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3. Results
Fig. 2 shows the analysis of sensitivity of summer (JJA)
time-averaged precipitation to a moderate perturbation of the
threshold vertical velocity in the trigger function of the
Kain-Fritch deep convection scheme. This parameter is
perturbed from its standard value of 3.4 cm/s to 4.8 cm/s. In
Fig. 2 the noise standard deviation (a), the signal (b) and the
level of rejection of H 0 (c), all computed for summer
precipitation are shown. Results obtained with the large
domain are on the left and those with the small domain on
the right.
It can be seen that internal variability noise standard
deviation in the large domain (Fig. 2a, left) exhibit very
large values that locally reach 2 mm/day. In the same time,
in response to the parameter perturbation (Fig. 2b, left),
there is a decrease of precipitation over the continent and the
southeast portion of the domain, comparable to the noise
level. The signal is scarcely distinguishable from internal
variability since the level of rejection of H 0 is predominantly
below 90% (Fig. 2c, left).
Clearly, more simulations with perturbed initial conditions
are necessary to provide statistical significance in the large
domain. Some more confidence can be attributed to the
signal pattern over the continent, since a perturbation of
the same parameter and magnitude but of the opposite sign
produces a similar pattern of model response but with the
opposite sign (not shown). This indicates a strong linear
component of the sensitivity to the given parameter. It is
also worth noting that the internal variability exhibits a
pronounced annual cycle, with maximum in summer and
minimum in winter. Hence, the rejection level of H 0 is
larger in the remaining seasons. However, the magnitude
and the pattern of the signal also considerably vary
through seasons.
Reduction of domain size decreases the noise
magnitude roughly by factor of 3 (Fig. 2a, right). In the
same time, comparison of Figs. 2b left and right shows no
evidence that the sensitivity of summer precipitation is
reduced. One exception is the area of the positive
sensitivity off the coast of Virginia. Over land the absolute
change in precipitation is locally even larger in the smaller
domain. This is reflected in a substantial increase in
statistical significance of the signal: the level of rejection
of H 0 becomes larger than 99% over a considerable part of
the area of comparison (Fig. 2c, right).
4. Discussion and concluding remarks
It is clear that reduction in domain size decreases the
computational cost of the PPE, both directly through
reduction of the number of computational points and
indirectly – reducing the number of identical simulations
with perturbed initial conditions needed to provide the
statistical significance of the signal. Given that the
computational resources are usually limited by external
funding and thus non-negotiable, the lowering of noise is
an important advantage of the small domain because it
allows allocation of resources to the PPE size (e.g., more
parameters and perturbations can be included in the
study). Furthermore, noise can render very difficult the
quantification of non-linear components of the model
response to simultaneous multiple-parameter
perturbations.
There are, however, two important disadvantages of the
small domain to be noted. Firstly, the results obtained in
the small domain are representative for a small region and
are of little value for typical RCM domains to which this
study is addressed. Secondly, it can be argued that in a too
small domain, control of the large-scale interior flow by
the LBC may be excessive and suppress the signal.
Suppression of the model sensitivity to parameter
perturbations would decrease spread among the members
of the PPE and yield an underestimation of the uncertainty
range with respect to that in typical RCM simulations. Our
results provide no evidence that this happens with seasonal
precipitation but the concern still remains when less smallscale
dominated variables are considered.
References
Figure 2. Summer seasonal-average precipitation
(left – large domain, right – small domain): (a)
internal variability noise standard deviation (mm/day);
(b) change in response to positive perturbation of the
threshold vertical velocity in the Kain-Fritch deep
convection (mm/day), and (c) the statistical
significance of the change (%).
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