  The streaming instability (SI) provides a promising mechanism for
planetesimal formation because of its ability to concentrate solids into dense
clumps. The degree of clumping strongly depends on the height-integrated solid
to gas mass ratio Z in protoplanetary disks (PPDs). In this letter, we show
that the magnitude of the radial pressure gradient (RPG) which drives the SI
(characterized by $q\equiv{\eta}v_K/c_s$, where ${\eta}v_K$ is the reduction of
Keplerian velocity due to the RPG and $c_s$ is the sound speed) also strongly
affects clumping. We present local two-dimensional hybrid numerical simulations
of aerodynamically coupled particles and gas in the midplane of PPDs. Magnetic
fields and particle self-gravity are ignored. We explore three different RPG
values appropriate for typical PPDs: $q=0.025, 0.05$ and 0.1. For each $q$
value, we consider four different particle size distributions ranging from sub
millimeter to meter sizes and run simulations with solid abundance from Z=0.01
up to Z=0.07. We find that a small RPG strongly promotes particle clumping in
that: 1) At fixed particle size distribution, the critical solid abundance
$Z_{\rm crit}$ above which particle clumping occurs monotonically increases
with $q$; 2) At fixed Z, strong clumping can occur for smaller particles when
$q$ is smaller. Therefore, we expect planetesimals to form preferentially in
regions of PPDs with a small RPG.
