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Jack J. Katzfey

Abstract

The extreme precipitation events with peak observed rainfall of greater than 700 mm over the South Island of New Zealand were simulated using the DAR hydrostatic mesoscale model nested within the ECMWF analyses. The ECMWF analyses for two of the events showed a low-level jet with mixing ratios greater than 12 g kg−1 crossing the South Island of New Zealand during the heavy precipitation near a cold front. The third case, which had smaller mixing ratios, occurred as a low-level jet and crossed the South Island while a low redeveloped downstream.

Three different orographies were used with the 30-km horizontal resolution model runs, with progressively increased terrain heights. The highest orography was created by artificially inserting the effective barrier of the Southern Alps to northwesterly flow in the model grid. Orography had a strong influence on the amount of precipitation: the peak precipitation was related to orographic slope while the area-averaged precipitation was related to the maximum orographic elevation. The model successfully simulated nearly half the peak observed precipitation and over half the area-averaged precipitation (determined by hydrological means) in two of the cases and much less in the third case. Refining the horizontal resolution from 30 to 15 km also increased the peak precipitation amounts. However, the area-averaged precipitation in the 15-km runs was not significantly larger than in the 30-km runs, suggesting more concentrated precipitation over a smaller area.

All simulations, except the artificial barrier orography case, produced a mountain wave consistent with linear theory, in spite of the nonsteady flow, irregular orography, and the large amount of diabatic heating present. The amplitude of the mountain wave increased with mountain height and resolution. The absence of a mountain wave in the run with the artificial barrier orography indicates unrealistic flow for that configuration.

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Jack J. Katzfey

Abstract

The extreme precipitation event that occurred on 27 December 1989 over the South Island of New Zealand was simulated using the DAR hydrostatic mesoscale model nested within the ECMWF analyses. The model simulated nearly half of the peak observed rainfall for this storm (greater than 700 mm) and captured the location and timing of the intense precipitation.

The heavy precipitation developed while a deep layer of moist subtropical air along a cold front ascended the high terrain of the South Island. The intense orographic ascent was associated with a low-level jet core with wind speeds of over 20 m s−1 ahead of the cold front. An upper-level trough and jet streak entrance region were also present upstream of the South Island during the event, aiding the ascent over the mountains and deepening the layer of moist air. The air crossing the mountain was nearly saturated throughout the troposphere and had only weak moist vertical stability near the cold front. Almost all of the simulated precipitation formed in the low troposphere through forced ascent, with only minimal convection behind the cold front.

Two sensitivity experiments were conducted to investigate the effects of orography and latent heating on the development of precipitation in the simulations. Weak upstream blocking by the orography was present, enhancing the ascent upstream and causing a slight moistening of the midtroposphere. The latent heat, maximized near the surface on the upwind side of the mountain, caused increased upward motion and precipitation over the orography and decreased ascent upstream, tending to dry and stabilize the air there. The latent heat release weakened the blocking effect of the orography and altered the mountain wave through reduced effective dry static stability.

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Isidoro Orlanski
and
Jack J. Katzfey

Abstract

A nested global, limited-area model was used to predict the President's Day cyclone of 18-19 February 1979. Both a low (∼150 km) and a high (∼50 km) horizontal resolution version were used. The model has full physics with a planetary boundary layer, moisture, moist convective adjustment, and radiation.

The low-resolution model using a global analysis for initial and boundary conditions (termed a simulation), was able to capture the general development and movement of the cyclone. Some discrepancies were noted for the intensity of upper-air features between the analyses and the model solution during the first 24 hours. The primary focus of this paper is to determine the effect of initial and boundary conditions, as well as model parameterizations on the accuracy of the predictions. The evolution of the storm is discussed with an emphasis on the quality of the numerical simulation.

The impact of the initial conditions on the model solution was tested by using four different global analyses. It was found that the variability between the solutions was less than the variability between the analyses. Varying the horizontal diffusion in the model produced stronger development with weaker diffusion, but the character of the development did not change significantly. The sensitivity of the simulation to latent heat was tested by running the model without latent heating. A low did develop in this model solution, although it was much weaker and it did not develop vertically as in the cases with latent heating.

The most significant improvement in accuracy in this sensitivity study occurred when the horizontal resolution was increased from 1.25° × 1.0° (∼150 km) to 0.4° × 0.32° (∼50 km). The position and intensity of the surface low were much close to reality, as indicated by comparison with a mesoanalysis and to satellite pictures.

The nested model was also run in forecast mode with boundary conditions for the limited-area model supplied by the (Geophysical Fluid Dynamics Laboratory) GFDL global model forecast. In general, the quality of the limited-area forecast compared very well with the simulations. The overall character and intensity of the development were similar.

The role of lateral boundary conditions was demonstrated by comparing forecasts and simulations with identical initial conditions. The results suggest the increasing importance of the boundary data with time in the limited-area forecast and show high correlation between the errors in the limited-area forecast and the global forecast within the limited-area domain.

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Leon D. Rotstayn
,
Brian F. Ryan
, and
Jack J. Katzfey

Abstract

A scheme for calculation of the liquid fraction f l in mixed-phase stratiform clouds has been developed for use in large-scale models. An advantage of the scheme, compared to the interpolation in temperature that is typically used, is that it makes it possible to simulate the life cycles of mixed-phase clouds, and the differences between deep and shallow clouds. The central part of the scheme is a physically based calculation of the growth of cloud ice crystals by vapor deposition at the expense of coexisting cloud liquid water, the so-called Bergeron–Findeisen mechanism. Versions of this calculation have been derived for three different ice-crystal habits (spheres, hexagonal plates, or columns) and for two different assumed spatial relationships of the coexisting cloud ice and liquid water (horizontally adjacent or uniformly mixed). The scheme also requires a parameterization of the ice crystal number concentration N i .

The variation with temperature of f l looks broadly realistic compared to aircraft observations taken in the vicinity of the British Isles when the scheme is used in the CSIRO GCM, if N i is parameterized using a supersaturation-dependent expression due to Meyers et al. If Fletcher’s earlier temperature-dependent expression for N i is used, the scheme generates liquid fractions that are too large. There is also considerable sensitivity to the ice-crystal habit, and some sensitivity to model horizontal resolution and to the assumed spatial relationship of the liquid water and ice. A further test shows that the liquid fractions are lower in cloud layers that are seeded from above by falling ice, than in layers that are not seeded in this way.

The scheme has also been tested in limited-area model simulations of a frontal system over southeastern Australia. Supercooled liquid water forms initially in the updraft, but mature parts of the cloud are mostly glaciated down to the melting level. This behavior, which is considered to be realistic based on observations of similar cloud systems, is not captured by a conventional temperature-dependent parameterization of f l . The variation with temperature of f l shows a marked sensitivity to the assumed spatial relationship of the liquid water and ice. The results obtained using the uniformly mixed assumption are considered to be more realistic than those obtained using the horizontally adjacent assumption. There is also much less sensitivity to the time step when the former assumption is used.

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