Locally applicable information about climate and soil properties can help farmers identify opportunities and reduce risks associated with changing to new land uses. This article describes techniques for preparing high-resolution regional maps and GIS surfaces of agriculturally relevant climate Parameters. Ways of combining these climate surfaces with soil data and information about the physical requirements of crops to identify areas likely to be the most suitable for new high-value crops are then outlined. Innovative features include methods for merging observations from temporary climate stations installed for one to two years in conjunction with longer-term climate station observations to improve input data for the maps, and techniques for mapping quantiles of climatic factors that may constrain agricultural operations. Examples are the expected 'one-in-five year' first and last frost dates, and the 'one-in-five year' lowest and highest seasonal rainfalls. The use of night-time satellite infrared observations to improve spatial resolution of frost hazard maps is also described. Typical standard errors of these climate mapping techniques are summarised. The benefits of ongoing consultation with local farmers and local government staff during the design and implementation of climate/soil/crop potential studies are described. These include optimising products to meet local needs, quality control of the resulting maps and GIS surfaces through local knowledge, and improved uptake of information by users. Further applications of techniques described in this paper include products useful to the energy sector, preparation of daily gridded climate data estimates for use in water quality and plant growth modelling, and development of regional climate change scenarios.

We use new data from the SALPEX'96 campaign to examine the evolution of orographically modified flow over and around New Zealand's Southern Alps. We consider prefrontal situations for which the larger-scale synoptic pattern generates an incoming northwesterly flow perpendicular to the mountain axis, calculate the nondimensional parameters which control the wind flow and determine how long it takes orographic modifications to become established. We place flows over the New Zealand Alps in a Rossby number/Blocking number parameter space and compare them with flows over other mountain ranges on the globe. Using model and aircraft data, we show the region of modified flow extends over 100 kin upstream of the Southern Alps.We show 2D model simulations represent many aspects of the flow modification satisfactorily, although they tend to overdevelop the final state of the Barrier Jet flow. In 3D, using real data to force the boundaries, the migratory nature of the weather systems in the area would generally limit the time available for a modified steady state to develop. In addition, in situations where there is significant lateral variation of initial fields, the 2D results may only be applicable for a few hours.From the point of view of potential to influence alpine rainfall processes, for the case studied, the gradual uplift associated with the modified flow between I and 2 kin above sea level extended far enough upstream in saturated air to enable cloud drops to grow to drizzle drops before reaching the much more substantial updrafts at the foot of the Alps. This allows vigorous prefrontal precipitation to occur over the foothills and mountains due largely to warm rain processes. (C) 2002 Elsevier Science B.V. All rights reserved.

This paper describes a “top-down” approach for estimating regional surface fluxes of methane, and its application to a pastoral farming region in New Zealand. The approach is based on air sampling from aircraft and interpretation by mesoscale dispersion modelling. The goal is an independent cross-check for an agricultural region of “bottom-up” emission estimation methods like those used for inventory reporting under the United Nations Framework Convention on Climate Change (UNFCCC). The “top-down” strategy infers emissions over an agricultural region from differences between methane concentrations measured upwind and downwind of the region. The approach was trialed over the Manawatu agricultural region of New Zealand's North Island. As expected, measured concentration differences were largest at low wind speeds (2–3ms−1 ). However, during these low wind conditions the concentration differences could not be reliably inverted to provide emission estimates, because of the complex variability in air flow caused by topography and land–sea temperature contrasts. Useful emission estimates were obtained during days with higher wind speed (about 8ms−1), as the strong synoptic-scale flow then suppressed the development of complex local flows. The upwind–downwind concentration differences were smaller during these conditions of stronger flow, so that precision limits to concentration measurements became significant. Methane fluxes (in the range of 20–100mgm−2d−1) calculated under the stronger wind speed conditions were consistent with “bottom-up” estimates scaled from per-animal emission factors, enhancing confidence in the inventory-reporting methodology.

The most direct way to establish the level of surface emissions of greenhouse gases is to measure and interpret concentration gradients in the atmosphere. We have tested the efficacy of this approach for inferring average methane fluxes from regions of pastoral agriculture a few tens of km in extent In its simplest form, vertical concentration profiles are measured upwind and downwind of the target region, based on air samples collected from light aircraft. Using simple mass balance models, the profile contrasts can be related to the mean surface flux over the intervening region. The inferred flux can then be compared with 'bottom-up' estimates based on livestock density and per-animal emissions. However, such simple models may poorly simulate air flows over the New Zealand terrain, and as an alternative, we deploy a state-of-the-art mesoscale meteorological model, RAMS, coupled to an atmospheric dispersion model. RAMS is used prognostically to guide the timing and siting of measurement campaigns, and diagnostically to simulate regional wind fields which are validated against local meteorological data. Source-oriented and receptor-oriented dispersion modelling techniques, in combination with aircraft-based sampling and laboratory gas analysis, provide 'top-down' methane flux estimates that compare favourably with 'bottom-up' estimates. These techniques thus enhance confidence in national emission inventories based on bottom-up estimation. However, the challenge for similar verification of nitrous oxide emission is more imposing.

The most direct way to establish the level of surface emissions of greenhouse gases is to measure and interpret concentration gradients in the atmosphere. We have tested the efficacy of this approach for inferring average methane fluxes from regions of pastoral agriculture a few tens of km in extent In its simplest form, vertical concentration profiles are measured upwind and downwind of the target region, based on air samples collected from light aircraft. Using simple mass balance models, the profile contrasts can be related to the mean surface flux over the intervening region. The inferred flux can then be compared with ‘bottom-up’ estimates based on livestock density and per-animal emissions. However, such simple models may poorly simulate air flows over the New Zealand terrain, and as an alternative, we deploy a state-of-the-art mesoscale meteorological model, RAMS, coupled to an atmospheric dispersion model. RAMS is used prognostically to guide the timing and siting of measurement campaigns, and diagnostically to simulate regional wind fields which are validated against local meteorological data. Source-oriented and receptor-oriented dispersion modelling techniques, in combination with aircraft-based sampling and laboratory gas analysis, provide ‘top-down’ methane flux estimates that compare favourably with ‘bottom-up’ estimates. These techniques thus enhance confidence in national emission inventories based on bottom-up estimation. However, the challenge for similar verification of nitrous oxide emission is more imposing.

This paper identifies relationships between air mass properties and mesoscale rainfall when moist air blows over New Zealand's Southern Alps from the Tasman Sea. Around 50% of the variance in six-hourly rain volumes summed across three separate cross-mountain raingauge transects and in six-hourly rain volume spilling across the alpine divide are statistically explained by the following properties of the approaching air mass: relative humidity, wind velocity normal to the mountains, air mass stability and synoptically induced upward motion. These factors also explain about 25% (r≈0.5) of the variance in the downwind distance reached by the spillover rainfall. For the highest 10% of six-hourly rainfalls, spillover distance and magnitude are negatively correlated with the 700 or 500 hPa temperature. Multiple linear regression equations suitable for predicting rainfall intensity and spillover are developed. A progression is described in the magnitude and depth of vertical motion and resulting condensation rates over the mountains as the properties of the incoming air mass evolve through a storm. These changes, together with greater downwind advection of ice particles compared to raindrops, explain the observed statistical relationships between the air mass properties and mountain rainfall.

During October 1996 a series of intensive meteorological measurements were made along the 500km length of the Southern Alps of New Zealand. These measurements were made to investigate the physical processes responsible for producing heavy rainfall and to evaluate the performance of a meso-scale weather forecast model. A by-product of the work was sets of simulated hourly rainfalls over the entire mountain range. Rainfall estimates were made every 24h on a 20×20km resolution grid covering the whole of New Zealand. Successive sets of 24-hourly values were combined to produce a continuous 29-day sequence of model generated rainfalls. The study region covers remote mountain river basins in which there are few rainfall data but over 20 continuously recording river flow monitoring stations. For many of the basins, the runoff rate is an order of magnitude larger than the potential evaporation rate. To a first approximation, and over time periods of days, the river catchments act like large rain gauges. For each basin a rainfall–runoff model was built using the Topmodel assumptions that saturated hydraulic conductivity decreases exponentially with depth from the ground surface, the hydraulic gradient of the saturated zone is equal to the topographic gradient, and subsurface recharge is uniform. As the water table rises in response to rainfall over each sub-basin, increasing amounts of the ground surface become saturated, and rainfall falling directly onto these saturated areas generates much of the storm runoff. Results are presented for basins ranging in area from 12 to 3830km2 and that lie on both the windward and leeward sides of the mountain range.

A series of numerical experiments was made to search for the basic features of diurnal wind variation in the lee of a mountain range using a two-dimensional dry version of the MRI non-hydrostatic model. The features of diurnal wind variation were classified into four regimes according to prevailing wind speed with respect to the characteristic behavior of the lee convergence zone (LCZ). Under moderate prevailing wind, the LCZ propagates downwind far into the leeside plain during the daytime accompanied by a surge of strong wind. This is in qualitative agreement with the daytime advance of downslope wind observed in the Canterbury Plains in New Zealand, and the Kanto plain in Japan.

Rain gauge, radar, and atmospheric observations during a prolonged northwesterly storm in November 1994 have been used to study factors influencing the distribution of precipitation across the Southern Alps, Despite the persistent northwesterly flow, the location and intensity of precipitation varied markedly during this storm, providing an excellent dataset for these investigations. Data from 36 recording gauges in the northern half of the Alps were supplemented by data from 57 daily gauges, which were partitioned into 6-h values. These data were grouped according to distance from the alpine divide, and best-fit transect curves, normalized for rainfall intensity, were established every 6 h. The fraction of the total transect precipitation falling in leeside catchments varied between 0.11 and 0.70, while a ''spillover distance'' index varied between 6 and 29 km. Comparison with atmospheric profiles of temperature and wind from Hokitika on the west coast of New Zealand and with European Centre for Medium-Range Weather Forecasts analyses revealed that precipitation was confined upwind of the divide during a period of blocked flow near the start of the storm, and only extended into leeside catchments with the onset of stronger flow and reduced static stability. Regression equations involving these factors explained up to 93% of the spillover variations. It is suggested that ascent and precipitation maxima an shifted upstream during blocked flow, while spillover is enhanced during stronger and/or unstable flow as the upstream influence lessens and snow and ice particles drift farther downwind before falling below the freezing level. Further case and modeling studies are needed to demonstrate the wider applicability of these findings.

The Southern Alps Experiment is being mounted to study the influence of New Zealand's Southern Alps on local weather and climate. This paper describes these alpine influences and outlines proposed field and modeling experiments. Experiment goals include understanding and quantifying factors that govern the intensity and spatial distribution of heavy rainfall, the west to east distribution of precipitation across the mountains, and the intensity of lee wind storms and warming. Linked research will explore the use of deterministic rainfall models to predict river flows from mountain watersheds.