Hi all,

I am having some trouble to understand how WRF computes the pressure perturbation.

I ran a basic simulation using the "2D flow over hill" idealized case, I changed the terrain function in the fortran code in order to obtain a 3D circular mountain (I did also some other small modifications to the default input sounding (which is "no moisture" anyway)). I used open lateral boundary conditions (open_xs, open_xe, open_ys, open_ye) and a sponge layer at half-height of domain, also, I switched off every type of ground-atmosphere interaction (PBL, heat fluxes etc..).

At the end of this message you can find the namelist that I used.

The aim is to visualize only mountain waves and the related perturbations on the background state.

The results that I get are pretty good for all the wave perturbations (w, u, potential temperature) except for pressure. In fact, the pressure perturbation becomes, in time, positive in everywhere within the domain.

If I run, for example, a simulation of 24 hours and I take a horizontal cross-section of the pressure pert. at k=1, t=12 I cannot see: positive pressure pert. along the windward side of the mountain and negative pressure pert. along the lee side (as in the first hours of simulation (for t=1:10) ), but only positive pressure even if the dipole (increased-decreased pressure along the two sides of the mountain) is still present.

This simulation is very basic, and there isn't anything in the domain that can be responsible of a changing in pressure, if no waves. Therefore... what is going on in the domain?

More specifically my question is: when I visualize the perturbation of pressure p this value is the sum of a perturbation due to waves pw (the dipole is still present) plus something else puknw. What is this something else? and... am I right to worry about this?

p=pw+puknw

In the end: I am sure this problem is related to the boundary conditions used. If I convert open BC in periodic BC I haven't this problem any more (positive pressure on windward side and negative pressure on lee side for all the time of simulation), so.. it looks like a problem of mass conservation... but could it be??

Furthermore, I tried to run the default WRF simulation for a 2Dhill idealized case (with periodic BC in y and open BC in x) and my simulation using these same boundary conditions, but in both simulations (my and default) the pressure becomes negative (this time!) in everywhere within the domain, in time... ).

If someone who knows very well how WRF works, can explain me what is going on in the domain during the pressure perturbation calculation... I'll be grateful .

Thank you very much,

Namelist employed:

&time_control

run_days = 0,

run_hours = 24,

run_minutes = 00,

run_seconds = 0,

start_year = 0001,

start_month = 01,

start_day = 01,

start_hour = 00,

start_minute = 00,

start_second = 00,

end_year = 0001,

end_month = 01,

end_day = 01,

end_hour = 24,

end_minute = 00,

end_second = 00,

history_interval = 60,

frames_per_outfile = 1000,

restart = .false.,

restart_interval = 0,

io_form_history = 2

io_form_restart = 2

io_form_input = 2

io_form_boundary = 2

debug_level = 0

/

&domains

time_step = 20,

time_step_fract_num = 0,

time_step_fract_den = 1,

max_dom = 1,

s_we = 1,

e_we = 100,

s_sn = 1,

e_sn = 100,

s_vert = 1,

e_vert = 200,

dx = 2000,

dy = 2000,

ztop = 20000,

hypsometric_opt = 1,

numtiles = 8,

/

&physics

mp_physics = 0,

ra_lw_physics = 0,

ra_sw_physics = 0,

radt = 0,

sf_sfclay_physics = 0,

sf_surface_physics = 0,

bl_pbl_physics = 0,

bldt = 0,

cu_physics = 0,

cudt = 0,

isfflx = 0,

/

&fdda

/

&dynamics

rk_ord = 3,

diff_opt = 0,

km_opt = 1,

damp_opt = 0,

zdamp = 10000.,

dampcoef = 0.003,

khdif = 00,

kvdif = 0,

smdiv = 0.1,

emdiv = 0.01,

epssm = 0.1,

time_step_sound = 6,

h_mom_adv_order = 5,

v_mom_adv_order = 3,

h_sca_adv_order = 5,

v_sca_adv_order = 3,

non_hydrostatic = .true.,

pert_coriolis = .false.,

/

&bdy_control

periodic_x = .false.,

symmetric_xs = .false.,

symmetric_xe = .false.,

open_xs = .true.,

open_xe = .true.,

periodic_y = .false.,

symmetric_ys = .false.,

symmetric_ye = .false.,

open_ys = .true.,

open_ye = .true.,

/

&grib2

/

&namelist_quilt

nio_tasks_per_group = 0,

nio_groups = 1,

/