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.

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.

Abstract

The ability of the CSIRO-9 General Circulation Model (GCM) to capture surface cutoff lows over eastern Australia is investigated by comparing composites of ten GCM cases with ten observed lows. The lows are also studied individually to compare their development and movement, as well as synoptic features, which may have been smoothed out in the compositing process. Finally, the incidence of all such lows in the 1 × CO₂ and 2 × CO₂ simulations are examined to determine the possible effects a doubling of CO₂ will have on their occurrence.

The GCM surface lows were found to develop from an upper-level cutoff low in a manner similar to the observed lows. In both sets, this development took place between the upper-level subtropical and polar jets in all seasons except summer, where only one jet was evident. Latent heat release appeared to play an important role in the intensification of the surface lows. The main difference between the two sets of cutoff lows was that the GCM surface lows tended to develop farther to the east of the upper-level cutoff, the upper-level features were less intense and occlusion did not take place. As a result, the GCM lows had a greater eastward translation compared to the observed lows, which often meander along the east coast for several days while they intensify. These features appear to be related to the low resolution of the GCM.

The frequency of east Australian cutoff lows was underpredicted in the WM by about 45% in the 1 × CO₂ simulation, with the greatest underprediction occurring in autumn and winter. Analysis of upper-level jet structure indicated that the GCM produced a poor simulation of the dual jet structure aloft, which may account for this problem. The 2 × CO₂ simulation produced even fewer cutoff lows over eastern Australia. This was probably caused by the reduced baroclinicity due to increased warming of polar regions, which resulted in an even weaker dual jet structure. The cast Australian cutoff lows were found to be more intense in the 2 × CO₂ Simulation, suggesting the greater role played by latent heat effects once development has been initiated.

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.

Abstract

A regional climate model (DARLAM) is implemented over the Australian region and a 20-yr seasonally varying simulation is examined for the presence of tropical cyclone–like vortices (TCLVs). The horizontal resolution of the model is 125 km with nine vertical levels and is forced at its boundaries by the output of the Commonwealth Scientific and Industrial Research Organisation GCM using a mixed layer (or “slab”) ocean. Additional simulations are performed with a horizontal resolution of 30 km and with 18 vertical levels to examine the impact of increasing resolution on storm intensity. A sample of TCLVs from the 125-km resolution simulation is simulated at 30-km resolution to determine whether they reach observed tropical storm intensity at the finer resolution. It is found that stronger vortices in the 125-km resolution simulation are more likely to intensify when simulated at finer resolution than weaker vortices. In this way, a detection threshold for vortices generated in the 125-km resolution simulation is established and then used to detect TCLVs in that simulation. The regional climate model DARLAM provides a good simulation of both cyclogenesis and its seasonal variation under the current climate. The response of the model under enhanced greenhouse conditions is studied. Under 2 × CO₂ conditions, there is no statistically significant change in regions of formation of TCLVs, with only a slight southward shift simulated. Nevertheless, there are statistically significant effects on the poleward movement of TCLVs, with storms generally tending to travel farther poleward in a warmer climate once they have formed. An analysis is undertaken to determine the reasons for this behavior. While the dynamical constraints on the maintenance of TCLV intensity under 2 × CO₂ conditions (e.g., vertical wind shear) are similar to those in the current climate, thermodynamic conditions (e.g., sea surface temperatures) are quite different and are likely to be at least partly the cause of this effect. Other causes include the combination of the slight southward shift in formation and a tendency for TCLV tracks to be more southward in enhanced greenhouse conditions, a consequence of more southward steering winds.

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.

Abstract

The authors evaluate the performance of current regional models in an intercomparison project for a case of explosive secondary marine cyclogenesis occurring during the Canadian Atlantic Storms Project and the Genesis of Atlantic Lows Experiment of 1986. Several systematic errors are found that have been identified in the refereed literature in prior years. There is a high (low) sea level pressure bias and a cold (warm) tropospheric temperature error in the oceanic (continental) regions. Though individual model participants produce central pressures of the secondary cyclone close to the observed during the final stages of its life cycle, systematically weak systems are simulated during the critical early stages of the cyclogenesis. Additionally, the simulations produce an excessively weak (strong) continental anticyclone (cyclone); implications of these errors are discussed in terms of the secondary cyclogenesis. Little relationship between strong performance in predicting the mass field and skill in predicting a measurable amount of precipitation is found. The bias scores in the precipitation study indicate a tendency for all models to overforecast precipitation. Results for the measurable threshold (0.2 mm) indicate the largest gain in precipitation scores results from increasing the horizontal resolution from 100 to 50 km, with a negligible benefit occurring as a consequence of increasing the resolution from 50 to 25 km. The importance of a horizontal resolution increase from 100 to 50 km is also generally shown for the errors in the mass field. However, little improvement in the prediction of the cyclogenesis is found by increasing the horizontal resolution from 50 to 25 km.

Abstract

Two questions motivated this study: 1) Will meteorological droughts become more frequent and severe during the twenty-first century? 2) Given the projected global temperature rise, to what extent does the inclusion of temperature (in addition to precipitation) in drought indicators play a role in future meteorological droughts? To answer, we analyzed the changes in drought frequency, severity, and historically undocumented extreme droughts over 1981–2100, using the standardized precipitation index (SPI; including precipitation only) and standardized precipitation-evapotranspiration index (SPEI; indirectly including temperature), and under two representative concentration pathways (RCP4.5 and RCP8.5). As input data, we employed 103 high-resolution (0.44°) simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX), based on a combination of 16 global circulation models (GCMs) and 20 regional circulation models (RCMs). This is the first study on global drought projections including RCMs based on such a large ensemble of RCMs. Based on precipitation only, ~15% of the global land is likely to experience more frequent and severe droughts during 2071–2100 versus 1981–2010 for both scenarios. This increase is larger (~47% under RCP4.5, ~49% under RCP8.5) when precipitation and temperature are used. Both SPI and SPEI project more frequent and severe droughts, especially under RCP8.5, over southern South America, the Mediterranean region, southern Africa, southeastern China, Japan, and southern Australia. A decrease in drought is projected for high latitudes in Northern Hemisphere and Southeast Asia. If temperature is included, drought characteristics are projected to increase over North America, Amazonia, central Europe and Asia, the Horn of Africa, India, and central Australia; if only precipitation is considered, they are found to decrease over those areas.