P4.21 Multiscale Analyses of Moisture Transport by the Central Plains Low-Level Jet during IHOP

1. Platform/Flight Level (FL)/ Instruments June 3 2002 June 9 2002 Learjet/ ~21000 ft/ Dropsondes 1100-1330 UTC 1440-1730 UTC clockwise circuit from NW corner of box 44 sondes (18 w/o winds) 1246-1454 UTC 1653-1850 UTC clockwise circuit from NW corner of box 58 sondes DLR Falcon / ~23000 ft/ Dropsondes DIAL HRDL 1100-1330 UTC 1440-1730 UTC clockwise circuit from SE corner of box 45 sondes 1210-1522 UTC 2nd Flight cancelled clockwise circuit from SE corner of box 21 sondes (2 w/o winds) some bad HRDL data on Eastern N-S leg some DIAL data missing on NE part of E-W leg NASA Proteus/ ~56000 ft/ NAST- I 1313-2133 UTC some cirrus contamination 1210-1700 UTC repeated southern E-W leg NASA DC-8/ ~17000-25000 ft/ LASE 1700-2200 UTC flight for Convective Initiation only. 1320-1723 UTC repeated passes over Northern E-W leg Ground based special obs SPOL Doppler radar reflectivity and radial velocity SPOL is located within dropsonde a/c flight box Data avail. all day SPOL is on South border of E-W leg of flight box Data avail after 1415 UTC Aeri hourly (0-3km) T/q profiler sites Homestead, Vici sites are within flight box Homestead site located on south border of E-W leg ARM central facility Located within Flight box Located south and east of a/c flight box Special NWS radiosondes 15, 18, and 21 UTC 15,18, and 21 UTC P4.21 Multiscale Analyses of Moisture Transport by the Central Plains Low-Level Jet during IHOP Edward I. Tollerud1, Brian D. Jamison2, Fernando Caracena1, Steven E. Koch1, Diana L. Bartels1, R. Michael Hardesty3, Brandi J. McCarty2, Christoph Kiemle4, and Gerhard Ehret4 1NOAA Research-Forecast Systems Laboratory, 2Cooperative Institute for Research in the Environmental Science, and 3NOAA Research-Environmental Technology Laboratory, Boulder, CO; 4German Aerospace Center (DLR), Germany 3. The Structure of the LLJ during the 9 June Mission The low-level jet core on June 9 was well-developed and intense, extending from the southwest corner of the flight pattern to a region just east of the northwest corner. The Falcon HRDL data displayed in Fig. 5 illustrates this horizontal pattern across the flight box. Below the level displayed, the wind maxima along the southern leg extended further east, presenting a more diffuse and wide LLJ than suggested by the figure. A dropsonde-derived wind and thermodynamic profile near the LLJ core on the northern leg (Fig. 6) reveals the vertical extent of the jet, which is also shown in the vertical section of windspeed along the northern leg shown in Fig. 7. At the core along the northern leg, the jet has a slight westerly component, a characteristic which is markedly stronger along the southern leg (not shown). Dropsonde sections at 55 km intervals appear to capture the main features of the LLJ. Analyzed at a larger scale (cf. the radiosonde-derived section near the mission start time in Fig. 8), the jet is still clearly shown and of even slightly greater magnitude, but is more diffuse and unfocused compared to the dropsonde-derived feature. Sections of lidar observations of wind (Fig. 9) and specific humidity (Fig. 10) show both fine-scale (<1 km) and small-mesoscale (10-25 km). The significance of these small-scale variations in transporting moisture is a primary problem to be addressed in this project. 4. Comparison of Moisture Transport along Sections through the 9 June LLJ The results illustrated in this section represent qualitative answers to principal issue addressed in this research: What effect do differences in resolution of observations have on measurements of moisture transport by the LLJ? Moisture transport (v x q) orthogonal to the rectangular rectangular by different scales of motion is shown here as sections along the northern and southern legs of the flight pattern of Fig. 2. Although the LLJ was intersected along both legs, a more completeset of aircraft observations were obtained along thenorthern leg. Large-scale estimates (otherwise interpretable as the picture presented by operational data platforms) of moisture transport by the v wind component (used here because it is transverse to the the east-west-oriented northern and southern flight legs) are shown in Fig. 11. Compared to the section of dropsonde-observed transports along this same leg (Fig. 12), the large scale transports at the jet core are slightly larger (350 vs. 250) but not as focused. However, comparison of the two sections of transport estimates are complicated by the difference in observation times (nominally 1200 UTC for the radiosondes, and between 1350 and 1430 UTC for the dropsondes). Future analysis of the Learjet dropsonde observations (which were made along the northern track closer in time to 1200 UTC) will offer a better comparison. However,these latter observations are without supporting airborne lidar measurements, limiting their usefulness for other purposes. 1. Introduction and Objectives During the International H2O Project (IHOP) in the Southern U.S. Central Plains (Weckwerth et al. 2004) in May and June 2002, aircraft missions on June 3 and June 9 2002 made detailed lidar and dropsonde observations of intense phases of the low-level jet (LLJ) . Combined with standard and enhanced operational observations (primarily radiosondes and profilers) and other ground-based research observations, data from these missions represent an unprecedented opportunity to de-scribe moisture transport in the LLJ at scales rang­ing from the synoptic scales resolved by the radiosonde network to sub-mesoscale features in the moisture and wind fields observed by airborne lidar instruments. Two questions immediately present them-selves: (1) Do focused observations at exceptionally high resolution provide details critical to our physical understanding of the LLJ; and (2) would inclusion of these details in model initialization fields significantly alter model-generated forecasts of LLJ evolution, transport, and subsequent precipitation generation? A practical way of stating (1) is: do small-scale correlations between moisture and wind fluctuations within the LLJ significantly alter larger-scale estimates of LLJ moisture transport? To illustrate this possibility, we present vertical sections across the LLJ of wind, moisture, and resulting moisture transport from multiple observation sets including radiosonde only, dropsondes, and simultaneous lidar measurements of moisture and wind. Cumulative transport through the sections will also be presented to estimate possible scale effects. In future work, we will directly address (2) by comparing analyses and numerical predictions from a control run of the Weather Research and Forecasting Model (WRF) made with operational datasets with an otherwise parallel analysis and forecast that have the advantage of input from research dropsonde observations and other research data. Fig. 11. Moisture transport (v x q) (shaded, with contour interval of 50 gkg-1ms-1) transverse to the northern leg of the 9 June flight box. Estimates are based on analysis of radiosonde observations at 1200 UTC. Vertical dashed lines denote west and east ends of the northern leg. Potential Temperature contours also shown (contour interval of 1oK). Fig. 5. HRDL-derived winds near the top of the LLJ during the first Falcon circuit on 9 June. 2. Representative Scales of Motion and Corresponding IHOP Observations An important aspect of these investigations is a determination of the role that motions of various scales play in the transport of moisture by the LLJ. As Fig. 1 indicates, several scales of motion must be observed with the relevant IHOP instrumentation. With these potential scales and availablae data sets in mind, the following observational strategies were employed. Two dropsonde aircraft (the DLR Falcon and a Learjet) flew box patterns chosen to bracket the predicted location of the LLJ on a morning when a strong LLJ is predicted. An example is the box pattern flown on 9 June (Fig. 2). The box size, location, and orientation were also chosen with regard to possibilities for computing full-domain moisture budget terms (see Section 5). Mission days were selected during clear-to-partly cloudy conditions since lidar systems were flown. The NCAR GPS dropwindsonde flown on these aircraft provide wind accuracies of 0.5 -2.0 m s-1 with an impressive vertical resolution of ~5 m and detailed humidity measurements with accuracy of at least 2% all the way down to the surface.The aircraft thus provided full wind and moisture profiles below flight altitude at approximately 55-km intervals along the periphery of the flight box. It was the original intent to have each aircraft complete the full rectangular circuit, refuel, and then repeat the circuit a second time. Instrumental and aircraft constraints forced some adaptations to this original plans. A smaller-scale description of moisture and airflow around the rectangular domain were provided by the DLR Falcon aircraft lidar measurements of winds (from High-Resolution Doppler Lidar, HRDL) and specific humidity (from the downward pointing Differential Absorption Lidar, DIAL). Only winds transverse to the aircraft flight path were measured by the HRDL. Additional moisture observations were available along part of the flight patterns from the Lidar Atmospheric Sensing Experiment (LASE) on the NASA DC-8 and the NPOESS Airborne Sounder Testbed (NAST) on the NASA Proteus research aircraft. The major strength of lidars is their ability to provide very detailed vertical profiles unrestricted by ground clutter, but they are restricted in their utility to the absence of clouds and precipitation, and have limited range due to attenuation effects. With these measurements, water vapor variability within the domain in the middle to lower troposphere can be calculated at small (lidar measured) scales in the optically clear air to complement the larger effective scale of the dropsondes available in all weather conditions. The combined use of the DIAL, LASE, NAST, and HRDL allows us to address the question of the scale-dependence of the moisture transport in the presence of a low-level jet. To provide the large-scale setting to the LLJ, the operational data stream of radiosonde, profiler, and surface observations were analyzed. For the two MLLJ cases during IHOP, 3-hourly special radiosonde launches were also available at conventional NWS sites. Table 1 describes the full set of observations for the June 9 case. Table 1. Observations for the June 9 case. Fig. 7. Windspeed (m/s) along the northern leg of the 9 June flight box. The core of the LLJ is near the eastern end of the flight leg at a pressure of about 820 mb. Fig. 6. Jet Core Dropsonde Profile at jet core along northern leg of 9 June flight box. Standard units apply. Fig. 12. Moisture transport (v x q) (shaded, with contour interval of 50 g/kg-1ms-1) orthogonal to the northern leg of the 9 June flight box. Estimates are based on analysis of Falcon dropsonde observations. Left and right plot edges coincide with west and east ends of the northern leg; small tic marks are spaced at 10 km intervals. Potential temperature contours are also shown (contour interval of 1oK). The profiles of moisture transport within a column in Fig. 13 provide more detail and also allow comparison with parallel measurements made by the combination of lidar instruments (the HRDL and DIAL) on the DLR Falcon. To synchronize the time of measurement of the various observations as much as possible, this comparison is made for a point along the southern leg sampled by the Falcon just after 1250 UTC. There is surprising agreement between the platforms, both in vertical distribution and magnitude. Since lidar values along this southern leg were attenuated before they reached the ground, it is possible that measurements at the height of maximum jet velocity and below, when (and if) they can be retrieved, will show flux values larger than the other two platforms. The curve of along-track moisture transport (in this case, transport by the u-component of the wind) as specified by dropsondes is included to help assess the impact of having just one component of lidar-observed winds. At lower levels, the winds are very nearly meridional, so that along-track transports are small. However, as the wind turns with height, the other component of transport approaches the transverse component in magnitude. We thus conclude that estimates based solely on lidar winds and moisture measurements will significantly underestimate the total transports at some locations and heights along the legs of the aircraft flight tracks. Fig. 8. Radiosonde analyses at 1200 UTC 9 June along the northern flight leg. Wind, potential temperature, and height anomaly are shown. Fig. 9. HRDL-derived windspeeds normal to the flight path along the northern leg of the 9 June flight box. Radiosonde-Scale Dropsonde-Scale Fig 13. Comparison of cross-track transports (v x q; units as in Fig. 12) observed at 1251 UTC near the eastern end of the southern leg of the flight box on 9 June . Brown curve describes along-track transports (u x q) observed by dropsondes but not measured by the DLR Falcon HRDL system. Fig. 10. DIAL (onboard lidar, DLR Falcon) observations of specific humidity along the northern leg on 9 June. Anomalous rectangular regions near east end of section suggest cloud contamination. Lidar-Scale Fig. 1.Scales of Moisture Transport in the LLJ. Fig. 2. Flight pattern of the Learjet and Falcon on 9 June. 5. Moisture Budget for the Low-Level Jet The geometry and other flight characteristics of the LLJ missions suggest the applicability of a moisture budget analysis. Indeed, as indicated previously, the mission flight patterns were partly chosen to make this a possibility. A methodology for the computation of the horizontal transport terms of this budget, for example, is readily apparent in Fig.14; the integrated normal component vectors along the perimeter of the box describe the net gain/loss of moisture within the box. As illustrated in Fig. 15, the integrated values along the southern leg of the 9 June flight pattern (with the LLJ blowing into the domain) roughly match those out of the box along the northern leg. A mathematical framework for a budget is given in the equation panel to the right. Previous moisture budget studies over the central U. S. have been aimed at large scales that can be adequately resolved with the existing radiosonde network. Imbalances in these budgets have been attributed to observational limitations, especially to inadequate time resolution of the twice-per-day radiosonde releases. For a phenomenon like the LLJ with strong diurnal variation, this difficulty is particularly troublesome. IHOP offers some real advantages but suffers some disadvantages as well. On the positive side, the very detailed moisture flux measurements and surface observations will provide moisture fluxes needed to correct the broad-brush large-scale estimates. Nevertheless, the mix of observation types over a rather limited domain presents a challenging problem for assimilation and analysis of data into a form suitable for budget calculations. With these assets and liabilities in mind, we are pursuing a limited budget study that is specifically focused on the LLJ. Three natural scales emerge from the array of IHOP observations (cf. Fig. 1). Synoptic scale circulations can be diagnosed by the expanded radiosonde and conventional observations of the operational array. Moving down in scale, the dropsonde observations and profiler sites describe scales close to the size of the IHOP domain and the width of the LLJ, but are not suited for fine-scale flux measurements. This third scale can be described, however, by the lidar and other specialized observing systems over smaller regions within the IHOP domain. Our budget computations will concentrate on the middle of these scales, with input from smaller scales to estimate the magnitude of subscale covariances to the budget. The subscale contributions to the various budget terms are described in the equation panel to the right. We believe that dropsonde observations will provide accurate estimates of cross-boundary moisture fluxes at time and space scales relevant to the LLJ. On the other hand, difficulties with closure of this moisture budget are obvious. For instance, direct observations of cloud water (if any) moving out of the dropsonde array will not be available, nor will direct observations of the vertical transport of moisture. However, if these terms can be estimated, then budget calculations may give valuable insight into the important mechanisms affecting moisture transport in the LLJ. • Moisture Budget for IHOP Domain • At a point: • q/t + vq (q)/p  R where R=(errors, P, surface flux,etc.) • Over a volume: • q/t + vq (q)/p R • Term by term: •  vq =  vq + v’q’ • Budget domain is 3D box enclosed by data “curtains” beneath flight track • Compute average horizontal flux as  vq dl • Domain size and flight patterns are compromise chosen to legitimize assumption of stationarity The full set of available observations from the flights on 9 June are illustrated in Fig. 3. In addition to those onboard the Learjet and the Falcon, othe liddar observing platforms were also available on aircraft, and from ground-based observations are available for comparison and model initialization. However, the westward location in the Oklahoma panhandle and western Kansas meant that other surface observations in the ARM-CART array were not well-placed for this case. Fig. 4, however, shows doppler winds from the S-POL radar located in Homestead in the central Oklahoma pan;handle. These observations may provide a valuable source of wind measurements for comparison with dropsonde and lidar winds along the flight path. Fig. 15. Vertically-integrated density-weighted moisture transport normal to the flight track between the surface and aircraft flight level computed from dropsondes along the south (blue) and north (red) legs of the 9 June DLR Falcon flight path. . Fig. 14. Vertically-integrated density-weighted moisture transport along the perimeter of the 9 June mission flight path computed from DLR Falcon dropsondes. Multiply vector units by ten to get transport in kg/m/s. Terrain values are displayed in meters, with a contour interval of 100 m. Fig. 4. S-POL doppler winds measured on 3 June at Homestead (see Fig. 3) during research LLJ flights. Fig. 3. A survey of available data for the 9 June research mission. The yellow cluster of obslerving sites in the center are near Homestead.h