STUDY OF MOIST FLOWS OVER OBSTACLES
The effects of humidity and condensation on flow patterns is receiving a growing attention in the literature. The study of the effect of moisture over 3-D idealized stratified flows has been undertaken in a more systematic way using simply shaped mountains and simple upstream flows. The presence of moisture can determine the characteristics of the orographically modified flow, effectively changing the flow regime with respect to the dry case with the same upstream wind and dry stability profiles: this change can be related to latent heat release and explained, at least to some extent, as a consequence of the strong reduction of the effective stratification in cloudy regions and, in cases of persistent precipitation into dry air, of low-level evaporative cooling.Recently, the existence of different flow regimes near obstacles and the transitions among them have been examined with the BOLAM model. The effect of an advective change of the humidity content inside a channel was analysed: a dependence on the history of the flow was apparent. A comparison among cases of obstacles with the same aspect ratio and H=N h_max/U value (N= Brunt-Vaisala frequency, h_max= maximum mountain height, U= ambient wind speed), but with different horizontal cross sections, highlighted the importance of the obstacle geometry in perturbing the upstream flow, favouring the persistence of upstream blocked regions, in the case of an arc-shaped mountain (figure 1).
Figure 1: Advective solution, Gaussian mountain with arc-shaped horizontal section, H=2,
after 60 h: horizontal wind vectors and wind speed (in m/s) at 10 m. The horizontal
section of the mountain corresponding to h_max/3 is represented by the thick curve
near the centre of the domain.
However, there is still a real lack of knowledge of how latent-heat release affects orographically modified flow, particularly as the latter affects precipitation distribution and intensity. Recent papers are beginning to shed some light on the matter, but there is much more to be done, especially in understanding the effects of moisture for flows where the length scale L of the obstacle is in the regime L ~ U/f (f is the Coriolis parameter), h_max ~ U/N. Particularly interesting is the limit of moist neutral flow (N_moist --> 0): in fact, since the 1940's, the presence of nearly moist neutral lapse rates has been noted in cases of heavy orographic rain and identified as one of the conditions favorable for orographic enhancement of rainfall, as it allows for the complete lifting of oncoming air by the obstacle.
Only very recently the first systematic study of 2D orographic flow modification under moist neutral conditions has been performed. The limit N_moist --> 0 has been explored systematically with the Weather Research and Forecast (WRF) model recently developed at NCAR. Such simulations are carried out on an idealised, bell-shaped topography. A number of problems, ranging from the definition of a moist stability consistent with the model equations and the accuracy requirements for its computation to the odd behavior of the standard Kessler parameterization have been solved.
The responses can be summarized as falling into three categories:
1. Small-mountain regime (h_max = 0 - 250 m): If a mountain is small enough, it is always possible to add a small amount of initial cloud water to prevent the appearance of unsaturated regions. The numerical solutions are well described by the linear theory using the moist stability in place of the dry stability (figure 2). A novel feature of the latter is that the moist tropospheric stability used here is much smaller than that of the stratosphere so that wave reflection off the tropopause plays a prominent role;
Figure 2: Selected fields at t = 10h from a numerical simulation
of moist nearly neutral flow past a small (h_max = 50 m) obstacle:
a) vertical velocity w (c.i.= 0.0050 m/s), b) horizontal velocity
perturbation u (c.i.= 0.02 m/s). Results are shown only on a
display window 0 < z < 15 km and -120 km < x < +120 km.
An analytical linear solution is shown in the insets of each figure.
2. Intermediate-mountain regime (h_max = 250 - 1500 m): The flow in this regime is characterized by an upwind-propagating disturbance which desaturates the initial sounding (figure 3). The presence of this unsaturated layer of high stability at midlevel produces a flow consistent with that expected in the case of a 2-layer dry troposphere with a neutral layer surmounted by a stable layer. Experiments done in the course of the study show that for even smaller amounts of initial cloud water mixing ratio qc the upwind propagating signal can occur for smaller mountains.
Figure 3: Cloud water mixing ratio at t = 5h from a numerical simulation
of moist nearly neutral flow past a h_max = 700m obstacle (initial qc = 0.05 g/kg):
qc < 0.01g/kg (white), 0.01g/kg < qc < 0.1g/kg (green), 0.1g/kg < qc < 0.5g/kg
(cyan), and qc > 0.5g/kg (pink). Results are shown only on a display
window 0 < z < 15 km and -120 km < x < +120 km.
3. High-mountain regime (h_max > 1500 m): For tall mountains, the numerical solutions indicate that the upwind atmosphere remains saturated; in the unsaturated lower troposphere downstream, features associated with downslope windstorms characterize the solution (figure 4); convective cells form as a consequence of locally generated zones of moist instability.
Figure 4: Cloud water mixing ratio at t=19h from a numerical simulation
of moist nearly neutral flow past a h_max = 2000m obstacle (initial qc = 0.05 g/kg):
qc < 0.01g/kg (white), 0.01g/kg < qc < 0.1g/kg (green), 0.1g/kg < qc < 0.5g/kg
(yellow), and qc > 0.5g/kg (pink). Results are shown only
on a display window 0 < z < 15 km and -120 km < x < +120 km.
The thick contours inclose the regions where the rain water mixing ratio
qr > 0.2 g/kg.