utilizing calpuff for offshore and nearshore dispersion modeling analyses

Modeling Software for EHS Professionals

Utilizing CALPUFF for Offshore and Nearshore Dispersion Modeling Analyses

Prepared By:

Weiping Dai Christine

Otto Fei Bian

BREEZE SOFTWARE 12700 Park Central Drive,

Suite 2100 Dallas, TX 75251

+1 (972) 661-8881 breeze-software.com

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Utilizing CALPUFF for Offshore and Nearshore Dispersion Modeling Analyses

Weiping Dai, Christine Otto, Fei Bian Trinity Consultants 12770 Merit Drive, Suite 900, Dallas, TX 75251 Email: [email protected]

ABSTRACT As recommended in the U.S. EPA Guideline on Air Quality Models, the CALPUFF modeling system has been widely used for long-range transport dispersion modeling analyses, especially for applications related to Class I area analyses (e.g., regional haze evaluation and Class I PSD increment analysis). As a non-steady-state puff dispersion modeling system with the capability of utilizing multi-layer gridded meteorological conditions and performing simplified chemical transformation for certain chemical species, CALPUFF has also been applied to various near-field and special situations.

In this study, we will explore the use of the CALPUFF system in offshore and nearshore dispersion modeling analyses. This study directly evaluates the potential impacts from emissions related to offshore oil and gas exploration and production sources as well as nearshore emissions sources. For such an application, careful attention to meteorological data is necessary to ensure that the overwater and sea-land meteorological conditions are appropriately reflected in dispersion modeling analyses. In particular, this paper will share our experience of performing CALPUFF modeling analyses with the use of overwater data and MM5 data for offshore and nearshore applications.

INTRODUCTION Dispersion modeling is a technique used to estimate the potential impacts (e.g., airborne concentrations or deposition) of pollutants through the simulation of atmospheric physics and chemistry. Dispersion of emitted pollutants from various sources (e.g., industrial facilities) in the ambient air is complex and subject to many factors related to the conditions of the atmosphere. The atmospheric conditions in the offshore or nearshore areas may be significantly different from the atmospheric conditions over land. For example, the following unique characteristics or conditions related to or induced by a large body of water (e.g., ocean) in the offshore or nearshore areas should be considered in the dispersion modeling analysis:

• The overwater surface is relatively smooth and uniform compared to the land terrain;

• There is a constant source of moisture in the marine boundary layer;

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• Since water has a high heat capacity and is partially transparent to solar radiation, theoverwater atmosphere experiences relatively small diurnal temperature change andtypically relatively low mixing heights (about 500 m, as low as 100 m) resulting instable atmospheric conditions;

• Diurnal and annual variations of stability over water are unrelated to the typicaloverland behavior; and

• In coastal (nearshore) areas, sea-land breeze may result in the landward growth of thethermal internal boundary layer (TIBL) with strong convection (turbulencefluctuations) that may cause plume fumigation.

As such, dispersion of air emissions in the overwater (offshore) and coastal (near-shore) areas is unique and requires the proper treatment and utilization of the overwater and coastal meteorological conditions. The CALMET/CALPUFF modeling system is one model that applies these theories. The CALMET model is used to generate three-dimensional gridded meteorological data (such as hourly wind and temperature fields) in the modeling domain through sophisticated treatment and assimilation of available surface, upper air, precipitation, and overwater observations, prognostic wind field data from mesoscale models such as MM5, and geophysical data. In addition, two-dimensional fields such as mixing heights, surface characteristics, and dispersion characteristics are also developed. In the CALMET analysis, the overwater atmospheric conditions can be reflected through the treatment of overwater data (e.g., ship or buoy observations) and the utilization of mesoscale data (e.g., MM5). With the use of prognostic mesoscale data, the lake breeze circulation with a return flow aloft may be captured and introduced into the diagnostic wind field results.

The CALPUFF model is a Lagrangian puff model in which individual puffs of pollutant are released and are allowed to grow in the horizontal and vertical directions using the distribution coefficients in the Gaussian plume model in which pollutants spread outward from the centerline of the plume following a normal statistical distribution. The Lagrangian puff dispersion formulation treats plumes as a series of Gaussian puffs that move and disperse according to local conditions that vary in time and space. CALPUFF utilizes the CALMET treatment and processing of overwater and coastal conditions in the puff dispersion. In the coastal areas, the rapid changes of the atmospheric conditions (e.g., TIBL) can be reflected in the puff model formulation. As a typical stable coastal condition, the landward growth of the TIBL is caused by the sensible heat flux associated with solar heating of the land surface. The convection can cause rapid fumigation (but not well-mixed immediately) of the pollutants in the TIBL. In CALPUFF, the land-sea interface is typically resolved on the scale of the computational grid, i.e., the model computes turbulence and dispersion parameters during the transition from overwater to overland dispersion based on the land use properties (e.g., water or land) of each grid cell. Once a puff within the

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overwater layer enters the overland layer, the algorithm of puff growth is switched. For better spatial resolution, CALPUFF also implements a sub-grid scale TIBL module (SGTIBL) to divide the sampling step into smaller sub-steps, compute the change in the local TIBL height for each sub-step, and define proper dispersion properties for the puff.

Therefore, through the treatment and processing of overwater data, and the implementation of options to account for the characteristics of the overwater boundary layer and the coastal TIBL in the puff dispersion, the CALMET/CALPUFF system can be used for offshore and nearshore dispersion analyses.1, 2

DATA REQUIREMENTS AND AVAILABILITY CALMET implements the following components for overwater and coastal conditions:

• Marine (overwater) boundary layer to determine micrometeorological parameters

• Overwater temperature estimate

• Lake/sea breeze region option

The boundary layer models incorporated into CALMET include the overland boundary layer model and the overwater boundary layer model. The former utilizes the energy balance model to compute hourly gridded fields of the sensible heat flux, surface friction velocity, Monin-Obukhov length, and convective velocity scale and the latter uses a profile technique to compute the micrometeorological parameters (e.g., friction velocity, surface roughness, Monin-Obukhov length, and overwater mixing height) in the marine boundary layer based on the air-sea temperature differences and overwater wind speed. Note that this method is sensitive to the accuracy of the observations. In addition, in the interpolation of overwater observations, the radius of influence is defined by the user. The observation is excluded from interpolation if the distance between the grid point and the meteorological station exceed the radius of influence.

Since water bodies can affect the temperature and temperature gradients significantly, CALMET calculates the overwater temperature separately through the use of overwater data as input into the model. The overwater temperature interpolation is controlled by the selection of the land use categories for which the overwater data is applied. While the overwater surface temperature is based on the overwater observations, the temperatures for other vertical layers are based on the default or user-specified lapse rates below and above the overwater mixing height. The default lapse rates below and above the overwater mixing height are –9.8 K/km and –4.5 K/km, respectively. CALMET also allows only overwater stations to be used for overwater temperature based on the vertical temperature lapse rate.

In addition, CALMET provides the option to define a lake/sea breeze region within which

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the surface winds are calculated separately replacing the original winds. The winds within a lake/sea breeze region are calculated using an inverse distance squared interpolation based on the relative distance from the coastline.

CALMET uses the following overwater observations from one or more observation sites (e.g., ships and buoys) in the gridded meteorological data development:

• Air-sea temperature difference (K),

• Air temperature (K, use default value of 288.7 K if missing),

• Relative humidity (%, use default value of 100% if missing),

• Overwater mixing height (m),

• Temperature lapse rate below the mixing height overwater (K/m, use default value of –9.8 K/km if missing),

• Temperature lapse rate above the mixing height overwater (K/m, use default value of –4.5 K/km if missing),

• Wind speed (m/s), and

• Wind direction (degrees).

The above overwater data can be developed from overwater ship or buoy observations.

As discussed previously, prognostic meteorological data (e.g., MM5) can be used in the CALMET analysis. Table 1 presents the meteorological parameters in the MM5 data as input into CALMET. Since atmospheric dynamics and physics are reflected in the process of MM5 data creation along with overwater observations and the gridded MM5 data can be generated at a fine spatial resolution from the surface layers to many more upper layer levels, the utilization of MM5 in the CALMET analysis can provide a much better data representation for overwater and coastal areas. As such, the CALMET analysis with both overwater observations and MM5 data as input can satisfy the data requirements for offshore and nearshore dispersion analyses.

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Table 1. Meteorological Parameters Available in MM5 Data

Data Type Parameters

MM5 Data Pressure, elevation, temperature, wind direction, wind speed, vertical velocity, relative humidity, vapor mixing ratio, cloud mixing ratio, rain mixing ratio, ice mixing ratio, snow mixing ratio, and graupel mixing ratio

CASE STUDY WITH CALMET/CALPUFF Based on the availability of meteorological data (e.g., overwater data and MM5 data) and special treatment and utilization of such data in the CALMET/CALPUFF modeling system, it is reasonably expected that the CALMET/CALPUFF modeling system is appropriate for overwater and nearshore dispersion applications. In this paper, case studies were performed to evaluate the application of the CALMET/CALPUFF modeling system to the analysis of offshore and nearshore meteorological conditions and air pollutant dispersion. Specifically, the effects of utilizing the overwater data and MM5 data in the analysis are evaluated. The east Gulf of Mexico coast near Pensacola, Florida was selected as the study domain. In this case, the sea surface temperature is higher than the air temperature during most times of the year, and thus causes the overlying atmosphere to be unstable and convective. Figure 1 shows the stability parameter (z/L, where z in the height above the sea surface and L is the Monin-Obkhov length) for each month of the year based on buoy data at several locations.3 In this study, a 10-day period in the month of July (i.e. July 11 – July 20, 1996), was evaluated.

CALMET Meteorological Data Comparison The overall modeling domain for the CALMET meteorological data development is shown in Figure 2 along with the buoy station locations. In addition, about 21 land-based surface stations, 7 upper air stations and numerous precipitation stations are available along with the 36-km resolution MM5 data. The CALMET meteorological data was developed for the following four scenarios (note that all available upper air and precipitation stations are used in all scenarios): (1) Scenario 1 - CALMET analysis with all available land-based surface station data, overwater data, and MM5 data; (2) Scenario 2 - CALMET analysis with only land-based surface station data and MM5 data; (3) Scenario 3 - CALMET analysis with only land-based surface station data and overwater data; and (4) Scenario 4 - CALMET analysis with land-based surface station data only.

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Figure 1. Variation of Stability Parameter (S.A. Hsu)

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Figure 2. Case Study Modeling Domain with Overwater Stations

Figures 3 through 8 illustrate the wind vectors (at the bottom four vertical layers: 20, 50, 100, and 150 meters), mixing heights, atmospheric stability, and ambient temperature at mid-night and noontime on July 12, 1996 for Scenario 1 (which implemented all the available meteorological data including overwater and MM5 data). These figures demonstrate the diurnal changes of these parameters for the offshore, nearshore, and inland areas. Overall, these plots capture the following distinctions between offshore, nearshore, and inland conditions: (1) the mixing height is relatively low in offshore areas and increases rapidly in the nearshore areas starting from mid-morning through afternoon; (2) the diurnal change of atmospheric stability is less significant in overwater areas than in inland areas; and (3) the inland temperature may be higher than the overwater temperature during daytime and the pattern may reverse at nighttime. The variation of these atmospheric parameters for other scenarios are also compared and presented in Figures 9 through 17 for noontime conditions. Substantial differences may exist for wind condition, atmospheric stability, and temperature between different scenarios.

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Figure 3. Wind Vector and Mixing Height at 12am of July 12, 1996 for Scenario 1

Figure 4. Wind Vector and Mixing Height at 12pm of July 12, 1996 for Scenario 1

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Figure 5. Atmospheric Stability at 12am of July 12, 1996 for Scenario 1

Figure 6. Atmospheric Stability at 12pm of July 12, 1996 for Scenario 1

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100 200 300 400 500 600 700LCC East (km)

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200

300

400

500

600

700

LCC

Nor

th (k

m)

28.N

30.N

32.N

91.W 89.W 87.W 85.W

Figure 7. Atmospheric Temperature at 12am of July 12, 1996 for Scenario 1

Figure 8. Atmospheric Temperature at 12pm of July 12, 1996 for Scenario 1

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CALPUFF Dispersion Modeling Comparison The plume dispersion in offshore and nearshore areas was evaluated with CALPUFF for the

same four scenarios described in the previous section. An emission source located at (475

km, 415 km) was simulated with a constant value of emission rate (100 g/s), stack diameter

(1 m), exit velocity (10 m/s), and exit temperature (300 K) with three different stack height

values (10 m, 50 m, and 100 m). The highest ground-level concentrations are compared in

Table 2. Even under the same dispersion modeling options, the CALPUFF modeling results

show that the maximum concentrations could be substantially different with the use of

different CALMET data sets with or without the overwater and/or MM5 data. The

corresponding occurring date and time of the maximum concentrations are also very

different. Such differences are not completely unexpected because of the significant

differences in the CALMET atmospheric conditions for different scenarios.

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Table 2. Modeling Result Comparison

SUMMARY With the use of overwater and MM5 data, the CALMET meteorological model can develop

meteorological data sets that reflect the characteristics and variations of atmospheric

parameters (e.g., wind conditions, mixing height, atmospheric stability, and temperature) in

offshore and nearshore areas. CALPUFF dispersion modeling results can be substantially

different if overwater and/or MM5 are not used. Therefore, it will be critical to use both

overwater and MM5 data when modeling offshore and nearshore dispersion. Without the use

of overwater and/or MM5 data, the CALMET output meteorological data (e.g., wind

conditions, atmospheric stability, and temperature) may be notably not consistent with the

expected offshore or nearshore conditions.

REFERENCES

1. Earth Tech, Inc., “A User’s Guide for the CALMET Meteorological Model”, January

2000.

2. Earth Tech, Inc., “A User’s Guide for the CALPUFF Dispersion Model”, January

2000.

3. S.A. Hsu, “Estimating Overwater Convective Boundary Layer Height from Routine

Meteorological Measurements for Diffusion Applications at Sea”, Journal of Applied

Meteorology: Vol. 36, No. 9, pp. 1245–1248.

utilizing calpuff for offshore and nearshore dispersion modeling analyses

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