Arabian region. We use the WRF Model with 1-km horizontal grid spacing and test three PBL parameterizations: the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme; the Asymmetrical Convective Model, version 2, (ACM2) scheme; and the quasi-normal scale-elimination (QNSE) scheme. Convection initiates near the Al Hajar Mountains of northern Oman at around 1100 local time (LT; 0700 UTC) and propagates northwestward. A nonorographic convective band along the west coast of the United Arab Emirates (UAE) develops after 1500 LT as a result of the convergence of cold pools with the sea breeze from the Arabian Gulf. The model simulation employing the QNSE scheme simulates the convection initiation and propagation well. Although the MYNN and ACM2 simulations show convective initiation near the Al Hajar Mountains, they fail to simulate the development of the convective band along the UAE west coast. The MYNN run simulates colder near-surface temperatures and a weaker sea breeze, whereas the ACM2 run simulates a stronger sea breeze but a drier lower troposphere. Sensitivity simulations using horizontal grid spacings of 9 and 3 km show that lower-resolution runs develop broader convective structures and weaker cold pools and horizontal wind divergence, affecting the development of convection along the west coast of the UAE. The 1-km run using the QNSE PBL scheme realistically captures the sequence of events that leads to the moist convection over the UAE and adjacent mountains.

Open access