The Open Atmospheric Science Journal, 2008, 2, 117-130 117 Open Access Mesoscale FDDA Experiments with WTM and ACARS Data Chia-Bo Chang*,1 and Robert Dumais2

1Department of Geosciences, Tech University, Lubbock, Texas 79409, USA 2Army Research Laboratory, White Sands Missile Range, NM 88002, USA

Abstract: This is a real-data Four Dimensional (FDDA) study using MM5 in conjunction with West Texas surface observations and ACARS (Aircraft Communications Addressing and Reporting System) profile data collected by commercial aircraft during both en route and ascent/descent phases of their flights. The high-frequency mesonet data and ACARS and profiles are ideal for testing the effects of FDDA on short-term mesos- cale numerical prediction. The mesonet experiments involved 35 sites with an average horizontal spacing of about 30 km, while in the ACARS case ninety five profiles were used. Results indicated that nudging the MM5 model with the surface-based data over the relatively small area of the mesonet domain had limited impact on the model’s performance. In the ACARS runs, FDDA had long-lasting impact throughout the entire model atmosphere. FDDA appeared to improve the quantitative precipitation forecasting skill of MM5 and reduce slightly the model’s warm bias at the surface. The study suggests that ACARS has potential to significantly enhance our expertise in short-term mesoscale modeling and to sup- port the need to rapidly and accurately adjust high-resolution meteorological model forecasts to real-time observations.

INTRODUCTION (Newtonian relaxation) [6] was employed in the MM5 FDDA. The focus of this paper is on mesoscale modeling and FDDA. Because of its small spatial size mesoscale weather There are other more sophisticated data assimilation is strongly influenced by fast-changing local conditions such methods involving the Kalman filter or three-dimensional as cloud, friction, and surface heating. To catch these fast- variational (3DVAR) procedures [7], but these advanced changing local events so as to maximize the model perform- treatments are much more computationally intensive and ance, state-of-the-art mesoscale numerical weather predic- require background information generally not available in a tion (NWP) often makes use of FDDA of high-frequency fast-response scenario, e.g., prediction of atmospheric observations to update the model state during the time inte- boundary layer dispersion for emergency response. We be- gration [1]. The West Texas Mesonet (WTM) surface obser- lieve that the Newtonian relaxation technique represents a vations reported at five minute frequency, along with the good balance between complexity, timeliness, and accuracy, NOAA ACARS observations taken by commercial aircraft at which is an important guiding principle for short-term pre- about every ten minutes while crisscrossing the nation [2], diction and . offer valuable data for mesoscale FDDA. Other researchers Relevant scientific questions examined include: such as those at NCAR’s Research Applications Laboratory [3, 4] have provided some previous examination of the im- a) can data assimilation using high frequency surface pact and quality of ACARS in mesoscale FDDA. A better observations alone significantly influence the model understanding of how to take advantage of these high resolu- forecasts? tion data sets can significantly advance our expertise in b) how sensitive are the model forecasts to changes in mesoscale NWP. (1) the nudging parameters and (2) the input ACARS In this study, real-data assimilation experiments based on data density? the WTM surface tower data and ACARS profile data were c) how does the model’s quantitative precipitation fore- conducted using the MM5 prediction system [5] to reveal the casting (QPF) respond to FDDA? impact of high-frequency FDDA on mesoscale NWP. We were particularly interested in the data’s impact on boundary The rest of this paper is organized as follows: Sections 2 layer (BL) and precipitation forecasting. and 3 give, respectively, a brief overview of WTM and ACARS including the quality of the data sets. Section 4 de- As shown later, the WTM observations were confined to scribes the data assimilation procedures, and synthesizes the a relatively small domain over the Southern Plains, while the results of three real-data case studies. Major findings and ACARS observations covered a much more widespread area. conclusions are summarized in Section 5. The empirical method known as observational nudging WTM A detailed technical overview of the WTM surface sta- *Address correspondence to this author at the Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA; tions and completed site locations can be found in [8]. Fig. E-mail: [email protected] (1) shows a sample plot of WTM surface tower data over an

1874-2823/08 2008 Bentham Open 118 The Open Atmospheric Science Journal, 2008, Volume 2 Chang and Dumais area of about 300 km (west to east) by 350 km (south to data sets can be found on the website http://acweb.fsl.noaa. north). Each square box of approximately 50 km by 50 km gov. ACARS wind and temperature data are collected by represents a county. There are 45 observation sites with an many commercial aircraft during both en route and as- average spacing close to 30 km. The surface data are now cent/descent modes of their flights at a very high frequency. being distributed in real time via the Internet to users access- At flight altitudes of about 23,000 ft, data are generally taken ing the WTM homepage (www.mesonet.ttu.edu). The every 5-6 minutes. Nearer to airports the data spacing is de- mesonet online data are updated in real time every five min- creased by some airlines. Below 18,000 ft, a vertical resolu- utes. The (NWS) has been incor- tion of 1000 to 2000 ft. is quite common. More than 150 porating these observations into their hourly weather aircrafts provide data with a vertical resolution of 300 ft. roundup product since 2001 and is available to the public during the first minute after take-off. The temporal and spa- and news media. The redundancy of data sources will ensure tial distribution of the data is such that ACARS can provide that data used for any projects has a backup in case of a valuable up-to-date weather data for short-range forecasts of server failure at Reese Center, which is located in the box of atmospheric boundary layer . 4th row from the bottom and 3rd column from the Texas-New The quality of ACARS data had been examined by many Mexico state line. researchers [2, 9, 10]. Estimated wind vector accuracy was 84 1014 68 1017 68 7 6 + 1 5 65 65 67 about 1.8 m/s and estimated temperature accuracy was about 68 o 7472 0.5 C. When ACARS was compared to , root 66 72 76 6668 85 mean square (RMS) deviations were 7.4 degrees in direction 77 69 67 78 73 79 76 77 8371 1015 and 5.3 m/s in speed. In comparing just the ACARS as- 77 1017 66 69 80 4 3 +852 68 67 69 6967 65 80 69 88 cent/descent winds and with radiosondes, it 82 71 o 66 82 72 80 68 82 was found that temperature differences were less than 2 C on 66 83 82 87 o 66 67 70 94 percent of all occasions, and less than 1 C greater than 68 74 77 68 69 80 101478 68 8280 4 85 percent of the time. RMS deviations were 4.1 8269 78 67 67 82 816967 78 73 67 69 70 m/s, while direction RMS differences were 35 degrees 70 66 68 81 67 84 82 79 81 79 (mostly due to light and variable wind situations). 63 64 79 78 71 78 65 69 71 78 80 68 77 71 MM5/FDDA EXPERIMENTS 64 66 67 78 79 69 77 79 78 72 66 69 MM5 provides various options for parameterizing pre- 64 66 69 68 72 7110161 017 + 1 0 cipitation and BL physics. The following BL-related physi- 77 64 +10 72 70 79 66 cal parameterization options were selected for the simulation 66 77 101477 1013 experiments. 070802 1800 Panhandle Sfc P1ot • Grell moist-convection [11]. Fig. (1). A sample plot of WTM surface tower observations. As described by Schroeder et al. [8], robust quality con- • Atmospheric radiation with the effects of clouds [12] trol (QC) procedures for the WTM data have been devel- • MRF [13] oped. In developing a QC algorithm, care is taken to remove • Surface heat and moisture fluxes from the ground the erroneous data while maintaining the integrity of the valid data as much as possible. The first step in the QC pro- • Surface energy budget to calculate the ground tem- cedure is to replace any missing data with interpolation from perature the nearest two data points. Small sub-sets of the time series • Multi-layer thermal diffusion are examined to identify spikes in the data. This is done by moving a 7-point data window through the data, computing We experimented with various nesting configurations in mean and standard deviation data for that window, and look- MM5 including one-, two-, and three-grid nest runs in con- ing for data points that are significantly out of line with the junction with horizontal grid spacings ranging from 3 km to statistics. The filtering pass is done three times to remove the 20 km. The model appeared to be quite reliable and robust in anomalies. These procedures work very well at removing bad all nesting configurations. A close examination of the model data while not altering the valid data points. It is important to output did not reveal any indication of computational insta- note that the filtering process does not smooth or average out bility during the model integrations. The results of just the the time series, it only looks for and corrects spurious data single-nest (20 km grid spacing) FDDA simulations for the points. following three case studies are presented in this paper. ACARS (A) 23 November 2002: Tranquil synoptic conditions. These data are routed by several cooperating airlines to (B) 21-22 February 2003: Weak synoptic disturbance NOAA GSD (Global Systems Division) for quality control. with light precipitation. General information about ACARS and how to access the (C) 8-9 May 2005: Severe outbreak in Cen- tral Texas

Mesoscale FDDA Experiments with WTM and ACARS Data The Open Atmospheric Science Journal, 2008, Volume 2 119

• Nudging factors (N) for wind, temperature, and hu- midity is 4 x 10-4 s-1. • Horizontal radius of influence (R) is 100 km from the observation site. • Vertical radius of influence () is 0.001 centered at the level of = 0.995. • Time window (t) is 60 min centered at the observa- tion time. In each case, the MM5 simulations included a 24-h con- trol run (CNTR) without invoking the WTM data along with a similar 24-h FDDA run with observational nudging be- tween 6 h and 12 h model time. In the CNTR run, MM5 re- produced the observed surface flow in Case A quite well, while in Case B the model over-predicted the surface wind speeds but did well in surface temperature and dew point forecasting [15]. Figs. (3) and (4) show separately the 12 h and 18 h simu- lated vector wind and temperature (T) differences (CNTR-

FDDA) at = 0.97 in Case A. As expected, at the end of Fig. (2). MM5 model domain and terrain. FDDA the most notable differences occur in the general area The version of the MM5 model used in the FDDA ex- of the WTM. The largest differences are over 2 m/s in wind periments is non-hydrostatic and has 24 terrain following speed and 0.8°C in T. However, these differences dissipate levels in the vertical. Other key model parameters are pre- rapidly after FDDA is turned off. In 6 hours (18 UTC) the sented in Table 1. Fig. (2) shows the MM5 domain and ter- maximum differences on the grid have dropped to less than rain with WTM data for Cases A and B. Note that WTM 0.6 m/s and about 0.1°C, along the eastern edge of the WTM (Fig. 1) situated in the middle of the domain encompasses domain. An eastward translation of the vector wind differ- roughly one tenth of the total area. The MM5 initial and lat- ences is noted between 1200 and 1800 UTC. Increasing the eral boundary conditions were derived from the NWS/NCEP length of FDDA to 9 h (from 3 h to 12 h) resulted in very Eta model [14], which had 50 vertical levels and a horizontal similar 24-h simulations. grid mesh size of 22 km. All model integrations were per- formed on a SGI Octane-2 workstation.

0 Table 1. Key Case Study Parameters 0

0 0 L 0 Horizontal Length of 0 Case Level Grid Size 00 Dimension Integration H H 1220 A 67 by 67 24 15 km 24 h 0 4450 L 0 0 B 67 by 67 24 15 km 24 h 0 0 19 L 858 C 67 by 67 24 20 km 36 h 500 500 L L 0 83 500 868

Starting Time Data Source/ 0 Case 0 H UTC/Day/Month FDDA Period 0 0 L 0 22 H 894 H 561 A 00/23/11 WTM/6 to 12 h 0 44 L B 00/21/02 WTM/6 to 12 h 0 0 0 0 13 0 0 0 C 00/08/05 ACARS/12 to 24 h 0 0

0 0 FDDA WITH WTM DATA L L H 0 11 0 12 003 The observational nudging package provided by MM5 is used in the FDDA experiments. The details of the MM5 ob- Fig. (3). Vector wind and T differences (CNTR-FDDA) at = 0.97 servational nudging scheme can be found in the NCAR re- ( 20 m above the surface) for Case A at 12 UTC, 23 Nov. 2002. port [5]. The nudging parameters selected in this study are as The contour interval for T is 0.25°C. follows: 120 The Open Atmospheric Science Journal, 2008, Volume 2 Chang and Dumais

servational nudging. The nudging parameters were similar to those mentioned earlier. However, some adjustments were made in order to test the sensitivity of the model’s response H .010 to changes in the nudging parameters and data density. L .041 H Table 2. Mean (M) and Standard Deviation (SD) of u, v, and .015 L T Differences (CNTR - FDDA) Over the Model Do- .005 H .060 main for Case A at Every 3 Hours at (a) Surface and (b) = 0.68 (p 700 hPa)

Table 2a. Surface L H .027 .114 .21 u (m/s) v (m/s) T(oC) H t(h) .013 M SD M SD M SD

H 6 -0.05 0.17 -0.02 0.09 0.00 0.01 H .003 H .018 9 -0.11 0.34 0.01 0.13 0.00 0.07 .052 12 -0.09 0.32 0.09 0.31 0.00 0.05 L 15 0.01 0.08 0.03 0.09 0.00 0.05 .004 18 0.01 0.04 0.00 0.06 0.00 0.03

H L 21 0.00 0.02 -0.01 0.03 0.00 0.02 .002 .005 24 0.00 0.03 0.00 0.03 0.00 0.02 Fig. (4). Similar to Fig. (2) but at 18 UTC 23 Nov. 2002.

Table 2 shows the means (M) and standard deviations Table 2b. = 0.68 (SD) of the u, v, and T differences computed over the model domain as a function of time for both the surface and lower- u (m/s) v (m/s) T(oC) troposphere at  = 0.68 ( 700 hPa) and for Case A. At both t(h) M SD M SD M SD levels, all means and SDs are zero at 3 h. At the surface, the means show mild temporal oscillations, while the SDs rise 6 0.00 0.00 0.00 0.00 0.00 0.00 rapidly at 6 h and then drop sharply in 6 hours (becoming quite small at 18 h). The maximum differences in surface 9 0.00 0.01 0.00 0.01 0.00 0.00 SDs occurring at the end of data insertion (12 h) are one or- 12 0.00 0.02 0.00 0.02 0.00 0.01 der of magnitude larger than their upper-level counterparts 15 0.00 0.03 0.00 0.03 0.00 0.01 occurring around 18 h. The effect due to inserting WTM 18 0.00 0.03 0.00 0.04 0.00 0.01 surface data on the upper-level flow is clearly negligible, and the effect on the surface flow diminishes within a few hours 21 0.00 0.03 0.01 0.04 0.00 0.01 after the termination of FDDA. Similar results for Case B are 24 0.00 0.02 0.00 0.02 0.00 0.01 noted in Table 3. The surface means and SDs have compara- ble sizes as in Table 2a, although the SD of T reaches its Table 3. Similar to Table 2a but for Case B maximum a little later. In both cases, results suggest that

FDDA with the WTM surface observations had very little impact on the model’s performance beyond the first few u (m/s) v (m/s) T(oC) t(h) hours. M SD M SD M SD FDDA WITH ACARS DATA 6 -0.01 0.05 0.02 0.09 0.00 0.00 The severe weather case of 5-6 May 2005 (Case C) was 9 0.03 0.14 0.06 0.21 0.00 0.02 selected for the FDDA using ACARS experiments. The model domain was similar to that shown in Fig. (2) except 12 0.03 0.15 0.05 0.20 0.00 0.03 covering a larger area. First, we carried out a 36-h simulation 15 0.02 0.07 0.00 0.05 0.00 0.04 of the severe weather environment which included a dryline 18 0.00 0.06 -0.01 0.06 0.01 0.09 perturbation in West Texas and intense convection in Central 21 0.00 0.05 0.00 0.05 0.00 0.05 Texas from 1200 UTC to 0000 UTC of 8- 9 May. Again, this simulation without FDDA was regarded as the control run 24 0.00 0.06 0.00 0.06 0.00 0.04 (CNTR) to provide a benchmark for evaluating the impact of ACARS data. In the subsequent FDDA runs, more than 90 The distribution of the ACARS data used in this case ACARS profiles over an area across several mountain and study is shown in Fig. (5). In the lower and middle MM5 south central states were processed for use in the MM5 ob- Mesoscale FDDA Experiments with WTM and ACARS Data The Open Atmospheric Science Journal, 2008, Volume 2 121

(a) model layers clusters of ACARS reports oriented in the north-south direction are found throughout central Texas and . The data coverage spreads more uniformly in the upper model layers. Each data point recorded during a flight at a given time, latitude and longitude, and altitude is treated as one single independent observation. The locations of indi- vidual observations in terms of the model grid system must first be determined, and they are then organized in order of increasing time as required by the MM5 nudging program. Explanations for the input data structure can be found in the MM5 tutorial notes under the section of “Data Used in FDDA” [5]. The ACARS observations used in the case study initially consisted of 1928 data points. The number dropped to 1213 after averaging observations within the same proximity, meaning that they were reported in the same time slot and at the same vertical level while within 10 km (half the grid 1090 km size) of the individual model grid points. Many more obser- vations were inserted in the model upper layers than in the (b) middle and lower layers. The first FDDA experiment (E1) was a 36-h run with 12-h observational nudging applied be- tween the 12 h and 24 h model integration periods, and based on the averaged ACARS data set. The nudging parameters as defined earlier were R = 100 km, N = 4 x 10-4 s-1, = 0.001, but with t = 30 min. The three additional experiments conducted to test the model’s response to changes in R, N, and data density were: • E2 similar to E1 but R = 50 km. • E3 similar to E1 but N = 2 x 10-4 s-1. • E4 similar to E1 but with all 1928 data points or without the averaging procedure. a. CNTR vs E1 Figs. (6) and (7) show the MM5 initial state (00 UTC 8 1000 km May) at both the surface and 700 hPa, respectively. A north- (c) south oriented dryline is evident in the middle of the model domain. The major features at the surface are southerly flow to the east of the dryline and southwesterly flow to the west (near the Texas-New Mexico state line), creating a conflu- ence of air near the dryline. Notable changes in the wind directions from southerly at the surface to westerly at 700 hPa (veering profile) occur in South Texas. The 700 hPa confluent zone is located in Central Texas and Oklahoma, which was well to the east of the dryline, and there is a counter-clockwise turning of 700 hPa winds from westerly to southerly in the eastern half of Texas where the outbreak of severe weather was observed later. The average wind speed at 700 hPa is on the order of 10 m/s, while at 300 hPa (not shown) the flow is predominantly southwesterly with a rela- tively narrow zone of winds in excess of 30 m/s stretching from Mexico to the northern boundary of the modeling do- main. This stronger zone of 300 hPa flow generally occurs in

1000 km the vicinity of the dryline.

Fig. (8) shows the observed surface vector winds and Td Fig. (5). ACARS observation sites, 12 UTC, 8 to 00 UTC, 9 May at 12 UTC 9 May. Throughout the 36-h period, southerly 2005: (a) upper model layers centered at 300 hPa, (b) middle model flow persists across Central Texas and continues to advect layers centered at 500 hPa, and (c) lower model layers centered at 700 hPa. 122 The Open Atmospheric Science Journal, 2008, Volume 2 Chang and Dumais

(a) 15.0 L 9.46 2.20

H 8.09 L 10.0 15.0 L 4.12 H H 18.4 .60 10.0 17 10.0 15.0 H 14.7 15.0 H N 3 10.0 8.97 10.0 10.0 N 3 10.0 L 10.0 3.26 L 5.26 H L H 10.0 H 9.77 5.81 9.25 13.4 L

10.0 8.88 0.0 0.0 10.0 L H 5.75 3 9.37 N H 7.5 N 3 16.9 L 10.0 H 9.22 L H 15.0 0 12.7 . 5.21 0 H 10.0

H Fig. (7). MM5 Initial 700 hPa vector winds. The contour interval is 10 m/s. (b) Figs. (9) and (10) show 24-h and 36-h simulated surface winds and Td from the CNTR run, respectively. Throughout the first 24 h, a significant weakening of surface circulation occurred in the western half of the domain. The MM5 pre-

0.00 H dicted a large increase in the Td gradients in West Texas, and 15.4 L 0.0 a northeastward intrusion of dry air and resulting dryline -8.52 bulging into regions of western and the Oklahoma

N 3 Panhandle. At 36 h, the middle section of the dryline re- treated slightly westward and aligned along the Texas-New 16.0 Mexico border. The observed double-boundary structure

8.00 (Fig. 8) and tight Td gradients were well predicted, but the L model dryline in the area of the Texas Panhandle was pre-

-8.73 0.0 16.0H dicted too far to the west. This was likely related to the MM5 16.7 over-prediction of southerly flow in this region, which 16.0 caused an advection of moister air further northwestward into the Texas Panhandle. Clearly, surface convergence N 3 0.00 played a key role in the intensification and maintenance of 0.0 16.0 L 16.0 the sharp Td gradients. -5.67 The model convective precipitation rates (not shown) in 0.0 the CNTR run reveal that no organized convection takes place before 18 h, and that convection in Oklahoma and o Fig. (6). MM5 Initial state (a) vector winds (m/s) and (b) Td ( C) at West Texas (along the eastern edge of the dryline) took place the surface. from 18 h to 30 h. However, no convection was present near the dryline at 36 h. The major convective activity was found moist air from the Gulf of Mexico inland. The T gradients d quite a distance from the dryline in Central and East Texas, along the in West Texas tighten, although they do where severe weather was reported. The observed convective weaken considerably in the Panhandle. The result is a double activity reached its peak intensity around 24 h, and tailed off boundary surface T structure enclosing a diffluence area of d over the next 12 h. relatively weak surface winds. Again, there is confluence associated with the dryline. Significant changes take place in Fig. (11) shows the MM5 simulated 24-h and 36-h accu- the upper-air flow patterns (not shown). At 700 hPa, the mulated precipitation. At 36 h, some negative impacts of the counter-clockwise turning flow (Fig. 7) 12 h earlier is re- eastern grid boundary on the model precipitation field are placed by a clockwise cyclonic turning with southerly flow noted. Fig. (12) shows the observed 24-h accumulated pre- in South-Central Texas transitioning to westerly in East cipitation ending at 12 h (1200 UTC 8) and 36 h (1200 UTC Texas. At 300 hPa a westerly flow regime dominates the 9) of the model integration. The general patterns of the 24-h whole area with no upper-air jet features appearing in the simulation compare quite well with the observations regard- model domain during this time. ing the broad areas of heavy precipitation. Mesoscale FDDA Experiments with WTM and ACARS Data The Open Atmospheric Science Journal, 2008, Volume 2 123

(a) (a)

L H 381 L 13.0 L H .667 .225 8.85 L H .075 H 10.0 H 8.73 8.18 H .609 10.1 L 11.4 .097 10.0

10.0 N 3 3.99 N L 3 L .179 .116 10.0 .428

H H L 12.7 7.8 .409 .264 H H 7.31 L H 11.3 .237 11.0 7.28 10.0

10.0 N 3 L N L 10.0 3 .127 .211 L L .264 H .237 2.73 H 7.07 10.0 9.03 17. 3 L L L 1.70 2.17 .317 (b) (b)

8.00 0.0

L 0.0 -8.10

-9.4709 H B 6.1 8.00 H

8.00 16.0 14.4 0.0 0.0 N 3 N 3 L L -7.86 -8.53 L -8.00 H

-3.34 8.00 16.0 0.0 0.0 8.00 H

17.2 16.0 N 3

8.00 16.0 6.0 N 3 0.0 8.00

16.0

8.00 0.0

L o Fig. (8). Observed surface conditions at 12 UTC 9 May: (a) vector Fig. (9). 24 h simulated (a) surface winds (m/s) and (b) Td. ( C). winds (m/s) and (b) T (oC). d Although the nudging of ACARS data ends at 24 h, its Fig. (13) shows the E1 simulated Td at 36 h. The large Td impact on the MM5 simulations persists through the next 12 differences between CNTR and E1 occurred in West Texas h. Fig. (15) shows the difference (CNTR-E1) maps of sur- and eastern New Mexico e.g., due to the modeled location face vector winds (V), surface temperatures (T), and spe- and orientation of the dryline and surface confluence zone). cific (q) at 36 h. Note that no q data were used in Fig. (14) shows the 36-h simulated accumulated precipitation nudging. Clearly, V is compounded by latent heating. The in E1. The overall patterns resemble those of CNTR (Fig. locations of the V, T, and q centers are well correlated 11). However, some differences between the two runs are with each other at 24 h, but not as well at 36 h. At 24 h, over noted. In CNTR, an area of over 5 cm of rainfall is predicted 15 m/s V are found in many locations, and T and q re- in North Texas at 24 h, while in E1 a similar maximum is veal several maximum and minimum centers with values as centered further to the south. At 36 h, basic features such as large as 6°C and 7 gm/kg respectively dispersed across the location of heavy rainfall (> 2.5 cm) in the Central Texas and Oklahoma. Twelve hours later there still exist Texas, appear to be in better agreement with the observations areas of large V. However, we see clear fading of T In E1 (Fig. 12) than in CNTR. 124 The Open Atmospheric Science Journal, 2008, Volume 2 Chang and Dumais

(a) vicinity of the Oklahoma-Texas border. The bulk of P be- tween E1 and CNTR occurs throughout the final 12 h of the 1.77 H model simulation period, indicating significant influences of 9.83 ACARS data on the model precipitation processes many 10.0 L hours after the “turning off” of FDDA. .173 H 15.4 .906 (a) L L H .060 1.07 7.90 H .602 9.34 N 3 H L 10.2 .296 L L .331 3.00 H 0.047 1.27 9.50 H L 16.6 .000 .317 L

H .317 4.37 15.1 9.51 H 317 9.92 L 2.87 4.27 L H .317 .635 .494 11 8 H .317 2.5 N H 3 .29 L 635 9.74 L 5.08 .159 .000 1.27

L 2.54 .321 H H H 11.2 .827 635 8 .317 3.64 635

.317 635 1.27 (b)

.317 .635

8.00 .317 0.0 H H 8.50 H

10.0 0.0 (b) 8.00 105 W 100 W 95 W N 3 .32 L 11.5 L 317 2.84 12.6 . 1.04 L .635 L

.000 .317 H -6.49 0.0 16.0 -1 66 .317

N .317 3 L H 16.0 13.5 N -1.68 -6 3 1.27 2.54635 16.0 16.0 0.00 .635 2.54 L 1.27 L H 635 10.2 .000 5.08 7.70 16.2

.317 .635 5.08

2.54 Fig. (10). As in Fig. (9), except for 36 h. .317 2.54 N 3 .635 .27 during the 12-h period. At 36 h, the most pronounced q are .317 L 1.27 found in the vicinities of the Kansas- Oklahoma and Texas- .000 New Mexico state lines, likely due to the discrepancy in the .317 model’s predicted location and orientation of the dryline. H .000 Fig. (16) shows (CNTR-E1) and (E1-CNTR) in the 36-h accumulated precipitation (P). The CNTR run produces a Fig. (11). (a) 24 h and (b) 36 h accumulated precipitation (cm) for broad area of much higher rainfall in North Texas with the the CNTR run. largest P over 5 inches, and smaller areas of positive P Table 4 summarizes time sequences of means and SDs of further south. The E1 simulation produces significantly the (CNTR-E1) differences in u, v, T, and q over the model heavier rainfall in the Central Texas. The observed patterns domain at the (i) surface, (ii) lower-troposphere ( = 0.68), in Fig. (12) appear to support the high positive (E1-CNTR) and iii) upper troposphere ( = 0.32) for Case C. The basic P in Texas, in addition to the area of positive P in the structure of means and SDs at all levels is rather similar.

Mesoscale FDDA Experiments with WTM and ACARS Data The Open Atmospheric Science Journal, 2008, Volume 2 125

(a) 105 w 100 w 90 w

Inches

.317 2.43

H 3685 10 .740 8.0 .317 6.0 H 5.0 .265 L .635 4.0 .000 L 3.0 .000 .317 2.5 N 3 2.0 1.5 .317 1.0 1.27 .635

0.75 2.54 .635

0.50 .317 L .317

0.25 .06 .000 .635 2.54 0.10 2.54 .635 0.01 5.08 1.27 .3171.27 Missing H 1.27 17.5 2.54 2.54 N 3 2.54 .635 (b) .377

.635 .317 1.27 Inches H w Mexico .317 10 .000 8.0 Fig. (14). As in Fig. (11b), except for E1. 6.0 5.0 4.0 Some oscillations are revealed in the mean values between 3.0 12 h and 36 h of the simulation period. The amplitudes are 2.5 2.0 not particularly large, and there is no coherent relationship in 1.5 the frequency and phase between any two variables or two 1.0 0.75 levels. The SDs of u, v, and T increase rapidly after FDDA is 0.50 invoked 12 h into the model simulation. The small but non- 0.25 0.10 zero SDs of u, v, and T at 12 h are probably due to the use of 0.01 the 60-min time window centered at the starting point. For q Missing there are no changes at 12 h and its SD values increase gradually before 21 h (because the ACARS profiles used do not include humidity data). The SDs at the individual levels Fig. (12). 24-h accumulated precipitation observed at (a) 1200 reach their highest values after 24 h, although at different UTC, 8 and (b) 1200 UTC, 9 May 2005. (after www.srh.noaa.gov/ times depending upon level and parameter.. It is interesting rfcshare/precip_analysis_new.php). to note that more often these maximums occur a few hours

105 w 100 w 90 w after the termination of FDDA (24 h into the simulation). Also, at the end of simulations, all SDs remain quite high and close to those noted at 24 h. It indicates that the model is able to retain the profile information even hours after ending 8.00 8.33 H FDDA. In contrast the model tends to forget quickly the in- 15.0 0.0 serted surface data as seen in the WTM experiments. Table 4 shows that there are no significant differences in 0.0 00.8 the SD values of u and v between the surface and upper lev- N L 3 12.3 H els. Considering that generally lower wind speeds exist near 15.4 the surface, we may argue that the ACARS profiles have larger impact on air motion near the surface than in the upper L levels. The close correlation between V and the model pre- H-6.40 -1.74 cipitation (Fig. 12) suggests that latent heating likely has an L important role in defining SDs. For these experiments, the 12.5 16.0 FDDA appears to result in a cooler and wetter lower and

16.0 upper troposphere, while a warmer and drier middle tropo- N 3 sphere (not shown). 16.0 16.0 Fig. (17) shows the means and SDs of forecast errors L 0.0 16.0 7.97 (observations-model) at the model levels for CNTR. The mean curves suggest that the model over-predicts u except near the surface, but generally under-predicts v. The SD o Fig. (13). 36 h simulated Td ( C) for E1. 126 The Open Atmospheric Science Journal, 2008, Volume 2 Chang and Dumais

Table 4. Mean (M) and Standard Deviation (SD) of u, v, T, tional forecast error statistical measures. Nevertheless, the and q Differences (CNTR - FDDA) Over the Model somewhat warmer surface T in the CNTR run shown in Ta- Domain at Every 6 Hours at (a) the Surface (b) = ble 4a hints that the ACARS profiles in this case tend to 0.68, and (c) = 0.32 (p 300 hPa) slightly reduce the model’s warm bias.

(a) Surface b. E2, E3, and E4

These three experiments are designed to examine how u (m/s) v (m/s) T (oC) q (gm/kg) t(h) MM5 FDDA responds to changes in the magnitude of (i) M SD M SD M SD M SD nudging parameters, (ii) radii of influence, and (iii) data den- sity. Inter-comparison based on the means and SDs of fore- 12 -.01 .06 .00 .06 .01 .01 .00 .00 cast errors show minimal differences between CNTR and 18 .06 1.20 .10 1.34 .04 .91 .01 .31 these three experiments. However, some interesting varia- tions are present in the simulated 36-h accumulated precipi- 24 .08 1.85 .04 1.86 .01 1.20 .03 .70 tation pattern as shown in Fig. (18). The basic structure and 30 .07 2.05 .00 2.34 .07 .99 .00 1.12 magnitudes of (CNTR-E4) and (E4-CNTR, which is not 36 -.09 1.90 .03 2.14 .07 .74 -.09 1.28 shown) strongly resemble those of (CNTR-E1) and (E1- CNTR) depicted in Fig. (16). We also note that (CNTR-E2)

and (E2-CNTR) shown in Fig. (18) resemble their counter- (b) = 0.68 parts in E3 (not shown). However, Fig. (18) does differ con-

siderably from Fig. (16). In particular, (E2-CNTR) shows a u (m/s) v (m/s) T (oC) q (gm/kg) much less organized configuration with many isolated pre- t(h) M SD M SD M SD M SD cipitation centers. All experiments except CNTR produce an area of heavy rainfall observed on the Texas-Oklahoma bor- 12 -.04 .19 .02 .12 -.01 .05 .00 .00 der. The MM5 model appears to not be sensitive to the re- moval of proximate data in E4, while the model’s QPF re- 18 .15 1.09 .17 1.42 .02 .29 .01 .47 sponds significantly to both the reduction of horizontal ra- 24 -.04 1.78 .33 2.39 .04 .47 .01 .70 dius of influence in E2 and the change in the nudging factor 30 -.11 1.71 -.24 1.87 .00 .49 -.03 .52 in E3.

36 .09 1.65 -.18 1.65 .01 .79 .03 .48 SUMMARY The study is designed to test the underlying hypothesis (c) = 0.32 that high frequency mesoscale FDDA using the observa- tional nudging method can significantly improve mesoscale u (m/s) v (m/s) t (oC) q (gm/kg) NWP. The work was carried out based on MM5 real-data t(h) simulation experiments in conjunction with WTM surface M SD M SD M SD M SD data and ACARS profile data. Three case studies were se- 12 -.00 .03 .00 .03 .00 .00 .00 .00 lected for the experiments. For each case, we conducted a control run without invoking data assimilation, several 18 .12 1.43 -.02 1.49 -.02 .42 .01 .09 FDDA runs, and then a subsequent forecast validation. 24 -.08 2.72 .78 2.55 .20 .62 .00 .18 The two WTM cases represented rather tranquil synoptic 30 .44 2.76 .18 2.77 .20 .61 .00 .23 conditions over West Texas. Our findings indicated that 36 .11 1.81 -.41 2.55 .13 .53 -.04 .26 nudging the MM5 model with surface-based data alone over such a relatively small area such as the WTM domain had limited impact on the model’s performance. It is basically a curves suggest MM5 performs better in v than u below Level three-dimensional (two horizontal dimensions plus time) 6 (e.g; lower boundary layer and near surface). For T and q, assimilation. The impulses caused by the data insertion dissi- the relatively large SDs are found near the surface (below pated quickly within three hours after the “turning off” of Level 18) suggesting lower forecast skill in the model PBL. nudging regardless of the length of the nudging/assimilation Also for this experiment, the mean values reveal that the period. The WTM case study indicates that worthwhile model surface air is too warm and wet by about 1°C and 1 mesoscale data assimilation probably requires additional gm/kg, respectively. The 36-h root mean square (RMS) er- vertical profile observations of the atmosphere. rors of u, v, and T have a similar size to those of 24-h errors The severe weather event of 8-9 May 2005 which oc- (for typical regional model forecasts) summarized by Anthes curred over Central Texas was selected for the ACARS ex- [16]. There are only very minor differences in the means and periments. Widespread precipitation was reported over Cen- SDs of forecast errors between CNTR and E1. The FDDA tral and East Texas during the 24-h case study period. The does clearly alter the model wind, T, and precipitation pat- areal means of the model’s difference fields (for various me- terns, but it is difficult to determine whether such changes teorological parameters) between the control and individual truly represent improvement in terms of the more conven- FDDA runs showed some temporal oscillations with rela-

Mesoscale FDDA Experiments with WTM and ACARS Data The Open Atmospheric Science Journal, 2008, Volume 2 127

(a) (a) 105 w 100 w 90 w 100 w 100 w 90 w

.317 L L -.184 .016 L H .170 1.00 .207 H 15.5 1.11635 L L -.155 -.553 N 3 N 3 H 15.1 H L L 1.27 317 1.27 10.0 371 .012 575 .635 L 635 H -3 L .000 .055 .317 H 5.08 .317 12.0.54 L .168 10.0 L N H 3 8.63 21.3 9.31

N 3 317635 H 4.11

(b) .317 105 w 100 w 90 w 0.0 H 0.0 0.0 0.0 .039

0.0 0.0 0.0 0.0 (b) L 0.0 H H 105 w 100 w 90 w .813 1.15 -3.38

0.0 L 0.0 -1.64 H N .184 0.0 3 0.0 0.0

0.0

0.0 L L 0.0 H -.207 L 1.45 -1.11 0.0 -.4190.0 H L H .155 H 1.61 1.05 .553 H 0.0 3.38 H

2.75 N 3 0.0 0.0 0.0 317 635 .27 H L 3 N L 0.0 0.0 -.265 6.210.00 3.54 H 0.0 L 3.37 H 0.0 0.0 0.000 H H 1.27 5.96L 2.40 L 3.46 635 -12.0 0.0 0.0 -1.98 0.0 H 317 635 (c) 1.27 635 2.54 1.27 105 w 635 100 w 90 w 317 N 317 3 0.0 317 0.0 0.0 0.0 0.0 635 L H H 4.11 L .326 3.57 -.051 0.0 0.0 H L .328 0.0 3.24 0.0 0.0 0.0 0.0 Fig. (16). 36 h differences in accumulated precipitation (cm): (a) N 0.0 3 0.0 0.0 L CNTR–E1 and (b) E1-CNTR. 0.0 1.63 L 0.0 .233 H tively small amplitudes, suggesting that the model was un- 4.00 1.31 0.0 H 0.0 dergoing only mild adjustments due to the insertion of the L 4.00 4.64

.060 0.0 L 0.0 H 7.4 ACARS data. There was no indication of a data rejection .505 H problem. The corresponding standard deviations showed that 0.0 0.0 0.02.70 L impulses caused by insertion of the ACARS profile data had 4.00 0.0 1.36 N 3 lasting impact on the MM5 forecasts throughout the entire 0.0 0.0 L

7.69 0.0 model atmosphere, even 12 h after the termination of FDDA. H The ACARS data distribution was not very uniform in the 2.70 H .391 middle and lower layers. Nonetheless, some large changes 0.0 0.0 0.0 (control minus FDDA) in the model surface flow and tem- Fig. (15). 36-h differences (CNTR-E1): (a) surface winds (m/s), (b) perature fields were found far away from the areas where the T (oC), and (c) q (gm/kg). 128 The Open Atmospheric Science Journal, 2008, Volume 2 Chang and Dumais

(a) (b) 6.0 10

B

B 5.5 9 B

B 8 B 5.0 B

B 7

4.5 B B

B B B

6 B 4.0

B B B B

B 5 B B B B 3.5 B 4 B B B B B B 3.0 B B B 3 B B

2.5 2

2.0 A A 1

A A 1.5 0 A A A A A A A A A A -1 A 1.0 A A A A -2 .5 A A

A -3 A 0 A -4 -.5 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24 (c) (d)

2.4 2.4

2.2 2.2 B B

B B 2.0 B 2.0

B 1.8 B 1.8

B

B 1.6 1.6

B

1.4 1.4 B

B B B 1.2 B 1.2 B B B 1.0 B B 1.0

B A B .8 .8 A A A .6 .6 B A

A .4 A .4 B .2 A .2 B A 0 B 0 A B A A A A

A -.2 A A -.2 -.4 A -.4 A A A -.6 -.6 A A -.8 A -.8 A A -1.0 A 024 6 8 10 12 14 16 18 20 22 24 -1.0 12 0 2 4 6 8 10 14 16 18 20 22 24 Fig. (17). Mean (Curves A, the upper ones) and standard deviation (Curves B) of differences (Observation-CNTR) at 36 h as a function of model levels 1 to 23 (top to bottom). (a) u in m/s, (b) v in m/s, (c) T in°C, and (d) q in gm/kg.

ACARS data were concentrated. While ACARS in this in- verified better with the observations. The sensitivity experi- stance caused substantial variations in model temperature ments revealed a rather significant response of MM5 QPF to and wind forecasts, it is clearly difficult to make general the changes in the nudging parameters. Some ACARS data statements about forecast impact (either positive or negative) points were located very close to each other. This could be and identify precisely such based on just this single case due to multiple reports of the same data. However, averaging study. One positive note is that ACARS data slightly reduced data within the same proximity in E1, in an attempt to avoid the MM5’s surface warm bias for this case. the possible multiple usage problems, appeared to have little FDDA appeared to improve somewhat the QPF skill of effect on the model’s QPF performance. MM5. In comparison with the control run, the locations of An advanced mesoscale model may perform well in heavy rainfall areas simulated with ACARS data in E1 some cases but not in others, and the model responses to data

Mesoscale FDDA Experiments with WTM and ACARS Data The Open Atmospheric Science Journal, 2008, Volume 2 129

(a) systems, high wind events) are desired, especially those oc- 105 w 100 w 90 w curring in the vicinity of the WTM domain. This will enable us to carry out a comprehensive MM5 model validation us- L .317 ing the WTM tower T, wind, and rainfall observations. This - .110 H H research will hopefully allow us to reveal the most likely .111 1.03 areas for greatest MM5 (and later, WRF) model improve- L -.655 ment in West Texas, through the use of composite local forecast statistics derived from a large number of diverse mesoscale weather events. It may also help us to determine N 3 the optimal nudging parameters for use in our West Texas 3.17 MM5 FDDA implementation in order to improve local QPF 1.27 and BL forecasting. We would gain a physical understanding H of how to capitalize upon the locally available high fre- . 000 .635 .317 .635 quency observational data, and develop real time guidance in L 3172.5485 using these valuable data sources. Of course, running the L N -5 -.875 4.7 67 model in other geographical regions using different sources 163 .36.35 of local observations will likely lead to different conclusions N 3 and physical/statistical relationships, so the future focus here

.317 L is on model applications over our West Texas areal region. -2 55

.317 ACKNOWLEDGEMENTS The authors wish to acknowledge the data provided by

the West Texas Mesonet team, Dr. John Schroeder and Mr. (b) Wes Burgett at Texas Tech University, and the program- 105 w 100 w 90 w matic and management assistance of The Institute for Envi- ronmental and Human Health at Texas Tech University. Part H of the study was supported by Research Development and L.110 L -.111 -1.03 Engineering Command (RDECOM), Department of the H Army. .655.317 REFERENCES

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Received: April 12, 2008 Revised: April 16, 2008 Accepted: May 16, 2008

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