RADAR OBSERVATIONS DURING NAME 2004PART II: PRELIMINARY RESULTS

Timothy J. Lang, Stephen W. Nesbitt, Steven A. Rutledge, and Rob CifelliColorado State University, Fort Collins, Colorado

David Ahijevych and Richard E. CarboneNational Center for Atmospheric Research, Boulder, Colorado

1. Introduction

2. Methodology

4. Defining Regimes

5. Precipitation Feature Statistics

6. Environmental Influences

7. Conclusions

8. ReferencesAhijevych, D. A., R. E. Carbone, T. J. Lang, and A. V. Manzanillo, 2005: The diurnal cycle of rainfall and the identification of rainfall regimes within the North American monsoon of NW Mexico. 32nd Conf. on Radar Meteorol., Amer. Meteorol. Soc., Albuquerque, NM.Higgins, W., et al., 2005: The North American Monsoon Experiment (NAME) 2004 field campaign and modeling strategy. Submitted to Bull. Amer. Meteorol. Soc.Lang, T. J., R. Cifelli, L. Nelson, S. W. Nesbitt, G. Pereira, S. A. Rutledge, D. Ahijevych, and R. Carbone, 2005: Radar observations during NAME 2004. Part I: Data products and quality control. 32nd Conf. on Radar Meteorol., Amer. Meteorol. Soc., Albuquerque, NM. Mesinger, F., G. DiMego, E. Kalnay, K. Mitchell, P. C. Shafran, W. Ebisuzaki, D. Jovic, J. Woollen, E. Rogers, E. H. Berbery, M. B. Ek, Y. Fan, R. Grumbine, W. Higgins, H.

Li, Y. Lin, G. Manikin, D. Parrish, and W. Shi, 2005: North American regional reanalysis. Bull. Amer. Meteor. Soc, in revision.Nesbitt, S. W., E. J. Zipser, and D. J. Cecil, 2000: A census of precipitation features in the Tropics using TRMM: Radar, ice scattering, and lightning observations. J. Climate, 13, 4087-4106.Nesbitt, S. W., R. Cifelli, and S. A. Rutledge, 2005:  Storm morphology and rainfall characteristics of TRMM precipitation features.  Submitted to Mon. Wea. Rev. 

Contact Info: Timothy Lang, CSU Atmospheric Science, Ft Collins, CO 80523 (970) 491-6944, tlang@atmos.colostate.edu

JP3J.6

The enhanced observation period (EOP) of the North American Monsoon Experiment (NAME; Higgins et al. 2005) took place during July and August of 2004. A major component of the EOP was observations from a multi-radar network placed in Tier I (Lang et al. 2005), the core monsoon region consisting of the Gulf of California (GoC) and the Sierra Madre Occidental mountain range (SMO) in northwestern Mexico (Higgins et al. 2005).

The primary goal of the radar network was to characterize and understand convective and mesoscale processes in the complex terrain of the core monsoon region. We examine several outstanding research questions and hypotheses relevant to the North American Monsoon (NAM): a. Diurnal cycle of precipitationRadar-based assessment of the spatial and temporal variability of precipitation in a tropical region of significant orography (e.g., the SMO) has never been attempted before. Thus, we will use the NAME radar network to observe and describe statistically the regular daily cycle of precipitating systems over the high SMO, the western slopes, the Gulf of California coastal plain, and the southern Gulf region.

b. Intraseasonal variability of precipitation A major goal of NAME is to better understand regimes associated with intra-seasonal variability of convection during July-August in the Tier I region and its linkages to precipitation in the southwestern U.S., including influences of Gulf surges, jets, easterly waves, surface fluxes, and topographic blocking. c. Relative importance of organized systems for precipitationIt is hypothesized that mesoscale convective systems and other modes of organized convection contribute significantly to total rainfall within the Tier I domain. We will examine the relative importance of organized systems as a function of both the diurnal cycle and meteorological regime (i.e., intraseasonal variability). d. Role of terrain in initiating and organizing convectionIt is hypothesized that terrain plays a major role in organizing convection over the Tier I domain. S-Pol and SMN radar observations can be used to assess the morphology and organization of storms relative to major terrain features.

Radar composite domain (Lang et al. 2005) rotated and cropped to examine along-coast and cross-coast variability.

Exclude Baja and Pacific Ocean areas due to potential sea clutter problems with Version 1 composites.

Use Precipitation Feature (PF) analysis technique based on Nesbitt et al. (2000). Contiguous areas (including corner pixels) of composite equivalent radar reflectivity ≥ 15 dBZ are considered as a PF within each composite.

An ellipse-fitting procedure, developed by Nesbitt et al. (2005), was applied to each PF to objectively estimate the maximum dimension (i.e. twice the major axis length of each ellipse) of each feature. Features were subsetted geographically, by regime (defined below), and by their characteristics.

A subset of PFs has been identified as organized features. These features meet the criteria that their maximum horizontal dimension is ≥ 100 km, and the storm contains at least 16 km2 of convective area as determined by convective-stratiform separation.

Composite reflectivity from 5 August 2004 at 2315 UTC (1715 LT). The best-fit ellipse of each feature is plotted over each identified PF within the composite.

We observe at least two distinct precipitation regimes, as explained by Ahijevych et al. (2005).

Figure to left shows an example of reduced dimension time series from Ahijevych et al. (2005). LHS is Hovmoller in cross-coast dimension; RHS is along-coast.

Regime A is mainly characterized by a coherent progression of enhanced rainfall from the SMO to the GoC. Precipitation over the GoC is mainly nocturnal. Propagation is ~7 m s-1.

Regime B is defined as conditions when a northward progression of precipitation is prominent. Propagation is ~10 m s-1. Regimes A and B overlap considerably.

Location of all organized features during the day. Organized features mainly appear over land during afternoon, with some propagation toward coast. During undisturbed periods organized features are mostly confined to land. The majority of over-Gulf early morning organized features occur during the disturbed AB Regime, and in the south.

Time series of PF statistics. During disturbed Regimes, there are increases in feature rainfall, max dimension, and organized feature fraction. Mean convective fraction is approx. constant.

Diurnal cycle of PF stats by Regime. Regime AB behavior is similar to Regime A or Regime B only. Bi-modal behavior - early morning and evening peaks

Diurnal cycle of PF stats by cross-coast location. SMO Peaks and Foothills peak in evening, while GoC peaks in morning. Coastal Plain has combination of signals from afternoon sea-breeze convection and evening PFs drifting off SMO.

Diurnal cycle of PF stats by along-coast location. Morning signal only important in southern portion of domain.

Regime-composite skew-T/log-p diagrams at Los Mochis. Regime AB soundings are the moistest on average. All Regime-averaged soundings contain at least 1500 J kg-1 of CAPE. However, there is a significant cap in the averaged soundings, with 40-70 J kg-1 of CIN present. It is apparent that thermodynamic profiles are sufficient for deep convective initiation if the cap could be broken.

Regime-averaged wind profiles composited as above, and rotated 35° to look at coast-normal and coast-parallel shear profiles. Steering winds blow toward the coast at 3-4 m s-1. Values of 0-4 km shear are larger at Mazatlan, nearly by a factor of two during disturbed Regimes. This increased shear may explain the prevalence of the longer-lived convective systems over the southern portion of the radar domain, which were observed to last into the early morning hours.

To examine large-scale flow patterns corresponding with these Regime periods, North American Regional Reanalysis data (NARR – Mesinger et al. 2005) are used. During Regime AB, the radar domain is located in close proximity to a tropical easterly wave (EW) trough (in a composite sense).

Terrain played a key role in initiating and organizing convection in our study domain. Precipitation along the SMO initiates in the late afternoon and advects west toward the GoC, where it peaks overnight. The GoC maximum mainly occurs in the south, under disturbed meteorological conditions (Regimes A and B), when there is more shear. During propagation there is gradual growth in size of PFs.

During the NAME EOP, there were at least two significant Gulf surges (Higgins et al. 2005). Gulf surges tended to overlap with disturbed precipitation Regimes. But there were many disturbed Regime periods not associated with a Gulf surge. However, this study suggests a possible link between tropical EWs and precipitation regimes. The development and propagation of organized systems (e.g., MCSs and other large precipitation features) are a key component of the diurnal cycle of precipitation in this region, particularly during disturbed Regimes. Organized systems are particularly important for rainfall in the evening along the SMO, and during the early morning along the southern portions of the coast and GoC.