layer. If the hourly sensible heat, H, is known, the mixed

height z at time t+1 can be estimated from time t in a i

stepwise manner.

The sounding at the nearest rawinsonde station to the grid cell

is used to determine the lapse rate  . 1

The daytime mechanical mixing depth is calculated from:

The nighttime mechanical mixed layer is determined from:

Examples of a daytime and nighttime MESOPUFF-II generated

mixing depth field are shown in Figures 4a and 4b. The daytime

mixing depths generated by MESOPUFF-II exhibit some extreme

discontinuities from one grid cell to the next. This is a

result of using only the nearest rawinsonde station sounding to

calculate the convective mixing depth, rather than an

interpolated field. The nighttime mixing depths make a much

smoother transition from one cell to the next. The daytime

mixed depth values range from 552 meters to 1759 meters,

whereas the nighttime mixed depth values range from 10 meters

to 1360 meters.

In the ARM3 system, the daytime convective mixed depths

are determined as the height of the intersection of the hourly

surface potential temperature and the morning potential

temperature sounding. Cold or warm air advection is accounted

for by adjusting the hourly surface potential temperature

values according to an advection rate. The advection rate is

determined from the difference in potential temperature between

Hmech

53UFree Stream

where: UFree Stream is taken to be the wind speed

at 3000 meters

3-22

Figure 5a - ARM3 Mixing

Heights for July 21, 1984 1200

LST.

Figure 5b - ARM3 Mixing

Heights for July 21, 1984

0000 LST.

the afternoon and morning sounding at a height above the

convective mixing height.

The ARM3 mechanical mixing depth is the same for both

daytime and nighttime conditions

 

 

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