Colorado State University Mesoscale Model Research Articles

A three-dimensional urban canopy model for mesoscale atmospheric simulations and its comparison with a two-dimensional urban canopy model in an idealized case

Most urban canopy models adopt the two-dimensional (infinitely long) street canyon as the basic unit to represent the city texture. This paper presents a new urban canopy model, the 3DUCM (three-dimensional urban canopy model), which accounts for every single building of the city to determine the air temperature and heat fluxes involved in the surface energy budget. 3DUCM has been coupled with the Colorado State University Mesoscale Model, and an idealized city has been considered as case study. Among the main results, it is shown that the air temperature simulated by 3DUCM within the canopy is lower compared to that calculated using the two-dimensional approach, mainly because of crossroads modelling. The new model is suitable for predicting the temperature field down to the building scale and it is potentially easy to apply to realistic scenarios, for which the actual shape, orientation and constructing material properties of every single building constituting the canopy can be taken into account.

Influence of the synoptic circulation on the local wind field in a coastal area of the Tyrrhenian Sea

In this paper the statistical analysis of the wind field measured by a Doppler sodar is used to characterize the local circulation at the Tyrrhenian sea coastal site of the Pratica di Mare airfield. Daily behaviour of the wind direction and intensity, as a function of the season, is highlighted and the existence of two nocturnal alternative components of the local circulation is evidenced. Rawinsonde data were used to determine geostrophic wind values. The comparison between low level and geostrophic wind directions allowed to establish the influence of the synoptic scale forcing in determining both which of the two nocturnal currents can be observed at the site and how their directions evolve during the day. Numerical simulations carried out by using the Colorado State University Mesoscale Model (CSUMM) confirm the statistical analysis results and allow to depict the low level wind field patterns in the area. The use of data from ECMWF analysis is proposed in cases in which rawinsonde data are not available to enquire on the synoptic scale forcing.

Improvement of urban thermal environment by managing heat discharge sources and surface modification in Tokyo

This paper analyzes the implications of anthropogenic heat discharges into the urban thermal environment of Tokyo. Heat discharges by the representative office, commercial and residential buildings were simulated with the help of the DOE-2 building energy simulation model. The approach used in this paper also takes into account the heat storage within building structures. The geographical information system based technique was used to estimate the heat discharge distribution all over Tokyo. The mesoscale analyses of the urban climate were carried out with a model that was based on the Colorado State University Mesoscale Model. The improvements in the urban thermal environment via the various measures were analyzed for two types of scenarios, namely, scenarios related to the management of heat discharge sources and urban surface modifications. The maximum improvement in average temperature for daytime was found to be 0.47°C (at noon) as a result of greening the areas around the buildings of Tokyo. Similarly, the maximum improvement in average temperature for the evening was found to be 0.11°C by discharging all heat to the ground.

エネルギー消費からみた都市内土地利用配置の評価

This report evaluates five land use patterns with respect to energy consumption. Two types of energy consumption are considered: one depends on the atmospheric temperature, that is, energy consumed by air-conditioning and hot water supply systems; and the other is that consumed by transportation systems. Energy consumption by air-conditioning and hot water supply systems is estimated using a mesoscale meteorological model called the modified Colorado State University Mesoscale Model (CSUMM). Calculation of energy consumption by transportation systems is based on two types of trip behavior models. Comparison of energy consumption shows that the mixture of land use reduces energy consumption, and that excessive concentration of urban functions is not desirable with respect to energy consumption.

Modifying a Mesoscale Meteorological Model to Better Incorporate Urban Heat Storage: A Bulk-Parameterization Approach

Abstract A simple bulk-parameterization scheme is implemented in modifying a mesoscale meteorological model to better incorporate urban heat storage. The objective is to improve the quantification of the fluxes associated with heat storage change and to more explicitly account for the integrated effect of urban canopy layer fluxes on the overlying boundary layer. The approach involves integrating an Objective Hysteresis Model with a meteorological model, which, in this case, is the Colorado State University Mesoscale Model (CSUMM). When the resulting modified version of the model (MM) is applied to the Atlanta, Georgia, region, larger daytime urban heat islands are predicted (∼1.5°C larger) than with the nonmodified version of CSUMM. The largest simulated summer daytime Atlanta heat island for the modeling episode of 25–27 July is 1.4°C with CSUMM and 2.9°C with MM. The impacts of surface modifications, such as changes in urban albedo, on air temperature are slightly larger in MM than in CSUMM.

Sea breezes and advective effects in southwest James Bay

Observations from a transect extending 100 km inland during the Northern Wetlands Study (NOWES) in 1990 show that the sea breeze develops on approximately 25% of days during summer and may penetrate up to 100 km inland on occasions. The sea breeze exhibits a marked diurnal clockwise rotation as a result of the Coriolis effect along the unobstructed coastline. The marine advective effect is shown to depend on gradient wind direction. With northwesterly upper level flow the sea breeze tends to be northeasterly in direction and is associated with decreased temperatures and vapor pressure deficits (VPD). With southwesterly upper level flow the sea breeze tends to have a southeasterly direction and less effect on temperatures and VPD. This is attributed to shorter residence times of air parcels over water. For two cases, Colorado State University mesoscale model simulations show good agreement with surface wind observations and suggest that under northwesterly gradient flow, Bowen ratios are increased in the onshore flow along western James Bay, while during southwesterly gradient flow these effects are negligible. These results have implications for the interpretation of local climate, ecology, and hydrology as well as land‐based and airborne turbulent flux measurements made during NOWES.

Observations and numerical modelling of Lake Ontario breezes

Abstract Analysis of two years of land‐based data shows that the Lake Ontario breeze develops on 30% of the days during summer. It typically develops in mid‐morning and persists until the late evening when it is replaced by a well developed land‐breeze regime. Simulations of 4 cases with the Colorado State University mesoscale model show good agreement with observations and suggest that local lake breezes are strongly influenced by adjacent water bodies (e.g. Lake Erie), the elongated shape of the lake, and the large‐scale wind direction. With gradient flows across the long axis of the lake, a broad band of along‐lake flow develops during the afternoon (easterly winds during southerly gradient flows and westerly winds during northerly gradient flows). Furthermore, during west‐to‐northwesterly gradient flow a nocturnal cyclonic eddy is predicted at the western end of the lake. These results imply that wind‐field models applied in the vicinity of Lake Ontario should incorporate the entire lake in their mode...

Numerical simulation of sea breeze interactions over the Auckland region, New Zealand

Abstract The Colorado State University mesoscale model is used to investigate the influence of gradient wind direction on migratory sea breeze convergence zones (SBCZs) that develop over an irregular coastline incorporating two narrow peninsulas. Results, which are in good qualitative agreement with observations, reveal that SBCZ dynamics are strongly influenced by the extent to which flow is either subparallel or perpendicular to the orientation of the major coastlines. The local coastal configuration is also shown to be the dominant factor controlling the location of zones of intense vertical motion associated with SBCZs. A notable aspect of the study was the simulation of a late afternoon mesoscale cyclonic eddy caused by the interaction of sea breezes during southeasterly gradient flow. Further observations are recommended to verify the existence of this phenomenon.

Numerical Investigation of Mesoscale Circulations over the Los Angeles Basin. Part I. A Verification Study

Abstract This paper describes some improvements to the Colorado State University mesoscale model and the evaluation of the model's accuracy in simulating airflows in the Los Angeles Basin. Model output was compared with observations taken on two meteorologically distinct days. The statistical method of Willmott was used to evaluate the predicted surface winds. The model was generally accurate in simulating the winds, but failed to replicate the observed southeasterly flow from Los Angeles into the San Fernando Valley. The simulated surface winds were snore accurate during the daytime than during the night, since the model did not reproduce the nocturnal winds' spatial variability. The model is a good predictor of mixing heights, which is an important variable in predicting pollutant concentrations, and successfully replicated observed daytime boundary layer averages of wind and potential temperature.

Numerical Study of the Influence of Environmental Conditions on Lake-Effect Snowstorms over Lake Michigan

Numerical simulations are used to examine the influence of environmental parameters on the morphology of lake effect snowstorms over Lake Michigan. A series of model sensitivity studies are performed using the Colorado State University mesoscale model to examine the effects of lake–land temperature difference, surface roughness, atmospheric boundary layer stability, humidity, and wind speed and direction on the morphology of simulated storms. Four morphological types of lake effect snowstorms have been identified: (i) Broad area coverage, which may become organized into wind parallel bands or cellular convection; (ii) shoreline bands with a line of convection roughly parallel to the lee shore and a well developed land breeze on the lee shore; (iii) midlake band with low-level convergence centered over the lake; and (iv) mesoscale vortices with a well-developed cyclonic flow pattern in the boundary layer. The model is able to reproduce all four morphological types. Simulations varying environmental parameters independently define the thermodynamic and wind conditions for the occurrence of each morphological type. In particular, the limiting conditions of lake–land temperature difference, upwind wind speed stability, and humidity for development of a land breeze on the east side of Lake Michigan are defined for lake snow conditions. The effects of wind direction, surface roughness, and latent heat release are also described.

Numerical simulation of coastal clouds when solar radiation is blocked by smoke

Large fires ignited in a major nuclear exchange would inject a considerable amount of smoke into the atmosphere. This smoke could block the incident solar radiation from reaching the ground. Without the input of solar radiation on the ground, the temperature over the land surface would drop more rapidly than the temperature over the ocean, leading to formation of a thermal gradient at the continental coastlines. This temperature gradient has been hypothesized to induce a flow field that would initiate precipitation which could rapidly scavenge much of the smoke. This hypothesis is being investigated using an enhanced version of the Colorado State University Mesoscale Model to simulate air flow, clouds and precipitation along continental coastlines. The model has been modified to include the formation of fog and clouds, removal of moisture by precipitation, and the transport and diffusion of smoke. Also, the long wave radiation parameterization has been changed to take into account the effects of clouds. For both east and west coasts, with a moderate westerly synoptic wind and with solar radiation blocked by smoke, continental cooling leads to the formation of fog or low clouds over land. When fog forms, the layer of most rapid cooling moves to the top of the cloud while the potential temperature inside the cloud becomes nearly constant. As higher layers become saturated, the cloud expands vertically, and the liquid water content increases until precipitation falls from the cloud. Over the ocean, the atmosphere cools radiatively while the sea surface remains at constant temperature thus reducing the thermal stability and enhancing the vertical mixing of moisture which is injected into the atmosphere from the sea. Eventually a layer of precipitating cloud forms over the ocean that is very similar to the clouds over land. Once these similar cloud layers have formed, the cooling rates over land and sea become nearly identical, and there is no mechanism to produce the horizontal temperature gradients needed for vertical growth of clouds and precipitation into the upper levels of the atmosphere where they can scavenge the injected smoke.

Numerical Simulation of Coastal Flows when Solar Radiation is Blocked by Smoke

Abstract Smoke from fires ignited in a large-scale nuclear exchange would greatly reduce the flux of solar radiation at the ground and lead to rapid cooling over continental regions. Because of its large heat capacity, the ocean would cool more slowly so that a thermal gradient would tend to develop at the continental coastlines. In order to investigate the hypothesis that these thermal gradients would lead to zones of persistent precipitation along the coastlines, a modified version of the Colorado State University Mesoscale Model has been used to simulate the evolution of atmospheric flows for both West and East coasts assuming a moderate westerly synoptic flow of 5 m s−1. In both cases a layer of fog forms over the ground, effectively limiting the rate of cooling over land. This fog layer grows in height as the atmosphere aloft cools, forming a stratus cloud. Moisture from the ocean surface, mixed vertically into a radiatively cooled atmosphere, causes a cloud layer to form over the ocean as well. With...

Thermally Forced Surface Flow and Convergence Patterns over Northeast Colorado

Numerical model simulations have been performed with the Colorado State University mesoscale model to determine the regions of most likely occurrence of first cumulonimbus activity. It is shown that during the day, convergence along the eastern slopes of the Continental Divide and along the Cheyenne Ridge and Palmer Lake Divide coincides with the regions of most moist and unstable air. During the evening the flow reverses and the main convergence is located in the Platte River Valley. The interaction of these features with the nocturnal jet and Denver convergence/vorticity zone is also discussed.